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From Gold Coins to Cadmium Light

On this page, we will take a brief look at some of the complications encountered when attempting to define units of measurement with high accuracy.

Time and Length

The second is defined as the time taken by 9,192,631,770 oscillations of the microwave radio frequency produced by an atom of Cesium-133 when the electrons in that atom are in the ground state, except for one that has emitted this radiation by making the transition from the upper hyperfine level of this state to the lower one. This definition was chosen to make the length of the SI second the same as that of a second of Ephemeris Time and so when this second began to be used in civil timekeeping (the changeover to "atomic time"), the use of "leap-seconds" became necessary immediately.

The 1952 definition of the second of Ephemeris time was based on the instantaneous length of the tropical year being 31,556,925.9747 seconds at the beginning of the year 1900, which was considered to be Noon GMT of December 31st, 1899, but according to Ephemeris Time rather than civil time. However, while the length of the year at that time was the basis for the standard, the length of the second was not 1/86,400 of the length of the day at that time, but was rather based on the average of the length of the day from 1750 to 1892 as previously worked out by Simon Newcomb.

At one time, the metre was defined as 1,650,763.73 wavelengths of the orange red line in the spectrum of Krypton-86 which corresponded to the radiation emitted by an electron moving from the orbital

2 p     to the orbital  5 d
   10                      5

in an unperturbed fashion.

However, a scientist proceeded to measure the speed of light by performing an accurate measurement of the ratio between the wavelengths (and/or frequencies) of these two types of radiation. In Zen-like fashion, this convinced those responsible for the standards of the fundamental absurdity of the situation, and so now the definition of the second stands, but the definition of the metre has been replaced; now, the metre derives from the second, through the speed of light, which is, by definition,

2.99792458 * 10

metres per second.

This leads to the length of a metre being approximately 30.6633189885 wavelengths of the Cesium-133 microwave radiation noted above: the fact that the wavelength is not a tiny fraction of a metre is why it had previously been considered more suitable to a standard of time, for which a radio frequency which can be manipulated by electronics is more accessible than an optical frequency, than one of distance.

The speed of light in a vacuum can also be given in units of the uniform US/British inch of 2.54 centimetres, which leads to light travelling 186,282 miles, 698 yards, 2 feet, and 5 21/127 inches every second in a vacuum.

Note that it is more convenient to measure the length of waves of light through interference fringes, and the time between oscillations of radio waves through electrical circuitry, however. So the dual definitions allowed both time and distance to be more accurately defined.

Incidentally, as far back as 1927, a definition of the metre in terms of a light wavelength existed, but that definition was based on a line in the spectrum of cadmium: the length of the metre was defined as 1,553,164.13 times the wavelength of the 6436.4696 Ångström cadmium red line.

A. A. Michaelson had found the cadmium red line to be particularly monochromatic; he had made a measurement of the International Prototype Metre in 1892 which indicated the length of the metre as 1,553,163.5 times the wavelength of the cadmium red line, the wavelength of which was taken to be 6436.472 Ångström units.

The figure used in the 1927 standard of 1,553,164.13 wavelengths per metre corresponds closely to a wavelength of 6436.4696 Ångströms for the cadmium red line; that the cadmium red line is exactly 6436.4696 Ångström units long was adopted as a standard for the definition of the international Ångström unit in 1907, based on measurements in 1906 by Benoit, Fabry, and Perot of the length of the metre in terms of the cadmium red line.

Cadmium, however, has eight stable isotopes, the most common of which, Cadmium-114, has a natural abundance of 28.73%. so the accuracy of a standard based on natural cadmium would be limited by the small variations in the wavelengths of the same spectral line between different isotopes.

Prior to the use of Krypton-86 for the standard length of the metre, another possibility that was considered was to use Mercury-198. This isotope of mercury was created in 1946 by bombarding gold with neutrons, as gold, like aluminum, has only one stable isotope, and this method of creating isotopically-pure mercury was simpler than attempting to separate the isotopes of mercury by their miniscule difference in weight. Its 5460.753 Ångström spectral line was the one considered for use as a standard. Also, a current secondary standard for the metre is an iodine-stabilized helium-neon laser, the light from which has a wavelength of 6329.9139822 Ångströms.

It is necessary at this point to add that 3515.3502 wavelengths of the cadmium red line would, by the 1927 definition, subtend some 2.2633475317 millimetres (as opposed to 2.2633485174 millimetres), which seems to indicate a discrepancy in the definition of the Potrzebie. As a little arithmetic shows that this definition of the Potrzebie would lead to a metre of 1,553,163.45 wavelengths of the red line of cadmium in length, it seems apparent that Donald Knuth, then a 19-year old high school student, had converted from millimetres to wavelengths of cadmium light using a reference giving the 1892 figure.

In my efforts to sort out the mystery of that discrepancy, which led me to finding out about the 1892 figure, I had encountered a biographical essay on A. Michaelson by Robert A. Millikan giving 6438.472 Ångströms as the wavelength of the red line of Cadmium as determined by Michaelson which would indeed lead to a standard of about 1,553,163.5 wavelengths per metre. In the essay, the resulting standard as 1,555,165.5, which I took to be a misprint. Thus, the mystery of the difference between the length of the Potrzebie as defined in terms of the metre and that as defined in terms of the cadmium red line appeared to be solved, and further searching led me both to confirmation that 1,553,163.5 was the figure arrived at by Michaelson in 1892 and to find that the later standard derived from the 1906 measurements of Benoit, Fabry, and Perot as noted above.

It may also be noted that in 1964, an agreement was reached between the U.S. and Britain to define the inch as 2.54 centimetres.

Prior to 1964, the inch was defined in the U.S. on the basis that a metre was exactly 39.37 inches long, which led to the inch being about 2.540005 centimetres long, and in Britain the inch was 2.539997 centimetres in length. (One older reference gives the metre being 39.37079 inches, and the inch therefore being 2.539954 centimetres in length.)

Or is the British inch 2.539996 centimetres? There are sources giving both values for the British inch. I found the 2.539997 figure in the Encyclopedia Britannica article on weights and measures, while the 2.539996 figure occurs in news stories from 1938 and 1958 about the ongoing negotiations to establish agreement between Britain and the United States on the inch of 2.54 centimetres, so I would have been inclined to believe the 2.539997 figure to be the more accurate one. Yet another source gives the length of the inch based on the British "Bronze Yard #11" to be 2.53999944 centimetres.

Since this was written, I have found a document on the Web, entitled "Which Inch", which sorts out a considerable amount of confusion.

