io6 



NATURE 



[April 5, 191 7 



been very fully investigated by Prof. O. W. 

 Richardson, who has collected most of the known 

 facts in an excellent manual on the subject. None 

 of the investigators of this subject made any 

 practical application of this knowledge until it 

 occurred to Dr. Fleming in 1904 to employ an 

 incandescent electric lamp having one or more 

 plates or cylinders of metal sealed into the bulb 

 as a means of detecting high-frequency electric 



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. ». — Various forms of Fleming oscillation valve or thermionic detector 

 used in wireless telegraphy. 



oscillations, as used in radiotelegraphy. Accord- 

 ingly he constructed such electric glow-lamps with 

 carbon filaments and a metal plate or cylinder 

 surrounding, but not touching, the filament, the 

 said cylinder being attached to a platinum wire 

 sealed through the bulb (see Fig. i). He em- 

 ployed this device as follows : — The carbon fila- 

 ment in the lamp O (see Fig. 2) is rendered in- 



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Fic. 2.— One mode of employing the oscillation valve as a detector in a 

 radiotelegraphic receiving circuit. A, antenna ; P S, oscillation trans- 

 former ; C, coiidenser; O, oscillation valve; B, battery; G, current- 

 detecting instrument. 



candescent by a suitable battery of storage cells, 

 B; most usually a 12-volt or 4-volt filament is 

 employed. A circuit is formed external to the 

 bulb by connecting the metal plate or cylinder to 

 the negative terminal of the lamp. In this circuit 

 is placed a current-detecting instrument, such as 

 a galvanometer, G, or a telephone. In the circuit 

 is also inserted the secondary circuit of an oscilla- 



NO. 2475, VOL. 99] 



tion transformer, P, S, the terminals of which 

 are closed by a condenser, C (see Fig, 2). 



If electric oscillations are created in the above 

 circuit, the alternations of current are rectified ; 

 that is to say, a unidirectional current flows 

 through the galvanometer or telephone. The 

 highly vacuous space between the incandescent 

 filament and the metal cylinder inside the bulb 

 possesses a unilateral conductivity. When the 

 filament is at a very high temperature negative 

 electricity can pass from the filament to the plate, 

 but not in the opposite direction. Hence the 

 device acts as a valve and was called by Dr. 

 Fleming an oscillation valve. Another way of 

 viewing the effect is as follows : — When the elec- 

 tric oscillations take place through the condenser,, 

 the plate or cylinder in the bulb tends to become 

 charged alternately positive and negative. The in- 

 candescent filament is continuously emitting nega- 

 tive ions or electrons, and these at once discharge 

 any positive charge on the metal plate, whereas 

 they do not discharge a negative charge. There 

 is, therefore, a continuous movement of positive 

 electricity to the plate from outside the lamp. If 

 the electric oscillations are in trains of damped 

 groups, then the effect is to convert them into 

 gushes of electricity in one direction which pass 

 through the telephone. If these groups come at 

 the rate of several hundred per second the tele- 

 phone receiver emits a continuous sound of corre- 

 sponding pitch, and if the groups are cut up into 

 Morse signals at the sending end, the listener at 

 the telephone hears these signals as long and 

 short sounds. 



An electric incandescent lamp with metal plates, 

 grids, or cylinders in the bulb is now called a 

 thermionic detector, because it serves to rectify 

 and render detectable by a galvanometer or tele- 

 phone receiver the feeble electric oscillations used 

 in wireless telegraphy or telephony. It depends 

 for its action upon the emission by the incan- 

 descent filament of electrons, or -thermions as they 

 are termed. 



Dr. Fleming found that a tungsten filament 

 was of special utility for this purpose. The 

 thermionic receiver has great advantages in that 

 it is not injured or put out of adjustment, like 

 crystal detectors, by powerful electric oscillations 

 or atmospheric discharges acting on Its receiving 

 circuits. 



In some of Dr. Fleming's experiments he em- 

 ployed an incandescent lamp with two plates 

 sealed into the bulb carried on separate terminals. 

 An illustration of such a double-anode or two- 

 element valve was given by him in a paper pub- 

 lished in the Proceedings of the Royal Society 

 early in 1905 (see c, Fig, 1). The new thermionic 

 detector naturally attracted the attention of radio- 

 telegraphists, and amongst others of Dr, Lee de 

 Forest in the United States. After adopting the 

 detector in substantially the same form, Dr. 

 de Forest patented in 1907 a modification In 

 which the two metal electrodes were sealed into 

 the vacuous bulb, in addition to the metallic or 



