EVOLUTION OF QUARTZ CRYSTAL CLOCK 581 



approach to timekeeping. The idea of utiHzing such resonance phenomena 

 for the measurement of time was suggested in January, 1945 by Professor 

 1. 1. Rabi of Columbia University at an address before the American Physical 

 Society and the American Association of Physics Teachers. 



These resonance phenomena, involving the interaction of microwave 

 electromagnetic radiation with atoms or molecules of matter, have been dis- 

 covered only quite recently and it is likely that a great deal more will be 

 learned about them in the next few years. The results already obtained are 

 very promising and investigations already under way may well lead to the 

 means for creating an entirely new type of standard of time interval and 

 rate — both of prime importance in Physics. 



The studies of greatest significance for such purposes now in progress fall 

 in two main branches involving quite different techniques. The actual 

 means for regulating a clock would be quite different in the two methods, 

 but would be possible in either. With what is known up to the present time, 

 however, the construction of such a clock would be a considerable under- 

 taking, especially to make one that would operate over long periods. The 

 two chief phenomena involving atomic or molecular resonances are: (1) the 

 absorption of high-frequency energy in certain materials, particularly in 

 gases, exhibiting ultra-fine absorption spectra; and (2) the defiection of 

 beams of atoms or molecules under special conditions of magnetic and 

 electric fields. The earliest reported work on the absorption of microwaves 

 in gases was done by C. E. Cleeton and N. H. Williams^^ in 1934. With 

 the development of improved high-frequency generators and measuring 

 techniques the work has been extended considerably during the last few 

 years by C. H. Townes^^, W. E. Good^^^ and others. It is believed that 

 with modifications of methods, such as used by them, it would be possible to 

 control the frequency of the short-wave generators such as used in making 

 these studies; and, if this can be done, the adaptation for use in time-measur- 

 ing devices would follow naturally as in the case of any other stable 

 oscillator. 



The general method using molecular beams has been a gradual develop- 

 ment over some years, but the first published suggestion of the applications 

 which relates closely to this work was made in 1938 when I. I. Rabi, J. R. 

 Zacharias, S. Millman and P. Kusch first used the beam deflection method for 

 measuring nuclear magnetic moments.^^^ Two articles^^** • ^^^ in Revieus of 

 Modern Physics in July 1946 give a good description of the molecular beam 

 method and the results of some studies of fine structure resonance phe- 

 nomena. The resonance curve shown in Fig. 35 obtained recently by P. 

 Kusch and H. Taub of Columbia University, and hitherto unpublished, 

 illustrates the resolution obtainable by molecular beam methods. According 

 to theory, the actual width of the resonance should be substantially inde- 



