﻿166 Messrs. E. W. B. Gill and J. H. Morrell on Short 



should occur when the above circuit is tuned to X, 2\, 3X, 

 etc., these positions being indicated by the deflexions of the 

 galvanometer connected to the thermo-j unction. Moreover, 

 the distances measured along the wires between successive 



positions of maximum oscillations should be equal to - , and 



all therefore should be equal. It was soon found that this 

 simplicity was not attained, in certain cases equi-spaced 

 positions were found, but in the majority of cases there were 

 at least two sets of positions forming two series of equal 

 spaces, which, as the spacing distance of the two sets was 

 different, appeared to indicate two optimum wave-lengths. 



These effects are due to the different modes of oscillation 

 of the system, and, according to the theory which we give 

 below, a grid voltage V will, under suitable conditions, sustain 

 oscillations of short wave-length between certain limits. Any 

 mode of oscillation corresponding to a wave-length between 

 these limits will be maintained. It was therefore desirable 

 to arrange the apparatus so as to avoid these complications. 



5. The most obvious improvement was to give up the idea 

 of finding the wave-lengths by moving the bridge, and to put 

 the bridge and its leads at the far end of the parallel wires 

 joined to the valve, and to measure the wave-lengths of the 

 oscillations by means of a loosely coupled secondary circuit. 

 The system of wires connected to the valve is thus fixed. A 

 second pair of long Lecher wires were set up with a loop 

 joining one end, and this loop was brought near the valve 

 circuit. When the secondary is in tune with an oscillation 

 in the primary the current in the primary is reduced. The 

 deflexion of the galvanometer connected to the thermo- 

 junction in the primary circuit may be reduced by 50 per 

 cent, when the bridge in the secondary circuit is in the tuned 

 position, and a movement of 0*5 cm. either way will restore 

 the deflexion to its original value. The distances between 

 the successive positions of the bridge on the secondary circuit, 

 for which the deflexions of the galvanometer attached to the 

 primary circuit are a minimum, are the same, and are equal 

 to halt the wave-length of the oscillation in the primary 

 circuit. All the wave-lengths quoted were measured on 

 this form of wave-meter and may be taken as accurate to 

 0*5 per cent. * 



With the condenser bridge and thermo-couple at the far end 

 of the Lecher wires the filament was heated to give an 

 emission of a few milliamperes (this is low r heating for an 



* Townsend and Morrell, Phil. Mag. Aug. 1921, pp. 266-268. 



