﻿The Heterodyne Beat Method. 479 



general quantized systems in §9, and examples are given. 

 In § 12 it is shown how it may be made to deal with as- 

 semblies composed partly of: free molecules and partly of 

 quantized systems. In §13 we deal with extensions possible 

 when only molecules are present. 



The methods we have described can also be made to throw 

 an interesting light on the statistical foundations of therrao- 

 dynamics; but in that connexion many points have arisen 

 which require rather careful discussion, and in order not to 

 make the present paper too long, we have deferred them to 

 a future communication. 



Cambridge, 

 May, 1922. 



XLV. The Heterodyne Beat Method and some Applications 

 to Physical Measurements. By Maurice H. Belz, M.Sc. 

 (Cantab.), Barker Graduate Scholar of the University of 

 Sydney *. 



IN a recent paper f, a preliminary account was given of 

 the application of the heterodyne beat method to the 

 measurement of magnetic susceptibilities. In virtue of the 

 importance of the method as a sensitive measure of physical 

 quantities, it seems desirable to give a more complete account 

 of the principle and of some of the difficulties encountered 

 in its application. 



Essentially the method consists of the following arrange- 

 ment shown in fig. 1. 



Two oscillating circuits, Set 1 and Set 2, are set up side 

 by side and arranged so as to have approximately the same 

 frequency. The two sets are loosely coupled so that in the 

 telephone included in one of the circuits a resultant beat 

 frequency is maintained equal to the difference between the 

 frequencies of the fundamentals or overtones in the two 

 circuits. If symmetry in the two circuits is essential, direct 

 coupling can be replaced by indirect coupling by means of a 

 third circuit in which the telephone is placed. In either 

 case, when the beat frequency is low enough, an audible note 

 will be heard in the telephone, and any changes in the 

 constants of either circuit will cause the frequency of 

 the audible note to alter by an amount equal to the change 

 in frequency of the responsible circuit. This at once provides 



* Communicated by Professor Sir E. Rutherford, F.R.S. 

 t Belz, Proc. Camb. Phil. Soc. vol. xxi. part 2 (1922). 