In the case of the United States, there were several standard values for the inch:

The Troughton Bar, given by Britain to the United States in 1815, was carefully measured, and based on that, in 1832, a standard inch 2.54006833 millimetres in length was adopted.

Then, in 1856, the U.S. received the British "Bronze Yard #11", which was conformant with the new standard British inch adopted in 1855. This gave the new standard American inch of 2.5399944 centimetres noted above.

The definition of the inch that appears in so many older textbooks, where one metre equals 36.37 inches, was adopted in 1892, and the yard based on this standard is called the "Mendenhall Yard".

And then the modern standard of 2.54 centimetres to the inch was adopted in 1959.

Unlike the case of the U.S. inch, where the inch was defined so that 39.37 inches equalled one metre, so that the U.S. inch must therefore be 2.54000508001016002032004064.. centimetres, the British inch was defined by a separate physical standard, so there is no inherent exact ratio between the British inch and the metre.

A 1928 scientific paper, titled "A New Determination of the Imperial Standard Yard to the International Prototype Metre", by Sears, Johnson, and Jolly, gave the ratio of 1 metre to 39.370147 British inches, which gives rise to an inch of 2.5399955596... centimetres. Incidentally, the International Prototype Metre was made in 1872, while the Imperial Standard Yard dates back to 1855.

An earlier measurement, from 1895, gave the length of the metre as 39.370113 British inches, leading to a British inch of 2.53999778969... centimetres. That rounds to 2.539998, and so it cannot alone be the source of the other value; despite the fact that one source notes that the 39.370113 figure was the de facto standard for the length of the British inch for many years.

The Imperial Standard Yard is made from a bronze alloy, 82% copper, 13% tin, and 5% zinc, and it is defined to have its standard length at a temperature of 62 degrees Fahrenheit; the International Prototype Metre is made of 90% platinum and 10% iridium, and its length is valid at 0 degrees Celsius (32 degrees Fahrenheit, or the freezing point of water).

Incidentally, the Imperial Standard Yard was noted as having shrunk by one part per million in a span of 20 years, calling its accuracy into question.

It may be noted that the Pyramid Inch was claimed to be 1.00106 English inches, so that would make it about 2.5426894 centimetres long. The Pyramid inch was said to be 1/25th of a royal cubit.

In fact, an ordinary cubit, about 18 inches long (so they were at least right that cubits related better to Imperial measure than to the metric system) was divided into six spans (each three inches long), which were in turn divided into four digits (each 3/4 of an inch; and, indeed, the keys on our typewriter keyboards have 3/4 of an inch spacing even today). A royal cubit is seven spans instead of six, and so, nominally, it should be 21 inches long, but then standards of measure were less accurate in those days. The royal cubit was used in the construction of the Great Pyramid; thus, its sides had a rise of one royal cubit for a run of five and one-half spans; which, multiplied by four, gives 3 1/7, giving the appearance that pi is involved in the construction of the Pyramids.

In fact, though, serious archaeologists and historians now know that the Egyptian royal cubit was about 52.6 centimetres (give or take 3mm), or 20.7 inches - so it was indeed near to 21 inches, and actually somewhat smaller, and thus not 25.0265 inches long. To the extent, therefore, that such a thing as an ancient Egyptian inch has any meaning, therefore, it would be about 98 4/7 percent of an inch, not 1.00106 inches, in length.

While the Egyptian measure relates to Imperial measure by a factor of about .986, the currently accepted value for the Roman foot makes it .971 feet long; thus, while the foot grew on its way to Britain, in the middle, as it passed through Rome, it shrank. And the Romans did divide the foot into 16 digits as well as 12 inches, and so linking the cubit to the inch as I have done is legitimate.

I think it is unfortunate that they missed their chance to define the inch as being about 2.540002 centimetres in length, so that the diagonal of a square 152 inches on a side would be exactly 546 centimetres, or the diagonal of a square 273 centimetres on a side would be exactly 152 inches. After all, supporters of the metric system have always criticized the Imperial system as irrational; and it would be convenient if having two systems of measurement allowed one, by using both of them, to measure exactly both the sides of a square and its diagonal.

Of course, lengths of 152 inches and 273 centimetres are somewhat unwieldy. However, as rulers measuring inches are often divided in tenths of an inch, one could relate 15.2 inches to 273 millimetres. But inch rulers are more often divided into sixteenths of an inch.

If one were to use the same method to relate the sixteenth of an inch to a millimetre, defining the inch as about 2.5399946 centimetres would lead to a square 284 millimetres on a side having a diagonal of 15 and 13/16 of an inch.

However, failing changes in our systems of measurement, one can always simply make use of the fact that 20 squared is 400, 21 squared is 441, and 29 squared is 841 to come reasonably close to a 45 degree angle and still use exact distances.

Also, even if redefining the inch is excluded, if only a single unit of measurement is used, comparable ratios to approximate the square root of two would be 239:169 and 577:408, which are in error by 0.000875 percent and 0.000150 percent respectively, while, using the inch of 2.54 centimetres, the ratio 152 inches to 273 centimetres approximates the square root of two with an error of 0.000078 percent.

The diagonal of a square 273 centimetres on a side is 386.0803025278548... centimetres, while 152 inches of 2.54 centimetres each are 386.08 centimetres, so there is an excess of 0.0003025278548... centimetres. The older U.S. inch, such that 39.37 inches equal one metre, is still in use for survey purposes, and this inch is equal to 2.54000508001016... centimetres. If, of the 152 inches of the diagonal, 59 and 9/16 of those inches were measured using the older U.S. inch, and the other 92 and 7/16 of those inches were measured using the current inch of 2.54 cm, an even closer approximation to the square root of two would be obtained.

A more approximate measurement of the diagonal of the square can be obtained using much simpler numbers. The diagonal of a square 9 centimetres on a side is 5.0109929... inches in length. For comparison, the diagonal of a square 7 centimetres on a side is 9.8994949... centimetres in length. The discrepancy, in addition to being in the opposite direction, is very nearly 3.6 times as large. So, the diagonal of a square 39.4 centimetres on a side, which is 55.7200143574999... centimetres, is very close to 10 centimetres (about the diagonal of 7 centimetres) plus 18 inches (about the diagonal of the other 32.4 centimetres) since 18 inches is 45.72 centimetres.

Since these are all even numbers, we can note that 9 inches plus 5 centimetres is approximately the diagonal of 19.7 centimetres.

Since those words were written, it occured to me that there might be another irrational number that should instead be used to define the relationship between the centimetre and a modified inch.

