﻿480 Mr. M. H. Belz on the Heterodyne Beat Method 



a very sensitive method. It is now easily possible to maintain 

 oscillations of frequencies up to 10 7 per second. Taking the 

 case when Set 1 has a frequency of 1,000,000 per second, 

 Set 2 a frequency of 1,001,000 per second, the audible note 

 will have a frequency of 1,000 per second. If the frequency 

 of Set 1 is changed to 1,000,001 per second, the frequency of 

 the audible note will now be 999 per second, and this change 

 in pitch can readily be observed by comparison with a note 

 of standard pitch. 



Fig. 1. 



This sensitive method has been successfully employed by 

 Herweg *, Whiddington f , Pungs and Premier J, Falcken- 

 burg §, and several others in physical researches, but the 

 precautions necessary for steadiness in the beat note have 

 never been completely specified. 



Precautions. 



With high frequency oscillations of the order 3 x 10 5 per 

 second to 5 X 10 5 per second such as were used in the present 

 investigations, electrostatic shielding from all external in- 

 fluence was of the first importance. This was ensured by 

 placing all the elements of the circuits in earthed metal- 

 lined boxes, one of the variable capacities, by means of which 

 final small adjustments were made, being provided with a 

 long ebonite spindle which projected beyond the containing 

 box. With the box closed the note from the telephone T 

 was considerably reduced in intensity, and in order to obtain 

 the maximum loudness, a small section was removed from 

 the box, shielding being maintained by means of a piece of 

 fine metal gauze. 



* Herweg, Zeit.f. Phys. vol. iii. p. 36 (1920). 



f Whiddington, Phil. Mag. vol. xi. p. 634 (1920). 



% Pungs and Preuner, Phys. Zeit. vol. xx. p. 543 (1919). 



§ Falckenburg, Ann. d. Phys. vol. lxi. 2, p. 167 (1920). 



