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



one part in 100,000 for several minutes in spite of small 

 unavoidable variations. With the frequencies employed in 

 the present experiments, however, such a condition could 

 not be established. The heating of the valve parts and the 

 consequent change in capacity in the system resulting from 

 changes in the filament current cause changes in wave-length 

 which certainly far outweigh any real change due to increased 

 thermionic emission alone. In order then to secure a constant 

 filament current, accumulators of 100 ampere-hours capacity 

 were employed. These were charged regularly after about 

 three days' use, and after the valve had settled down, the 

 current from them showed no variation during a single run. 



Faulty contacts of wires joining the elements of the 

 circuits were avoided by soldering, the only sources of 

 uncertainty being the sliding resistances in the filament 

 circuits. These, however, were good types with bright 

 surfaces and stiff springs so that the chance of error due to 

 change of contact was small. 



The principal cause of variation in the frequencies of the 

 circuits was found to be due to variations in the high tension 

 batteries. This trouble has been mentioned by Eccles and 

 Vincent *. In the present work the plate voltage was 

 obtained from trays of portable accumulators of fairly low 

 capacity, each tray providing 40 volts. After the valves 

 had been burning for an hour or so, taking a current of 

 about 10 milliamperes, this voltage began to vary and the 

 beat note consequently drifted. However, giving the valves 

 time to settle down, a matter of 15 to 20 minutes, it was 

 found possible to hold the heterodyne note quite steady for 

 intervals of 30 to 60 seconds, and this is ample time in which 

 to make a single observation. After about 90 minutes 

 burning the variation was too rapid and the batteries had to 

 be recharged. The size and consequent capacity of the cells 

 of these batteries is limited by the fact that they have to be 

 contained in a metal box, and thus this source of variation 

 can only be provided for in special cases. 



Technique. 



It is essential to maintain the oscillations generated in the 

 circuits at frequencies considerably different from the natural 

 frequencies of the coils alone f, that is to say with an 

 appreciable capacity in the system, and under these conditions 

 the frequency, n, of the oscillations in such a circuit containing 



* Eccles and Vincent, loc. cit. 



+ Cf. Townsend, Phil. Mag. vol. xlii. August (1921). 