If one decides that a computer keyboard should be tilted at an angle of ten degrees, then while the spacing of circuit traces for the different keys would be, horizontally, three-quarters of an inch, vertically they would be that distance divided by the cosine of ten degrees. So, if that could be made something reasonably easy to specify...

With an inch of 2.54 centimetres, the distance in question is 1.93438769564234419687... centimetres approximately. If, for example, we want that distance to instead be 1.9344 cm exactly, we would have to change the inch to 2.540016156569... centimetres.

On one page on this site, I note that the Egyptians had a royal cubit of seven spans as opposed to the regular cubit of six spans. But I note at least one page that claims the original form of the royal cubit in Egypt was a measuring unit for measuring diagonals.

Of course, seven spans would approximate the diagonal of a square that was five spans (rather than six spans, or one cubit) on a side. So I suppose one could imagine a royal cubit of about 7.0710678 spans in length.

On the other hand, the diagonal of a square 18 inches on a side would be 25.455844 inches, and one twenty-fifth of that would be 1.0182337649 inches, so it would be larger than a standard inch by slightly more than the "pyramid inch" proposed by pyramidologists.

I have since encountered, in a book from 1885, a scientific definition of the length of an inch from first principles instead of from a standard yard that, being made of iron, had a distressing tendency to shrink over time.

The claim is that at sea level, and at the latitude of London, the length of a pendulum which oscillates once a second is exactly 39.13929 inches.

The standard acceleration of gravity used in textbooks, 9.80665 metres/(second^2) is for the latitude of Paris, however.

Using the GRS-67 equation, and the latitude of 57.4769 degrees N for the Royal Observatory of Greenwich, one gets 9.812006677... metres/(second^2), leading to a length of 0.99416413 metres for the seconds pendulum, and thus an inch of 2.54006684956 centimetres - which is close to the 2.54006833 from the Troughton Bar. So it is possible to imagine a length for the inch which is defined independently of the metre directly from first principles; thus, the yard might be defined as 1,509,498.08 wavelengths of the same spectral line of Krypton-86 as used to define the metre.

Mass (and Weight)

The U.S. pound was redefined as 453.59237 grams in 1959, having previously been defined on the basis of 2.2062234 pounds equalling one kilogram exactly, leading to a pound of about 453.5924277 grams, nearly identical to the British pound, which was 453.59243 grams before also being redefined to the international standard avoirdupois pound of 453.59237 grams, which Britain adopted in 1963.

A pound is 7000 grains in weight, that is, the normal, 453.59 gram pound used to weigh food. Thus, if you have peas to weigh, you use this pound, which is called the avoirdupois pound. The troy ounce, which is used to weigh gold, however, is 480 grains in weight, and there are twelve troy ounces in a troy pound.

Thus, the relevant conversions are:

                     Grains       Grams

Avoirdupois Pound    7000         453.59237
Troy Pound           5760         373.2417216
Troy Ounce            480          31.1034768
Avoirdupois Ounce     437.5        28.349523125
Grain                   1           0.06479891

And so we can make this chart of ounces versus grams for some common chocolate bar sizes:

Ounces            Grams
1                  28.35
1 1/2              42.52
1.58733            45
1 3/5              49.36
1 3/4              49.61
2                  56.7
2 1/2              70.87
2.64555            75
3                  85.05
3 1/2              99.22
3.5274            100
4                 113.4

Here is a rough table of some values for gold, and the relative value of a dollar in terms of gold under those values:

                                -1834     -1933     -1971       2011
Up to 1834:          $19.38     $1.00     $1.07     $1.81     $81.80
Up to 1933:          $20.67        94¢    $1.00     $1.69     $76.69
Up to 1971:          $35.00        55¢       59¢    $1.00     $45.29
January 21, 1980    $850.00         2.28¢     2,43¢     4.12¢  $1.86
April 2, 2001       $255.95         7.57¢     8.08¢    13.67¢  $6.19
July 13, 2011:    $1,585.20         1.22¢     1.3¢      2.2¢   $1.00

The peak high in 1980 (New York), the peak low in 2001 (London), and the new record-setting price (New York) are noted.

People writing mediaeval role-playing games should note that due to the difference in density between gold and silver, if one uses the historic ratio that gold is 16 times more valuable than silver by weight, then it is also (approximately) 25 times more valuable than silver by volume. This will make it possible to determine more accurately how much treasure your characters can carry in their backpacks, while still using handy round numbers.

Prior to the devaluation of the U.S. dollar in 1933, which also led to the abolishment of gold coinage, the U.S. dollar was defined by defining the Gold Eagle, a coin with a denomination of $10, as being composed of 258 grains of 9/10 fine gold. In U.S. gold coins, the remaining tenth was originally made up of half silver and half copper, which were not without value themselves; however, shortly after the standard of interest was adopted in 1879 they switched to using only copper for the remaining tenth.

The Constitution originally defined the dollar as 371.25 grains of silver, and that is 90% of 412.5. In 1872, the weight of the silver dollar was reduced to 384 grains of 9/10 fine silver, because the price of silver had gone up again relative to that of gold, and gold was generally accepted as the international standard of monetary value. However, unlike the previous silver dollar, while this coin was still intended to be worth nearly a dollar, it was not expected to be worth exactly a full dollar; it was not token coinage, but it was subsidiary coinage.

If the situation between 1834 and 1872 is used as the basis, then, an attempt can be made to determine the exact nominal value of pure gold in U.S. dollars.

A gold dollar of 25.8 grains of 9/10 fine gold consists of 23.22 grains of pure gold, 1.29 grains of pure silver, and 1.29 grains of copper.

371.25 grains of pure silver and 41.25 grains of copper equal a dollar.

Thus, 1.29 grains of pure silver and 0.14333... grains of copper equal 0.3474747... cents.

So, 23.22 grains of pure gold and the remaining 1.15333... grains of copper equal 99.6525252... cents; thus, if we neglect the value of the copper, that works out to one troy ounce of gold (480 grains) being worth almost exactly $20.60 instead of $20.67.

If silver were worth 1/16th as much as gold, this would make 371.25 grains of silver worth about 99.58 cents instead of a dollar.

In 1805, the price of copper was considered high at 138 pounds a ton, or about $690 a ton. A ton being 2000 avoirdupois pounds of 7000 grains, this would be about 2.366 cents per troy ounce, compared to about $1.29 per troy ounce for silver and $20.60 per troy ounce for gold. Thus, the ratio of value by weight, historically, had been in the neighborhood of 50 to 1 between silver and copper, or perhaps higher.

