580 BELL SYSTEM TECHNICAL JOURNAL 



good standard for time measurements. A step in the right direction was 

 made later by A. A. Michelson whose precise determination of the wave- 

 length of this radiation made possible the redefinition of the International 

 Meter as a defipite number of such wavelengths, measured in vacuo. From 

 this definition, it is now possible to duplicate the primary standard of 

 length with great accuracy, and to check such secular changes as may occur 

 in the original standard, the distance between two marks on a metal bar. 

 The constancy of the standard, as defined by Michelson, depends upon 

 properties of primary particles of matter, and upon properties of space, 

 which, as far as human beings are concerned directly, appear to be quite 

 independent of time or location. A similar definition of rate, or time inter- 

 val, is very desirable. 



A ray of hope came out of the important work of Nichols and Tear^^^ who 

 proved that electric waves which could be produced electrically were of the 

 same stuff as radiation from hot bodies. They were able to detect radiation 

 of either sort by the same receiving device and showed that they both had 

 the same properties of refraction, polarization, etc. Later, Cleeton and 

 Williams^'2 were able to produce continuous electric waves at very high 

 frequencies — corresponding to about 1 cm. wavelength — and to show that 

 they also had the important properties of light waves. Now the range has 

 been extended somewhat more and there are reports^^^ of experimental 

 generators that can produce continuous waves of a few millimeters wave- 

 length. This is an active development and, of course, the end is not in 

 sight. From continuous waves of any frequency it is believed possible by 

 general techniques now well known to control lower frequencies, and from 

 them eventually all sorts of time measuring and indicating devices as pre- 

 viously described. 



Within the last few years, the missing link has been discovered which, with 

 Suitable instrumentation, may make it possible to construct a clock con- 

 trolled by atomic- or molecular-resonance phenomena. There are a great 

 number of resonance phenomena associated with the molecules in a gas, or in 

 molecular beams, which are responsive to electric waves that can be pro- 

 duced continuously by modern vacuum tube means. In some cases, the 

 sharpness of resonance is such that changes of frequency of one part in 10*^ or 

 less can be detected, leading to the idea that such resonance phenomena may 

 be utilized in some way to control the frequency of a suitable oscillator and 

 hence, through frequency conversion circuits, to control frequencies low 

 enough to operate clocks and other mechanisms. Some of the resonance 

 phenomena in point are in the one-centimeter region, a field that is rapidly 

 being exploited in radar and communication applications. It is to be 

 expected, therefore, that techniques for dealing with such high frequencies 

 will be developed in the near future thus facilitating a study of this new 



