Pilgrimage

Science has its holy places. This week one of them, in the suburbs of Paris, will be visited by a distinguished group of reverent pilgrims. Delegates to the International Conference on Weights & Measures, they will represent 31 nations, including the U.S.S.R.

The object of their veneration is enshrined in a white stone building in the Pare de St. Cloud. When the pilgrims have filed down three flights of basement stairs, they will be confronted by two iron doors. Behind those doors, in a massive concrete vault, lies the object.

With reverence (and three keys), the scientists will open the doors. Inside, in a triple case of glass, hard rubber and wood, they will find, safe & sound, they hope, the venerated object: a softly gleaming bar of platinum-iridium. On its polished surface are two fine lines. When the bar's temperature is zero degrees centigrade, the distance between those lines is exactly one meter (39.37 inches).

Lined Bar. This is THE meter: the standard length in terms of which all the world's measurements are defined. Even men who speak in pounds or poods, kilometers or versts, acres or mu depend ultimately upon the meter bar in Paris. The subtler units of measurement, such as dynes, electron-volts and curies, are based on it too.

When the metric system was devised by French revolutionists in 1791, its fundamental unit, the meter ("measure" in Greek), was defined as one ten-millionth of the distance between the earth's poles and its equator. Since this distance is hard to determine accurately, it was abandoned, and the standard meter bar put in its place. Most nations have copies of it. Even the stiff-necked British check their "standard foot" against the meter.

The Paris meter bar has served the world well, but there are two things wrong with it. Modern techniques of measurement make the fine engraved lines on the bar seem coarse and irregular. Then too, many scientists feel that the metric system should not be based on an arbitrary length, but upon some length taken from nature itself. Then, if all the meter bars were destroyed (by atomic war, for instance), the standard could be reestablished, as good as ever, when the radioactive smoke had cleared away.

Green Light. Many weights & measures experts have had their eyes on a kind of green light from glowing mercury vapor. But natural mercury contains seven different isotopes, each giving off light of slightly different wave length. Pre-atomic science had no practical way of making one-isotope mercury.

Last week Dr. Edward U. Condon, Chief of the U.S. National Bureau of Standards, told the Paris conference that the job had been done--by transmutation. Pure gold (atomic weight 197), placed in an atomic pile and exposed to a storm of neutrons, is transmuted into a single mercury isotope with atomic weight 198. This one-isotope mercury gives off green light waves of extraordinary uniformity, as measuring sticks. They are much better than cadmium light; they are vastly better than the meter bar. Scientists using mercury light should be able to measure with an accuracy of one part in 100 million.

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