In 1880, the price of copper peaked at 20 1/8 cents a pound (which I assume to be an avoirdupois pound); that price would work out to 1.38 cents per troy ounce, which is even lower, indicating a 100 to 1 ratio between silver and copper as a possible historical normal.

Given the composition of the Gold Eagle above, a half-eagle, with a $5 denomination, would be composed of 116.1 grains of pure gold, 6.45 grains of pure silver, and 6.45 grains of pure copper. A Gold Sovereign from Britain, on the other hand, is composed of 113.0015 grains of pure gold, and 10.2729 grains of copper, for a total weight of 123.2744 grains, or 7.9881 grams. Another source gives the weight of a British Gold Sovereign as 5 pennyweights, 3 171/623 grains; one pennyweight is 24 grains, so this is 123 171/623 grains, or approximately 123.2744783306581 grains. This works out to about 113.0016 grains of pure gold at 22 carats.

Of course, there is the matter of seigniorage; stamping out metal into coins, after all, costs money, so one would not expect the metallic content of coins, even when they are not a token or subsidiary coinage, to absolutely equal their value. I have not been able to find an admission that the United States engaged in such a practice, however; but one reference, which directly states that Britain did not have seigniorage either, then goes on to say that the mint price of gold there is 3 pounds, 17 shillings, and 10 1/2 pence per troy ounce, and that the Bank of England buys bullion at 3 pounds, 17 shillings, and 9 pence per troy ounce. I would take the 1 1/2 pence difference as the (very nominal!) seigniorage; that is, that gold is worth 3 pounds, 17 shillings, and 9 pence per troy ounce, but gold coins are denominated as if gold were very slightly more valuable.

If the halfpenny were exactly equal to the cent, making the British pound equal to $4.80 in value, 113 grains of gold to the pound would imply about 117.7 grains of gold to five dollars; thus, while a pound was not worth quite as much as five U.S. dollars, it was actually worth about $4.85 at the time of the gold standard. (Actually, the value of the mint par of exchange used at the time was $4.8665.) While the Canadian dollar was made to be equal to the U.S. dollar, the Newfoundland dollar was about 1 1/3 cents more than a U.S. dollar, as one would expect from a dollar equal to 100 ha'pennies.

With 116.1 grains of pure gold in a $5 half eagle, divided by five, being 23.205 grains per dollar, and 500/480 times 113.0016 grains of pure gold, divided by five, being 23.542 grains per Newfoundland dollar, it may also be noted that a Mexican 10 peso coin contained 7.5 grams of pure gold; this works out to 115.7427 grains of pure gold, thus, the Mexican 50 peso coin does not have exactly the same amount, but instead has slightly less, gold than a $25 U.S. gold coin would have: thus, two Mexican pesos would have been worth about 3/10 of a cent less than a U.S. dollar.

However, a metric dollar worth 1.5 grams of gold, even if slightly smaller than the pre-1933 dollar, is certainly attractive as involving round numbers.

In that era, other countries also had currencies which were fixed in value in terms of gold, and thus to one another. Thus, 19.2952 cents, at 4.48036 grains of gold, or 0.290322581 grams of gold, was the value of a French Franc, a Swiss Franc, a Belgian Franc, an Italian Lira, a Bulgarian Lev, or a Greek Drachma in those days; this was the standard fixed by the Latin Monetary Union. An American gold pattern coin, the Stella, denominated as a four dollar coin, was made to approximate the specifications of a 20 Franc gold coin (the coin commonly referred to as a Gold Napoleon, for Sherlock Holmes fans). Also, Austria-Hungary issued 4 and 8 Gulden coins which matched the specifications of 10 and 20 Franc coins.

There was also a Scandinavian Monetary Union during the same period, where the Danish Krone, the Swedish Krona, and the Norwegian Krone were valued so that 36 Kronor would equal 50 Francs, with both being approximately equal to 2 British Pounds. Note that a British Pound was closer to $4.80 in those days than to $5.

Finland was a member of the LMU rather than the SMU. Peru, Colombia, and Venezuela were also part of the LMU: the Venezuelan Bolivar was equivalent to 1 Franc, the Peruvian Sol and the Venezuelan Peso were each equivalent to 5 Francs

The Finnish Markka. when introduced in 1860, was originally defined as 1/4 of the Russian Ruble. This corresponmded well to the definition of the Russian Ruble in 1885 as being equivalent to 4 French Francs, but in 1897 the Russian Ruble was devalued to 2 2/3 Francs, so 8 Francs would equal 3 Rubles.

Neither Germany nor Portugal was a part of either of these monetary unions. The German Empire did not issue gold coins, but the individual German states had done so; thus, a 20 Mark coin weighed 7.965 grams, was composed of 900 fine gold, and thus contained 0.2305 troy ounces of fine gold, whether from Wurttemburg, Schleswig-Holstein, Schaumburg-Lippe, Saxe-Meiningen, Saxe-Weimar-Eisenach, or Saxony-Albertine, and so on, thus making the German Mark worth about 1.23 French Francs. A Portuguese Coroa was worth slightly more than 40 Kronor.

A proposal for an international currency, called the spesmilo (and with the plural spesmiloj, the proposal being advanced by proponents of the international language Esperanto), divided into one thousand spesoj, having a value of 11/12 of that of 0.8 grams of gold was advanced in 1907 by René de Saussure, and some banks, in Britain, Switzerland, and Czechoslovakia handled the currency, and there were coins minted which were denominated in it.

One problem with the Latin Monetary Union was that, at its inception, it was based on bimetallism: that is, silver coins were standardized as well, and were to be accepted by other countries based on their metal content. And this was based on a ratio where gold was worth 15.5 times as much as silver by weight, favoring silver even more than the "historic" 16 to 1 ratio that was also high even for the period prior to 1933.

Today, as I write this, gold is $1,257.40 per troy ounce, and silver is $16.07 per troy ounce, for a ratio of almost 78.25 to 1. I suppose I should not be surprised. Silver is no longer in demand for coinage; it is no longer in demand for photography - while black-and-white photography consumed silver, it was largely recoverable in the case of color photography, but then instant cameras consumed silver again, but now, of course, silver is no longer involved when CCD (and CMOS) sensors are used; and it is no longer in demand for tableware, given all the sulphur in the air these days. Nor, for that matter, is silver particularly fashionable for jewellery, perhaps for the same reason.

While there are great amounts of gold dissolved in the oceans, and along with platinum and iridium, gold is to be found in asteroids, I don't know of any science-fictional massive sources of silver beyond the horizon, though.

The St. Gaudens $20 Double Eagle gold coin, the last type of regularly circulating $20 gold coin in the United States, is regarded as one of the most beautiful coins ever made. Originally, the ultra-high relief version was only made as a proof, of which about twenty specimens are believed to exist.

