JOURNAL OF CHEMICAL EDUCATION
NICE TIMING
M ~ s slength, , and time are the basicvariablesin allphysicalprocTo describe such processes accurately, one must measure all three variables precisely and in terms of invariable standards. With new techniques of chronometry, scientists can measure the timing of natural phenomena with an accuracy far surpassing that of man's traditional timepiece-the earth. To measure time, it must be divided into suitable intervals by same repetitive process, such as the rotation of the earth about its axis (man's first unit of time). Also, onemust be able to count the repetition, for example, by marking off the days on a calendar. More precise measurements require periodic effects occurring a t a more rapid rate, cg., a clock pendulum oscillating once per second, or the quartz crystal of a radio transmitter vibrating millions of times a aecond. Even such devices are not reliable time standards, however; the length of a pendulum or the thickness of a quartz crystal can be machined t o only a limited accuracy. The dimensions of materidls c h a n g e h e n c e timing devices lose their accuracy-with changing external temperature, humidity, and pressure. Thus, the oscillations of such devices must be corrected occesionelly by means of astronomical observations. Although s. well-designed quarts oscillator can maintain its frequency t o one part in 100 million for a n hour or two, it is nevertheless only a means of h e l y subdividing the time we measure by the earth's rotation. A further difficulty is that this rotation itself is not steady enough for some astronomical observations. Because of the frictional drag of the tides, and for other reasons, theearth is slowing down, and our second-defined as '/as.lm of a mean solar dayis slowly getting longer. Another difficulty arises from the fact that the easth is a liquid contained in a more or less solid crust. Relative motion between the crust and the interior, and possible redistribution of matter in the crust itself. cause the length of the
The &st counting of molecular vibrations was done with ammonia molecules, which are built like triangular-based pyramids, with three hydrogen atoms a t the corners of the base and a nitrogen atom a t the top. The nitrogen atom vibrates toward and away from the base, and occasionally passes through the base t o a position on the other side. When radio waves are transmitted through a tube filled with ammonia gas, this inversion occurs more frequently if the radio waves have exactly the micronrave frequency a t which the nitrogen atoms are vibrating. Energy is absorbed in the process; the energy of the radio waves is measured after they have passed through the tuhe, and maximum energy is absorbed if the whole system is precisely in tune. The ammonia clock runs with a constancy of one part in 50 million, and may be improved t o one part in 100 million or more, considerably better than the accuracy of the earth's rotztiotion. A further refinement of technique uses a beam of ammonia molecules; here the fuaziness due to molecular collisions in a gas is overcome. I n this case, the heated molecules themselves emit a microwave signal, which is detected and used as the time standard. Another type of clock under development a t the Bureau may he controlled by the magnetio behavior of cesium atoms. Here the vibration is caused by applied magnetic forces acting upon the atom: since the cesium atoms are not free to collide wit,h ea.ch
cal observations as an apparent unevenness in the motion of the moon and planets relative to the earth. The ideal way to overcome such difficulties is to rely on a time standard that has some fundamental relation t o the structure of the universe. Thus, even the best man-made pendulums and quartz crystals are ruled out, and scientists have sought invariable and reproducible periodic phenomena within matter itself. Recentlv. the National Bureau of Stmdards develoued a n atomic cloak hnsed on the internal vibrations of a t o m within molecules. Ttwevibrntionx, unlike rlwsr of pmclulums orcrystalc, are largrly uninfluerwd hy rvtrrrul r m d i t i w s , a d are iwlcpvndmt of the
that of the earth. With such a clock, it should he possible t o study a number of problem of great scientific and practical imporiance, ranging from changes in the earth's rotation and checks on the theory of relativity to improvements in navigation. Also, atomic time may eventually come t o be recognized as the international standard, for scientific purposes. For one thing, it is universal, and would hold good if we should ever colonize Mars. But the earthbound work will continue from sun to sun, under the stimulus of the seasons.-Reprinled from the Industrial Bulletin of Arthur D. Litlle, Ine.
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ress in electronics now permits counting circuits to be operated a t frequenoies of 20,000 to 100,000megacycles per second. Also, some molecules and atoms have been discovel.ed that naturally vibrate a t just these relatively lower "micron.ave"frequencies. AMMONIA CLOCK
