174 



Professor J. A. Fleming 



[May 21, 



each one multiplies or amplifies the effect of the one before. Thus, 

 if the first valve multiplies potential variations ten times, or has an 

 amplification factor of ten, then two in cascade amplify 100 times, 

 and three 1000 times, and so on (Fig. 10). It is this use of three- 

 electrode valves in cascade that has given us recently such vastly 

 increased powers of detecting wireless waves. 



The last or final amplifying valve may be made to operate a 

 detecting or rectifying valve, or perhaps a crystal detector. 



I have on the" table a set of such cascade valves, six in series, 

 with one detector valve, which has been kindly lent by the Marconi 

 Company, and is used in their radio-telegraphic work. Fig. 16 

 shows the external appearance of this receiver, and Fig. 10 the 

 diagram of connections. 



Fig. 10. — Marconi Company's Valve Detectoe with 

 Valves in Cascade, Type 55. 



But there is an additional very valuable power possessed by 

 the thermionic valve, viz. that it can generate electric oscillations 

 as well as detect them. This property is common to the two- and 

 also to the three-electrode valves, but as time is limited I shall 

 confine my explanations to the mode in which the latter operates as 

 a generator of oscillations. 



We have already seen that the fundamental property of this valve 

 is that variations of grid potential create similar variations of plate 

 or thermionic current. Supposing, then, that this latter current is 

 passed through a coil over which is wound another secondary coil 

 which connects the grid and filament (Fig. 11). It is possible so to 

 make the connections that any increase in the plate current will give 

 the grid a negative charge and so immediately reduce the plate 

 current. Conversely, any reduction of plate current will give the 

 grid a positive charge which will again increase the plate current. 

 Hence the operations in the plate current when once started will be 

 maintained, the energy required being drawn from the battery B (see 



