EVOLUTION OF QUARTZ CRYSTAL CLOCK 579 



reasonable to suppose that an electrically-resonant circuit maintained at a 

 temperature in this region could be made to have a very high Q, and very 

 stable dimensions, and so have the chief desirable properties for rate control 

 that obtain in a quartz resonator. Resonant cavities used at high fre- 

 quencies have many of the properties of other electrical resonant circuits, 

 and in particular their energy dissipation for electric oscillations can be very 

 substantially reduced when cooled to superconducting temperatures. In 

 some experiments made recently at Massachusetts Institute of Technology^^Q 

 it has been shown that a cavity resonator made of lead, which for 3-cm. 

 waves has a Q of about 2,000 at room temperatures, is so much improved at a 

 temperature of 4 degrees absolute that the Q approaches a million. Such a 

 resonator could be used as the stabilizing element in an oscillator and hence 

 in a clock. The relative stability over long periods could, of course, be 

 determined only by experiment. 



Maintenance of the required low temperature would add considerably to 

 the complexity of such a system, but if the advantages were such as to pro- 

 duce a new order of stability, and particularly if it should make possible a 

 clock system with small or zero aging, it certainly should be justified for 

 future time measurement studies. 



The other avenue of approach is through the application of certain 

 resonance phenomena in atoms and molecules that do not depend upon 

 aggregates of matter as is the case with all mechanical systems used hereto- 

 fore in time measuring means. The extreme fineness of structure and the 

 constancy of atomic and molecular resonance phenomena have long been 

 recognized through studies of line spectra, and in the field of spectroscopy 

 these properties have been used as standards of wavelength ever since the 

 early studies of Joseph von Fraunhofer, reported in 1815.^^° Wavelength, X, 



and frequency,/, are associated by the simple relation/ = — where c is equal 



A 



to the velocity of light. For visible radiations / turns out to be extremely 

 large, for the red Hght, 6500A, it is 462 million million vibrations per second. 

 So far, such high frequencies have not been observable or measurable di- 

 rectly but can only be deduced from wavelength measurements as just 

 stated — which inevitably involve the use of man-made standards of 

 length and the combined errors of two quite different sorts of physical 

 measurements. 



It has long been the dream of physicists to find some way to tie in directly 

 with the natural frequencies of atoms and molecules and to derive from them 

 a direct measure of rate, and, of course, of time interval. It has been 

 thought, for example, that the red radiation from cadmium vapor, whose 

 wavelength was measured by C. Fabry and A. Perot in terms of the standard 

 meter as accurately as that standard could be defined, would also make a 



