182 



Professor J. A. Fleming 



[May 21, 



signal travelling through the aether. The plan usually adopted is to 

 give the relay so much inertia that the circuit is only closed by send- 

 in-- signals composed of several very long dashes or trains of electric 

 waves. 



I have recently devised a form of hell-call which depends upon 

 the use of a new type of four-electrode valve, made as follows: A 

 highly exhausted glass bulb contains a straight filament of tungsten, 

 which is rendered incandescent by a 6-volt battery. Around the 

 filament are arranged four narrow curved metal plates, having their 

 curved sides facing the filament and very near to it. Each of these 

 plates is carried on a wire sealed through the glass bulb. The plates 

 are arranged round the filament, as shown in plan in Fig. 17. 



oKSH^I'H'I'I'bJ 



Fig. 17. — Fleming Four-Anode Valve. 



1 and 2 are the collecting plates. 3 and 4 are the potential or deflecting 

 plates. B is the filament-heating battery, and the central dot is the end-on 

 view of the straight filament. G is a relay or galvanometer. 



Two of these plates on opposite sides of the filament, viz. 

 3 and 4 (see Fig. 17), are called the potential plates, and the other 

 two the collecting plates. The collecting plates are joined together 

 outside the bulb and connected to the positive terminal of the 

 filament-heating battery, and a galvanometer G or telegraphic relay 

 inserted in that circuit. The electronic emission from the filament 

 then creates a current which flows through the galvanometer or the 

 relay, as in the Edison experiment. If the two other plates have 

 a small potential difference made between them, either of constant 

 direction or else a high frequency alternating difference, this suddenly 

 reduces the thermionic current. The potential difference of the 

 potential plates introduces a new electric force into the field which 