In the first year of issue, what was actually minted initially was a more practical, but still high-relief, version, with the date of 1907 in Roman numerals; about 12,000 were minted, they were released into circulation in both 1907 and 1908. While the original ultra-high relief version was difficult to stack, and hence felt to be impractical as a coin, and a somewhat lower-relief version submitted by the artist was still not practical, even the more moderate high-relief version designed under the closer direction of the Mint from St. Gaudens' original by another artist proved difficult to strike, however.

Therefore, before the end of 1907, the low-relief version of the coin was designed and minted, which continued in use until 1932; thus, for 1907, both versions of the coin exist, and even for the date 1907, those of low relief far outnumber the high-relief ones.

In 2009, close to the 100th anniversary of the introduction of the St. Gaudens Double Eagle coin, the United States Mint offered for sale a 2009 coin, denominated as twenty dollars, following the ultra high relief style.

This special coin was made of pure gold, rather than 900 fine gold. Also, it was only 27mm in diameter, the same diameter as a $10 Eagle rather than the usual 34 mm diameter of a $20 Double Eagle. However, it weighed 31.109 grams, so this merely meant it was thicker; it still had the same gold content as one of the Double Eagles of the past.

Here is a rough table of coin diametres and values, where the values are in terms of the value of U. S. currency in precious metals between 1892 and 1933, and the upper value is for silver coins, the lower for gold coins:

13mm     15mm       16mm          17mm
      4        5          6 1/4         7 13/16
  $1.00    $1.25      $1.56 1/4     $2.45  5/16

17mm     18mm       19mm          20mm
      8       10         12 1/2        15 5/8 
  $2.00    $2.50      $3.12 1/2     $3.90 5/8

20mm     22mm       24mm          26mm
     16       20         25            31 1/4
  $4.00    $5.00      $6.25         $7.81 1/4

26mm     28mm       31mm          33mm
     32       40         50            62 1/2
  $8.00   $10.00     $12.50        $15.62 1/2

33mm     35mm       38mm          41mm
     64       80      $1.00         $1.25
 $16.00   $20.00     $25.00        $31.25

Moving one column to the right increases the weight and value of the coin by a factor of 1.25, moving one row down doubles the weight and value of the coin. The sizes have been formed on the basis of the currencies of several different countries, and thus they tend to be smaller than the sizes of U.S. coins, and/or larger than the sizes of British coins of corresponding value (1 shilling = 25 cents, 1 florin = 50 cents, 1 crown = $1.25, 1 pound = $5.00) up to the year 1919.

Another way to look at this is to go more directly to the raw data, shown below for a variety of gold coins.

What I have found is that when precious metal coins double in value, their diameter tends to increase by a factor of 1.25; when they increase in value by a factor of 2.5, their diameter tends to increase by a factor of 1 1/3.

These ideal proportions, if strictly followed, would mean that when the value, and hence the volume, of a coin increases tenfold, its diameter would increase in the proportion 25:12, and its thickness would increase in the proportion 288:125; the common logarithm of the former ratio being 0.31875876..., and of the latter ratio being 0.36248247..., it can be seen that the general tendency puts only a small amount more of the increase in the thickness. 2 raised to the 0.31875876... power is 1.247257..., and 2.5 raised to that power is 1.339204..., so it can be seen that both simple ratios are very consistent with the general trend.

Also, given the historical ratio, on which coin sizes were based, of 16 to 1 relating the values of gold and silver by weight, the ratio of their values by volume are approximately in the ratio of 25 to 1. If the values of coins of similar diameter in gold and silver tend to be in the ratio 20 to 1, this would indicate that a gold coin is usually about 4/5 as thick as a silver coin of similar diameter; the actual ratio lies somewhere between 20:1 and 25:1, so gold coins do tend to be thinner than silver coins, but by a smaller amount.

For coins of smaller size, they will sometimes be made thinner so they can have a larger diameter.

Relative United States                     U. S. Current Bullion Great Britain           Mexico             France              Switzerland Other

   1      9.4 mm - 9.93 mm    1/4 dollar
                              (California Gold)
   2     11   mm  1/2 dollar (est.)
                  (California Gold)
   4     13   mm - 15   mm    1 dollar coin                                              13   mm   2 Pesos
   5                                                                                     15.5 mm   2.50 Pesos

   8                                        16.5 mm 1/10 ounce                                               19   mm  10 Franc  18.5 mm     17.98 mm  2 Dollar Coin Newfoundland
  10     18   mm  Quarter Eagle ($2.50)      (1.62 mm thick)     19.30 mm Half Sovereign 19   mm   5 Pesos

  16     22   mm  Stella ($4, pattern only)                                                                  21.2 mm  20 Franc  21.2 mm     21    mm  Spain 80 Reales (2 Escudos)
  20     21.6 mm  Half Eagle ($5)           22   mm 1/4 ounce    22.05 mm Sovereign      22.5 mm  10 Pesos                                  21.59 mm (0.85") Canadian 5 Dollar Coin
                                             (1.78 mm thick)                                                                                 (1.82 mm thick)
  32                                                                                                         26   mm  40 Franc
  40     27   mm  Eagle ($10)               27   mm 1/2 ounce    28 mm    2 Pounds       27.5 mm  20 Pesos   29   mm  50 Franc              26.92 mm  (1.06") Canadian 10 Dollar
                                             (2.15 mm thick)                                                                                 (2.08 mm thick)
  64                                                                                                                                        35    mm  Spain 320 Reales (8 Escudos, Doubloon)
  80     34   mm  Double Eagle ($20)        32.7 mm 1 Ounce                                                  34   mm  100 Franc
 100                                         (2.87 mm thick)     36.02 mm 5 Pounds       37   mm  50 Pesos

 200     43.2 mm - 42.9 mm    Half Union ($50)
                 (Panama-Pacific Exposition/Wass Molitor California Gold)

Not included in the table are gold half union pattern coins with larger diameters of 51.1 and 50.8 mm.

The Eagle gold bullion coin of the U.S. Mint has been added; it is a 22 karat gold coin (.916 2/3 fine), and is interesting because precise thickness information is available, as well as for comparison with the sizes of the coins in the past circulating coin issue: the diameter changes more slowly with mass.

