﻿Electric Waves obtained by Valvesj 167 



M.T.5 valve) and the grid voltage was raised by two volts at 

 a time by means of batteries of small accumulators from 16 

 volts to 120 volts, while the plate was kept about 2 volts 

 positive, as this gave large deflexions. The corresponding 

 galvanometer deflexions are shown in fig. 3. 



The deflexions are plotted against grid volts ; as a thermal 

 detector was being used, the deflexions are proportional to 

 the mean square of the oscillating current. 



The curve shows that oscillations are occurring over nearly 

 the whole range, but that there are maxima for certain 

 voltages— viz., 16, 24, 42, 58, 82, 114, approximately. The 

 wave-lengths measured as above give from 16 to just below 

 24 volts X586 cm., from 24 to 40 volts X451, and so on, the 

 wave-lengths for successive portions of the curve being 366, 

 307, 262, 233. These correspond to the free oscillations of 



Fiff. 3. 



the system, the wires of which were 850 cm. long with 

 leads to the valve about 70 cm. long, with a slight addition 

 tor the leads within the valve itself. 



The system of wires connected to the valve therefore 

 present a selection of various modes of oscillation with wave- 

 lengths 586, 451, 366, 307, etc., cm., from which the valve 

 chooses the one suitable for the particular voltage V between 

 the grid and plate — the sharp rises just before the various 

 maxima showing that the system oscillates on the longer 

 wave-lengths by preference. For each particular wave- 

 length there is a certain grid voltage which gives the 

 strongest oscillations when the heating current in the filament 

 and the plate voltage is constant ; but the heating current 

 and the potential of the plate relative to the filament both 

 affect the optimum voltage for a given wave-length. In- 

 creased emission has the same effect, but this effect depends 

 on the degree of saturation of the emission current. 



In the preceding experiments the wave-lengths of the 

 oscillations were measured with a constant heating current 



