﻿Electric Waves obtained by Valves.- 165 



filament move outwards under the positive voltage, V, oL' the 

 grid, and a certain number go direct to the grid and arc 

 collected there, the remainder pass through the grid, and, if 

 the potential of the plate is just less than that of the filament, 

 they return to, and are finally collected on the grid. If, on 

 the other hand, the plate potential is a little above that of the 

 filament, a certain proportion of those getting through the 

 grid reach the plate. If the plate potentiometer is now ad- 

 justed till the plate current is just zero, and the bridge is 

 moved along the wires, it will be found that with the bridge 

 in certain regions a plate current appears. It was the 

 appearance of this plate current which led Barkhausen to the 

 discovery of the short waves. With the present apparatus 

 these oscillations are also made apparent by the deflexion of 

 the galvanometer attached to the thermo-j unction. The 

 positions of the bridge at which the galvanometer gave a 

 maximum deflexion were fairly sharply defined, and did 

 not always coincide with the positions for maximum plate 

 current. 



It is not necessary for the plate potential to be so adjusted 

 that the plate current is just zero when oscillations are not 

 occurring. The plate may be set at a considerable negative 

 potential, or the plate voltage may be positive. It was found 

 that for a given grid potential there is a certain plate potential 

 at which the oscillating current through the thermo-j unction 

 is a maximum. Also as the potential of the plate was 

 increased, for plate potentials only slightly positive, if oscil- 

 lations commence the plate current increases ; at a certain 

 plate potential no change is noticed in the plate current ; 

 and at higher potentials the plate current decreases. For 

 the M.T.5 valve this critical potential was about 4- 2 volts, 

 when the voltage drop down the filament due to the heating 

 current was about 4 volts. 



In the first experiments with this apparatus the position of 

 the bridge was varied and the current in the thermo-couple 

 observed when the grid voltage V, the heating current, and 

 the plate potential were all kept constant. 



The oscillating circuit consists of a condenser formed by 

 the plate and grid of the valve, the distributed inductance 

 and capacity of the Lecher wires up to the bridge, and the 

 capacities C, C in series with the wires and with the short 

 resistance of the thermal heater which connects them. Hence, 

 if there is an optimum wave-length A corresponding to the 

 grid voltnge V, and if, starting near the valve, the bridge is 

 pushed along; the wires, maximum amplitude of oscillation 



