Sep 282016


On this date in 1889 the first General Conference on Weights and Measures (CGPM) defined the length of a metre as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium, measured at the melting point of ice. The International Prototype Kilogram (IPK), a cylinder made of platinum-iridium and the International Prototype Metre, an X-cross-section bar also made from platinum-iridium were selected from batches manufactured by the British firm Johnson Matthey. Working copies of both artifacts were also selected by lot and other copies distributed to member nations, again by lot. The prototypes and working copies were deposited at the International Bureau of Weights and Measures (Bureau international des poids et mesures) in Sèvres.

Since that time the metre and kilogram have undergone more and more refinement in their definitions. I guess I’m just quirky but I find the history of weights and measures fascinating. I’ve actually had to do a great deal of research into the history of weights and measures for my writing. For example, to answer questions such as, “How high was the original Tower of Babel?” “What did a court dancer earn in the 16th century?” “How much beer did churches brew in the 15th century?” Yes, I have answered these and other similarly obscure questions using careful research into the history of weights and measure. This is because I have a very untidy mind, but I am a very precise scholar (you may have noticed).  I can’t write all that I want to about weights and measures in history. But I’ll ramble on for a bit. They are (sort of) important for cooking. I’ll get to that at the end.


Weights and measures appear to have been standardized first in ancient Mesopotamia, Egypt, and the Indus Valley somewhere  in the 4th or 3rd millennium BCE. Yes, I know, very precise !!! Actually, these civilizations had different needs for weights and measures. The ancient Egyptians, for example, required engineering techniques to build the pyramids of such precision that (so I am told by engineers) they cannot be replicated today using modern engineering techniques (and we have unbelievable refinement at our disposal). They were using the length of the forearm or the width of a palm as their standards and yet they built structures that are incredibly level and have endured for thousands of years. I suspect they had rulers, leveling tools, and such which are now lost. They were also using rule of thumb geometry which was eventually codified by Euclid centuries later. For example, they did not know Pythagoras’ Theorem but they knew that a triangle with sides in the ratio 3:4:5 was a right-angled triangle. Standards of weight and volume were of major importance in trade.


Since today is significant in the history of the metre, I am going to focus on standards of length primarily. I am going to use the spelling “metre” because it is more general than the US spelling “meter.” I’ve worried about standards of spelling in other posts. Early Babylonian and Egyptian records and the Hebrew Bible indicate that length was first measured there with the forearm, hand, or finger. The units of length used in ancient India included the dhanus (bow), the krosa (cry, or cow-call) and the yojana (stage). Measures from the system attributed to the Indus Valley Civilization (ca. 2600 BC) is sometimes cited as particularly accurate. Based on an ivory scale found in Lothal, their smallest unit corresponded to approximately 1.704 mm (0.067 in).

The common cubit was the length of the forearm from the elbow to the tip of the middle finger. It was divided into the span of the hand or the length between the tip of little finger to the tip of the thumb (one-half cubit), the palm or width of the hand (one sixth), and the digit or width of the middle finger (one twenty-fourth). The Royal Cubit, which was a standard cubit enhanced by an extra palm—thus 7 palms or 28 digits long—was used in constructing buildings and monuments and in surveying in ancient Egypt. The inch, foot, and yard evolved from these units through a complicated transformation not yet fully understood. Some believe they evolved from cubic measures; others believe they were simple proportions or multiples of the cubit. In whichever case, the Greeks and Romans inherited the foot from the Egyptians. The Roman foot (~296 mm) was divided into both 12 unciae (inches) (~24.7 mm) and 16 digits (~18.5 mm). The Romans also introduced the mille passus (1000 paces) or double steps, the pace being equal to five Roman feet (~1480 mm). The Roman mile of 5000 feet (1480 m) was introduced into England during their occupation. Queen Elizabeth I (reigned from 1558 to 1603) changed, by statute, the mile to 5280 feet (~1609 m) or 8 furlongs, a furlong being 40 rod (unit)s (~201 m) of 5.5 yards (~5.03 m) each.


Here my fascination deepens. The furlong is a contraction of “furrow long” which is now fixed at 220 yards. But in British history going back from the 19th century to the time of Alfred the Great (9th century) the “furrow long” was locally defined by the heaviness of the soil, how far oxen or horses could plough before being turned around. The rod or pole was the length of the ploughboy’s rod (used to goad oxen) which he laid on the soil to space the furrows, and an acre was a square that was a furlong by a furlong.  In documents stretching all the way from Alfred’s time to the early 19th century, an acre was the area of land that a ploughman could plough in a day. Thus a Norfolk acre was quite different from a Sussex acre. These were human measures, not standardized measures. However, both the Industrial and Scientific Revolutions required precision that farming did not. An acre could be off by feet and it did not matter, but you won’t split atoms that way.

The metre is part of the metric system, of course, which has its own peculiar history. The metric system’s primary function originally was to simplify calculations by making every unit divisible by 10, and, since we use base 10 for counting, this makes arithmetic simple. I labored over my sums in primary school using yards, feet, and inches; gills, pints, quarts, and gallons; pounds, shillings, and pence etc. How much will it cost to paint a room 12’6”x 8’3’’x 7’3” if 1 gallon of paint covers 9 square yards and costs 5/9d a quart? I never got the answers right. Interestingly dozens, and shillings and pence are easy because they are both base 12. So calculating the cost of eggs or buns sold by the dozens (not baker’s dozens) in shillings and pence is very easy, but complicated in decimal money. Yet . . . in the West eggs are still sold by the dozen. In China they are sold in 10s.