Oddly enough, when a 20 dollar gold coin was struck for Canada's Centennial in 1967, it was noted that because laws required a specific gold content for Canadian gold coins, the coin would have to be made with a metallic content worth more than 20 dollars, so it would not be available at face value; but this coin had a diameter of 26 mm, less than that of the long-past Canadian 10 dollar coin. However, the 20 dollar Centennial coin contained 18.2733 grams of 900 fine gold, whereas the regular issue 10 dollar coin contained 16.7185 grams of 900 fine gold. Thus, the 20 dollar gold coin did contain somewhat more gold than the 10 dollar coin, despite being slightly smaller in diameter, but the statutory requirement of gold content clearly had been adjusted downward; it seems to have been in the right ballpark to correspond to the post-1933 official U.S. gold value of $35 per ounce, and indeed, its gold content corresponds to that value of gold, with an additional reduction of about 7.5% for seignorage and the value of the other metals.

In the case of silver coins, for the United States and Canada, the diameters of coins were:

United States                           Canada                    Britain                              Switzerland            Other

15.5  mm   Half Dime (5 cents)          15.5   mm  5 cent silver

17.91 mm   Dime (10 cents)              18.034 mm (0.71")
            (1.35 mm thick)                                       19.41 mm Sixpence (1/2 shilling)     18.2 mm  1/2 Franc (0.835 fine)

22    mm   Twenty Cents                 23.3   mm                                                                             23.19 mm 20 cents Newfoundland
24.26 mm   Quarter Dollar (25 cents)    23.88  mm (0.94")         23.60 mm Shilling                    23.2 mm  1 Franc       23.62 mm 25 cents Newfoundland
            (1.75 mm thick)
30.61 mm   Half Dollar (50 cents)       29.72  mm (1.17")         28.5  mm Florin (2 shillings)        27.4 mm  2 Francs
            (2.15 mm thick)
38.1  mm   Dollar                       36.06 mm  (1.42")         36    mm Double Florin (4 shillings) 37   mm  5 Francs (0.900 fine)                     
                                         (2.84 mm thick)          38.61 mm Crown (5 shillings)

Note that the sizes of the Swiss silver coins compare closely to the sizes of the Mexican gold coins.

For comparison to the diameters of precious metal coinage, the sizes of the token coins may also be mentioned. In both Canada and the United States, the penny was three-quarters of an inch, or 19.05 mm, in diameter. The old Canadian large cent was exactly one inch in diameter. When the Canadian nickel changed from twelve-sided to round, its diameter was 0.835" or 21.21 mm, the same as that of an American nickel.

To illustrate how the round proportions of 1 1/3 for an increase in volume by 2.5, and 1 1/4 for an increase in volume by 2, approximate to the actual diameters of coins in practice, the following image

shows how the United States silver coins from the older type series - a 1941 Mercury Dime, a 1930 S Standing Liberty Quarter, a 1943 Walking Liberty Half Dollar, and a 1926 S Peace Dollar - compare in relative sizes to these proportions.

Incidentally, for comparison, 1 1/16" is 26.9875 inches, very close to 27 mm, a popular size, and 15/16" is 23.8125 mm, close to the size of a Canadian quarter. As well, .85" is 21.59 mm, and 1.06" is 26.924 mm, and .93" is 23.622 mm, and so the sizes of some coins may be explained by them being defined in inches rather than metric units.

The United States minted both a silver three-cent piece, which had a 14 mm diameter, called the "trine", and a larger cupro-nickel three-cent piece; as the silver five-cent piece was always referred to as a "half dime", I initially had trouble finding information about it in web searches.

The ideal size for a coin would seem to be something close to the size of a quarter, but a coin the size of a sixpence (about the size of a cent, or a 12 1/2 cent silver coin) or a nickel or a threepence (about the diameter of a 20 cent silver coin) is also within the ideal range.

Given that, the large difference in value between gold and silver creates a problem. Although coin denominations tend to advance in the ratio 1:2 or 1:2.5, a gap with a ratio of 1:5 is not unreasonable; thus, there was no two-cent coin.

To limit the gap to this size, a silver fifty-cent coin and a gold coin with a value of $2 or $2.50, both outside the ideal range, would be required, and at least one of those two would be needed even to limit the gap to a 1:10 ratio.

A number of solutions are possible. A coin with 25 cents worth of silver, and 75 cents worth of gold in a small dot in the center could be made. Or a 50-cent coin that is almost twice as thich as a quarter, so that its diameter is only as much larger than that of the quarter as needed to avoid confusion; perhaps around 26.5 mm, the diameter of the Canadian "loonie", and the American Susan B. Anthony dollar.

Initially, when most ordinary United States coins changed from being made of .900 fine silver to copper clad with cupronickel, the Kennedy half dollar continued to be made of silver, but with a reduced silver content.

The core of the coin was made of 21% silver, high enough to still be attractive in appearance; the cladding was 80% silver, the purity used for Canadian silver coins.

The coin's overall silver content was 40% silver.

A little algebra lets us determine that this means that the cladding made up 19/59ths of the coin's thickness, and the rest of the coin the other 40/59ths of the coin's thickness; or, rather, it would have if copper wasn't less dense than silver.

Even with a 16 to 1 ratio in the values of gold and silver, never mind a 78 to 1 ratio, the gap between the silver quarter and the gold half-eagle is a large one, that has to be filled either with inconveniently large silver coins, like the 50 cent piece, or inconveniently small gold coins, like the quarter eagle.

In the real world, this had never been a problem; the next stop after the quarter, in practice, was the one dollar bill; gold coins were only infrequently used for foreign exchange transactions involving banks or tourists.

Imagine, however, a coin with an inner layer of the usual silver coinage alloy, and cladding of the usual gold coinage alloy - using the 19/59 and 40/59 proportions of the Kennedy half dollar. Based on the 16 to 1 ratio, overall, the coin's metallic content would be worth about 5.83 times as much as silver. Note that the gold layer would actually be thinner than the .800 fine silver layer in the Kennedy half in proportion, since the difference in density between the two materials is greater; the calculation should have been based on volume, not weight.

Assuming, then, a coin which, by weight, is worth six times as much as a silver coin. And one wishes to give it the value of one dollar. So it would be the same size as a silver coin with a value of 16 2/3 cents.

A ten cent silver coin is the size of a dime, which is uncomfortably small, but one worth 12 1/2 cents is the size of a sixpence, or about the same diameter as a one-cent piece, which is comfortable, so such a coin could indeed be practical - assuming the cost of the cladding operation is not excessive, since presumably it would be a low-tech society that avoided paper money - and the method doesn't lend itself to counterfeit coins with a cheaper metal on the inside. (While gold is denser than lead, silver is less dense than lead; but lead is less electrically conductive than both of them.)

