April 5, 1917] 



NATURE 



107 



:arbon filament to be rendered incandescent. One 



f these electrodes was in the form of a plate, 

 and the other of a grid or zigzag of wire inter- 

 posed between the filament and the plate. In 

 using this double-plate thermionic detector, Dr. de 

 Forest connected the grid terminal to one side 

 of the receiving circuit condenser, and the nega- 

 tive terminal of the filament to the other side of 

 the same condenser ; but, instead of inserting the 

 telephone or current-detecting instrument in this 

 grid circuit, he included it in a separate external 

 circuit connecting the plate with the filament, and 

 placed in this circuit also a battery with negative 

 pole connected to the filament (see Fig. 3). 



Dr. de Forest called this arrangement an audiortt 

 and maintained that the physical action was dif- 

 ferent from that of the Fleming valve, though 

 valves with two anode plates had already been in 

 use for certain experiments. It has been shown, 

 however, to be essentially the same. It is 

 clear that the performance of the audion as a 

 radiotelegraphic detector depends entirely upon 

 the thermionic emission from the incandescent 

 filament. It has been demonstrated by Dr. 

 E. H. Armstrong in a paper in the Proceed- 

 ings of the Institute of Radio-Engineers for Sep- 

 tember, 1915, that the physical actions taking 

 place in the grid circuit of Dr. de Forest's audion 

 are precisely the same as in those in the Fleming 

 valve. 



The thermionic emission of negative ions causes 

 the grid to become negatively charged. On the 

 other hand, the battery in the external circuit 

 connected to the plate sends a thermionic current 

 through the vacuous space between the filament 

 and the plate inside the bulb, in virtue of the in- 

 candescence of the filament or negative electrode. 

 This current flows also through the telephone or 

 current-detecting appliance. When the grid be- 

 comes negatively charged, due to the rectification 

 of electric oscillations impressed upon the grid 

 circuit, it reduces the thermionic current flowing 

 between the filament and the plate, and therefore 

 varies the current through the telephone. The 

 physical actions which contribute to the operation 

 are therefore all dependent upon the thermionic 

 emission from the filament and upon the increased 

 unilateral conductivity of a highly rarefied gas or 

 vacuous space when the cathode or negative elec- 

 trode is rendered incandescent. 



This action is not necessarily dependent upon 

 the presence of any residual gas in the bulb, be- 

 cause even in a highly perfect vacuum the elec- 

 tronic emission from the incandescent filament 

 would take place. 



The double-anode Fleming valve, or the valve 

 with grid and plate, called an audion, has the 

 property that an amplification of current variation 

 can be produced by it. 



Thus, if the erid-plate thermionic detector is 

 arranged as in Fig. 3, feeble electric oscillations 

 taking place in the grid external circuit can be 

 made to produce large variations in the continu- 

 ous current flowing in the external plate circuit. 



Moreover, by connecting two or more such 



NO. 2475, VOL. 99] 



double-anode thermionic detectors in series, the 

 current in the plate circuit of one, acting induc- 

 tively on the grid circuit of the next, enables a 

 double amplification to be produced. 



Furthermore, such double-anode thermionic 

 valves can be used as generators of electric oscilla- 

 tions by inductively connecting through a suitable 

 transformer the grid and plate circuits g and h 



Fig. 3. — Dc Forest audion or form of thermionic detector. D, bulb of 

 glow-lamp : F, incandescent filament ; a, grid ; 6, plate ; T, tele- 

 phone ; A, B, batteries ; C, C", condensers. 



of one and the same bulb V (see Fig. 4). The 

 arrangement then acts as follows : — Feeble electric 

 oscillations set up in the external plate circuit by 

 any means create induced oscillations in the grid 

 circuit, and the latter sustain and enhance the 

 former, the energy to create these enhanced 

 oscillations coming from the battery in the plate 

 circuit. 



The process exactly resembles that in which a 



i d'i'l•l'l'i'!'l'l'I^ 



I 

 Is 



ctioooHodc 



3 L|.|.|.|J -J-; 5 



2 R. I ; * 



Fig. 4. — Mode of using doabie-aaode valve or thermionic detector a.s a 

 generator for electric oscillations. 



Bell telephone receiver in circuit with a battery 

 and carbon microphone transmitter emits a con- 

 tinuous musical note when the diaphragm of the 

 receiver is held near that of the transmitter. 

 Feeble vibrations are set up in the microphone 

 diaphragm by noises in the room, and these vary 

 the current through the telephone receiver, and 

 the sound so emitted keeps the transmitter dia- 

 phragm in motion. 