The metre as a standard of length was first proposed in 1668 by the English cleric and philosopher John Wilkins and based his standard on a pendulum with a one-second period. In 1670 Gabriel Mouton, Bishop of Lyon, also suggested a universal length standard with decimal multiples and divisions, to be based on a one-minute angle of the Earth’s meridian arc or (as the Earth’s circumference was not easy to measure) on a pendulum with a one-second period. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the phrase metro cattolico (“universal measure”), derived from the Greek μέτρον καθολικόν (métron katholikón), to denote the standard unit of length derived from a pendulum. As a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, and on 19 March 1791 advised the adoption of the term mètre (“measure”), a basic unit of length, which they defined as equal to one ten-millionth of the distance between the North Pole and the Equator. In 1793, the French National Convention adopted the proposal.


In the 1870s, in light of increasing precision, a series of international conferences was held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres in France. This new organization was to construct and preserve a prototype metre bar, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. However, it was later determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre. Regardless, this length became the standard. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.


In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of length. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1 650 763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

To further reduce uncertainty, the 17th CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second and the speed of light: “The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second.” Got it?

This kind of precision is necessary for modern scientific calculations, but not for cooking. In the kitchen we can return to the days of Alfred the Great and plough oxen if we want. If you go to the HINTS tab on this blog you’ll find under Weights and Measures:

In the bulk of my recipes I give a detailed and precise list of ingredients. This is really a matter of convention as opposed to how I actually cook.  In certain recipes, especially baking, exact measuring is very important.  But in the kinds of savory dishes I make most commonly I don’t measure anything.  I throw things in the pot by eye.  BUT . . .as the dishes simmer I am tasting, tasting, tasting, and making adjustments as necessary. So, what you should take from my recipes is not so much precise measurements as a general idea to play with.  Strictly speaking, all my herb and spice lists should be labeled “to taste” just as is typical with salt and pepper.

This comment is tempered slightly by a discussion in this post on this date last year.  My “stupid” question there was, “What is room temperature?” Is it the temperature of an igloo in Alaska or a mud hut in Kenya? It does make a difference – maybe about 90 degrees Fahrenheit (about 32 degrees Celsius). There we run into the problem of metric versus other units as well. The US stubbornly resists converting to metric, and even has its own set of volumes and capacities distinct from old style English units. A US gallon is different from the old imperial gallon. Hence, these days recipes get published in metric and US units to please everyone, even though conversion is a pain in the butt, and is far from precise. Oven temperatures, and to a degree all weights and measures, are a bit loose anyway. Ovens vary considerably anyway. But cooking sugar is a completely different matter.


In Victorian times you could use terms such as “soft ball” or “hard crack” for gauging the temperature of sugar when it is cooking, but nowadays a proper, accurate sugar thermometer is essential for jams or candies if you don’t want to land in a world of trouble. Trust me on that. Mrs Beeton does not even use well-known terms; she just describes the stages of cooking sugar and hopes for the best. To follow her recipes you need a lot of experience, or a conversion chart. He’s a rough one I made. It lists the common names of the stages and their temperatures. You start with a sugar syrup of sugar and water according to the recipe. Then you heat it over medium-heat and either use a thermometer clamped to the side of the pot or test for the stages. As the syrup heats, the amount of water reduces allowing the temperature to rise well above the boiling point of water. This is an abbreviated table, using Celsius only, and brief descriptions and uses. If you are going by appearances you need a bowl of iced water near the stove. As the syrup heats, take a little with a spoon and drop it into the water. Then check for appearance as follows:

Stage        Celsius (degrees C)  Appearance and Uses

Thread       106-112                     Loose thin thread. Sugar syrups.

Soft Ball     112-115                     Soft, sticky ball. Caramels, fudge, pralines,  fondant, and butter creams.

Firm Ball    116-120                    Firm but pliable ball. Caramels, nougat, marshmallows, and toffees.

Hard Ball    122-130                   Hard, sticky ball that holds its shape. Caramels, nougat, divinity and toffees.

Soft Crack  132-143                    Strands that are firm yet pliable. Butterscotch, firm nougat, and taffy.

Hard Crack 146-155                   Stiff, brittle threads. Brittles, toffees, hard candy.

After this you get caramel of various degrees, then burnt sugar. You’ll see in the descriptions and uses that there’s a lot of variety and wiggle room. Different kinds of nougat call for different stages, for example.

Here’s Mrs Beeton’s recipe for barley sugar, a hard candy that used to be very popular when I was a boy. Of course no one makes it these days. I used to buy it in the local sweet shop.



INGREDIENTS.—To every lb. of sugar allow 1/2 pint of water, 1/2 the white of an egg.

Mode.—Put the sugar into a well-tinned saucepan, with the water, and, when the former is dissolved, set it over a moderate fire, adding the well-beaten egg before the mixture gets warm, and stir it well together. When it boils, remove the scum as it rises, and keep it boiling until no more appears, and the syrup looks perfectly clear; then strain it through a fine sieve or muslin bag, and put it back into the saucepan. Boil it again like caramel, until it is brittle, when a little is dropped in a basin of cold water: it is then sufficiently boiled. Add a little lemon-juice and a few drops of essence of lemon, and let it stand for a minute or two. Have ready a marble slab or large dish, rubbed over with salad-oil; pour on it the sugar, and cut it into strips with a pair of scissors: these strips should then be twisted, and the barley-sugar stored away in a very dry place. It may be formed into lozenges or drops, by dropping the sugar in a very small quantity at a time on to the oiled slab or dish.

Time.—1/4 hour.

Average cost, 7d.

Sufficient for 5 or 6 sticks.

Apart from the temperature to reach for the sugar syrup, this is straightforward. But the temperature is definitely an issue. I assume she means the hard crack stage which is 146-155°C. Much easier to judge with a thermometer. You could also let the sugar caramelize very slightly. Then you know the temperature is right, although you are on thin ice.