A $2 coin would be easier to achieve; a gold coin with $1.75 worth of metal could be placed in the center of an enclosing coin of silver with 25 cents worth of metal, the result being larger than a quarter, but designed so the gold coin is firmly held in place, while being visible on all sides.

Coins have been made of an alloy of 10% silver and 90% copper. As they do not remain attractive in appearance, doing something similar with a gold coin does not seem like a reasonable alternative, although the ancient Greeks made coins of electrum, a gold and silver alloy, usually about 40% gold and 60% silver. And, of course, 10 karat gold can be used in jewellery.

To put this in perspective, though, let us consider the current case, where the ratio between gold and silver is not 16:1, but 80:1. If one were to go back on the gold standard, so that in terms of the new dollar, the quarter eagle, half eagle, eagle, and double eagle returned, then the silver dollar would be a 20 cent coin, the half dollar a 10 cent coin, the quarter a 5 cent coin, and the dime a 2 cent coin. So now the gap would be between a large 20 cent coin and a small $2.50 coin.

Presumably, instead of a 2 cent dime, one would have a 25 mill sixpence, with the dollar being divided into 1000 mills instead of 100 cents.

If, instead, the price of silver were used as the basis of revaluing the currency, then the double eagle would be a $100 coin, the eagle a $50 coin, and either the half eagle would be a $25 coin, or there would be a $20 coin the size an old $4 coin would have had (the $4 Stella, at 22 mm, was actually larger than the $5 Half Eagle at 21.6 mm; I'm thinking of something about 20.3 mm in diameter), and perhaps a small $10 coin the size of an old $2 coin.

But going back to a standard of gold and silver coinage at this time is clearly a fantasy, given the current volatility of precious metal prices. Also, it has been noted that the size of the world's economic activity, as reflected in GNP and GDP, relative to the value of the world's monetary stock of gold, is vastly higher than it was in the days of the gold standard. That, however, may be less significant than it seems, as it may simply reflect a move away from the use of gold to back currencies; I would be interested in knowing about the size of the world's total stock of gold, non-monetary as well as monetary.

Because copper or bronze coins tend to be token coinage, their size is quite variable; the value ratio of 20:1 for gold and silver at the same diametre is consistent, but that between silver and copper can vary from 25:1 to 50:1.

The density of gold is 19.32 grams per cubic centimetre, that of silver 10.5 grams/cc, that of copper 8.96 grams/cc. A ratio of 64:1 in value by weight would lead to a ratio of 75:1 in value by volume for silver and copper as metals. Copper coins tend, however, to be a token currency, that is, a metal form of paper money as it were, and so the ratio in weight of copper coin to silver coin would be more like 16:1 or 32:1.

The atomic weight of gold is 196.9665, and its one stable isotope is Gold-197. An atomic mass unit is

1.6605655 * 10


Thus, one gram of fine gold contains 3.0573954960689... * 10^27 atoms of gold.

Using the old value of 2.2062234 pounds to the kilogram, a kilogram would equal 15443.5638 grains exactly; so one grain would be 6.479384884502... * 10^-2 grams, and thus one grain of fine gold would contain 1.97972147858... * 10^20 atoms of gold.

Assuming a ten dollar gold coin made up of 258 grains of metal, being 9/10 gold, 1/20 silver, and 1/20 copper, taking gold to be worth 16 times as much as silver by weight, and taking copper to be worth 20 1/8 cents per avoirdupois pound, what can we infer about the value of gold?

One avoirdupois pound is 7000 grains. So the copper in such a coin is worth 0.0370875 cents, leaving the rest of the coin at 999.9629125 cents to be accounted for.

The rest of the coin consists of 232.2 grains of gold and 12.9 grains of silver; the silver, at 1/16 the value of gold, is equivalent to an additional 0.80625 grains of gold.

Thus, 233.00625 grains of gold equal 999.9629125 cents; and consist of 4.612875777685... * 10^22 atoms of gold.

That leads to a dollar being worth the same as 4.613045864... * 10^21 atoms of gold.

However, one wants the bank value of gold, not the coin value. Thus, a further adjustment needs to be made for seigniorage; the only figure I have available is the one that adds 1 1/2d of seigniorage to 3 pounds, 17s 9d. That works out to 933 pence, and so the factor desired would be 934.5/933, raising the "real" amount of gold in a dollar to 4.6204623364737787546789723 * 10^21 atoms of gold.

Thus, one could, if one wished, define a dollar as the pecuniary value of


atoms of Gold-197, or, as they say,


contained in a good delivery gold bar, which is a bar of gold that is at least 99.5% fine, and which has a mass of approximately 400 troy ounces, ranging from 350 troy ounces to 430 troy ounces (the actual fineness and the weight being marked on the bar). 400 troy ounces is about 12,441.4 grams, and the range would be from a low of 10,886.22 grams to a high of 13,374.49 grams.

But it might be more sensible to round up that last 0.9723 atom of gold to a whole atom.

Taking the pre-1933 U.S. dollar as the basis for a definition of the value of a dollar, of course, has the virtue of being something well-defined and close at hand. But if one is seeking out an ur-dollar, two other candidates come to mind.

The word "dollar", of course, is derived from "thaler", and the first thaler was the Joachimsthaler, originally minted in 1725. From it was derived the Reichsthaler of the Holy Roman Empire, used from 1566 to 1750, almost 200 years, with a silver content of 400.99 grains. (A Maria Theresa thaler, a trade coin deriving from this, contains 360.96 grains of silver, and is 0.833 fine.)

The value of silver, however, had a tendency to fall relative to the value of gold. Thus, an early attempt to establish a relationship between some gold coins and the thaler ended up requiring a readjustment: a Gulden, intended to be worth 72 Kreutzer, ended up having a value of 75 Kreutzer instead.

A gold coin contemporary with the thaler that was stable in metallic content was the Ducat as issued by Hungary; it continued to contain 3.53133 grams of 0.986 fine gold for a long period, from 1311 to 1915. This would be 53.738 grains of fine gold.

If gold is worth 16 times as much as silver, a ducat would be 2.14 thalers; if gold were worth 15.5 times as much as silver, a ducat would be worth 2.08 thalers.

So, if one accepts that the value given to silver was over-optimistic, one could define the ur-dollar as being the value of one-half ducat, or 26.869 grains of gold.

But that is one of only two possible candidates for the original value of the dollar.

