172 



Professor J. A. Fleming 



[May 21, 



connected to the plate, the stream of electrons emitted by the 

 filament will be drawn to the plate and give a thermionic current 

 of three or four milliamperes if the valve is highly exhausted. 



Pin terminals 



F IG , 7, — a Modern Three-Electrode "Hard " Valve 

 of a Type Usually Called the French Valve. 



This stream of electrons will reach the plate by shooting through 

 the holes or interspaces in the mesh or spiral grid G. 



Let us now suppose that we give the grid a small negative charge 

 by a battery B 3 . This will cause the electrons coming out of the 



Fig. 8. — Characteristic Curve of a Three-Electrode Valve. 



filament to be partly repelled, and therefore the thermionic current 

 in the plate circuit will be reduced perhaps even to zero. 



Again, let us give the grid G a small positive charge. This will 

 attract the emitted electrons and they will shoot through the grid 



