1920] Thermionic Valve in Wireless Telegraphy & Telephony 173 



with increased velocity. Therefore the thermionic current will be 

 increased. The important point to notice is that, owing to the small 

 electrical capacity of the grid, and also owing to the high voltage 

 acting in the plate circuit, a very small expenditure of power on the 

 grid circuit will vary or modulate a much larger amount of power in 

 the plate circuit. Just as the pressure of a child's finger on a switch 

 may start or stop an electric motor of several horse-power, or a feeble 

 current passing through a telegraph relay start or stop a large current, 

 so the three-electrode valve acts as a relay. 



If we plot a curve delineating the variation of thermionic current 

 with varying grid voltage or potential for such a three-electrode valve 

 we find that curve over wide limits to be nearly a straight line 

 (Fig. 8). This means that the change in plate current is propor- 

 tional to the change in grid voltage. 



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Fig. 9. — Two Thermionic Valves Coupled in Cascade. 



However rapidly the grid voltage may change, ? : so nimble are 

 these little electrons that the thermionic current copies on a mag- 

 nified scale the changes of grid potential. Hence the arrangement 

 is called a thermionic amplifier. 



"We can, however, advance further. If we cause the plate current 

 of one valve to pass through the primary coil of a transformer, and 

 then connect the terminals of the secondary coil of the latter respec- 

 tively to the grid and filament of a second valve, we find that the 

 fluctuations in the plate current of the first valve can be made to 

 generate exalted potential variations of the second valve, and this 

 again to create magnified variations of the plate current of the 

 second valve (see Fig. 9). 



This mode of connection is not limited to two valves ; we can 

 thus employ three, four or more valves in cascade, as it is called, and 