The first currency in the world that was decimal like the dollar, divided into 100 smaller units, was the ruble of Russia, divided into 100 kopeks. And it was close to a dollar in value. This was created by Peter the Great in 1704, with a 28 gram silver ruble being equivalent to 100 copper kopeks. Gold coins were later produced on the basis that gold was worth 15 times as much as silver, and so if we use that to define the value of the ruble, given that gold is usually worth more than 15 times as much as silver, a ruble becomes the value of 1.86666...67 grams of gold, or about 28.809319 grains of gold.

And if we use the specifications of the pre-1933 U.S. coinage in a simplistic way, by ignoring the value of the alloying metals, we get 23.22 grains of gold being equal to a dollar. Since 28.8 and even 26.87 are sufficiently larger than that to be unfamiliar and remote, using either as the basis for a new dollar is not appealing.

Whether or not the avoirdupois pound is a unit of force instead of a unit of mass, the troy ounce is definitely a unit of mass.

The kilogram is a unit of mass, and the metric unit of force is the Newton, which is the force required to accelerate a mass of one kilogram to a velocity of one metre per second in a second, thus it has the dimensions of one kilogram per metre squared.

It was more common to measure forces in terms of kilograms of force, based on a standard for the acceleration of the Earth's gravity of 9.80665 metres/second2, which means that a kilogram of force is equal to 9.80665 Newtons. This is based on the intensity of gravity, less the centrifugal force of the Earth's rotation, in Paris.

As the pound may be a unit of force (in the British unit of energy, the foot-pound), or a unit of mass, it has two companion units.

A slug is 32.174049 pounds of mass, and a pound of force is 32.174049 poundals.

This is based on an acceleration of gravity of 32.174049 feet per second squared; that many feet is 9.8066501352 metres, so that figure was clearly derived from the metric standard. However, I have also seen references that gave 32.174 exactly as the relation between pounds-force and poundals as well as slugs and pounds-mass; a different standard gravity, say that felt around London, could have been used for the British system.

In addition to the troy ounce, there were other alternative pounds used in Britain. Thus, the Tower pound was twelve Tower ounces, the Tower ounce being 15/16ths of a Troy ounce, or 450 grains. As well, a Merchant pound was 15 Tower ounces (6750 grains), and a London pound was 16 Tower ounces or 15 Troy ounces (7200 grains). Naturally, these units, being liable to cause confusion, tended to fall into disuse.

The Tower pound does have some historical significance. Prior to 1528 (!) the pound sterling was a Tower pound rather than a Troy pound of sterling (0.925 fine) silver; the Tower pound was originally established to be the weight of 120 silver dirhems, a coin from Arabia. Thus, while the British system of weights and measures largely, like most other European pre-metric systems of measurements, derives from that of ancient Rome, the fundamental standard on which the exact value of British weights were based came from Arabia.

Incidentally, when I note that the Avoirdupois pound is defined as 7000 grains, while the Troy ounce is defined as 480 grains, in both cases I am using the same grain, the troy grain. However, while there is no such thing as an Avoirdupois grain, there is also a Tower grain; so while a Tower ounce is 450 Troy grains, it is 640 Tower grains.

This would lead to the dirhem being defined as 45 Troy grains, or about 2.91595 grams. Incidentally, Wikipedia gives several modern values of the dirhem, 3.207 grams (the standard in the late Ottoman Empire), 3.088 grams (an Egyptian standard, defined as 47.661 Troy grains), and two modern proposals for reviving the dirham as a unit for measuring the weight of silver, 3 grams, and 2.975 grams, all heavier than the value preserved in the pound.

The name of the dirhem or dirham derives from that of the Greek drachma; in ancient Greece, a drachma was 4.3 grams of silver, and originally in modern coinage a drachma was 4.5 grams of 0.900 fine silver.

Liquid Measure

Speaking of the kilogram, I have read news items noting that the international standard kilogram has experienced changes in its mass, and that an effort was underway to make a more accurate standard from a sphere of silicon.

The metre and the second can be defined in terms of spectral lines or microwave emissions from specific substances. Could the gram be physically defined?

Originally, the litre was defined as the volume of a kilogram of water. As well, a litre was nominally 1,000 cubic centimetres of water. However, because water has a pronounced tendency to dissolve, at least to a limited extent, almost any other substance, it is not a good material to use for a physical standard. Thus, for a time, a litre, instead of being exactly 1,000 cubic centimetres or one cubic decimetre, was instead officially defined as 1.000028 cubic decimetres, because this was, as near as could be determined, the volume that a kilogram of water actually occupied; this anomaly was corrected in 1964, and thus now one millilitre (ml) is exactly the same as a cubic centimetre (often noted by the non-SI abbreviation cc instead of cm3).

Thus, if it were desired to give the kilogram a reproducible physical definition based on water, it would have to be redefined so that the new kilogram was 0.999972 existing kilograms, and this would not be acceptable, being too large a change, and affecting the derived units of energy and force, and hence the various electrical units.

Liquid measure has always been one of the more confusing areas of measurement.

In the United States, a gallon is 231 cubic inches by definition.

In the British imperial system, a gallon was originally defined as the volume of ten pounds of water. Subsequently, the British gallon was defined in terms of metric units as 4.54609 litres; this was in 1985, after the litre had become exactly a cubic decimetre.

The Imperial gallon is larger than the U.S. gallon, but the Imperial fluid ounce is smaller than the than the U.S. fluid ounce, due to one difference between the two systems:

U.S. Customary                                      British Imperial

1 gallon = 4 quarts =     3.785411784 litre         1 gallon = 4 quarts =    4.54609 litre
1 quart = 2 pints =         946.352946 ml           1 quart = 2 pints =      1.1365225 litre
1 pint = 2 cups =           473.176473 ml           1 pint = 2 1/2 cups =      568.26125 ml
1 cup = 8 fluid ounces =    236.5882365 ml          1 cup = 8 fluid ounces =   227.3045 ml 
1 fluid ounce = 8 drams =    29.5735295625 ml       1 fluid ounce = 8 drams =   28.4130625 ml
1 tablespoon = 4 drams =     14.78676478125 ml      1 tablespoon = 4 drams =    14.20653125 ml
1 teaspoon = 4 scruples =     4.92892159375 ml      1 teaspoon = 4 scruples =    4.73551041667 ml
1 dram = 3 scruples =         3.6966911953125 ml    1 dram = 3 scruples =        3.5516328125 ml
1 scruple =                   1.2322303984375 ml    1 scruple =                  1.18387760041667 ml

the space in the table dividing the part in which the U.S. units are smaller from the part in which the U.S. units are larger. Often, the tablespoon is approximated by 15 ml in recipies now, although I've read that in Australia, 20 ml is instead used as the approximation. Thus, one could speak of a metric scruple of 1.25 ml.

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