June 24, 1920] 



NATURE 



5^* 



current to be flowing in L and C, and that we 

 choose an instant when the grid is, say, at a posi- 

 tive potential relative to the filament and rising in 

 value ; in accordance with the properties of these 

 tubes the electromotive force transferred from the 

 grid to the anode circuit of the tube will be from 

 filament to anode outside the tube and rising in 

 value. Thus the electromotive force acting on the 

 portion of the coil in the anode circuit is in phase 

 with the potential difference postulated to exist 

 throughout the coil in virtue of the oscillating 

 current in it, and therefore the electrical motion 

 tends to be maintained. 



In such an oscillator the frequency of the oscil- 

 lations is mainly determined by the inductance 

 and capacity, but every other circumstance of the 

 circuits has its influence. Moreover, the ampli- 

 tude of the oscillations often varies when the 

 frequency does. Thus if acoustic variations be 

 imposed upon the current employed for heating 

 the filament, or upon the electromotive force in 

 the anode circuit, or upon a source of electro- 

 motive force between grid and filament, the high- 

 frequency output of the assemblage varies corre- 

 spondingly. An ordinary microphone will, it 

 need scarcely be said, be used for converting 

 the voice-made air vibrations into current varia- 



M'l 



KlG. 5. 



tions, and transformers will be used when of 

 advantage for introducing the current variations 

 into the various circuits mentioned above. 



One of the most interesting of these methods 

 was described by Major Prince in a paper 

 read recently before the Wireless Section of the 

 Institution of Electrical Engineers, the apparatus 

 being that finally adopted by the R.A.F. for aero- 

 plane work. 



The principles employed may be seen from 

 Fig. 6, which is drawn as free from detail as 

 possible. The antenna A is maintained in oscilla- 

 tion by the transmitting triode Rj in a manner 

 similar to that of Fig. 5 ; the modulator tube Ro 

 magnifies the voltages applied to its grid by the 

 microphone currents in the primary of the trans- 

 former T. The high voltage for the anode circuits 

 of each tube is supplied by the battery E, which is 

 in series with a large choking coil x- When the 

 microphone is quiescent the full voltage of E acts 

 steadily on the anode of Rj, but when the micro- 

 phone produces variable electromotive forces on 

 the grid of Rg these are multiplied, transferred 

 to the anode circuit of Rg, and if the choking coil 

 is large enough to be eff'ective, and the condenser 

 C not too large, they reach the anode circuit of 

 R]. In consequence the amplitude of the oscilla- 

 NO. 2643, VOL. 105] 



tions generated by Rj is varied in correspondence 

 with the microphone current variations, and to a 

 small extent the frequency is changed also. This 

 set of apparatus is styled a 20-watt set. 



When the normal range of transmission must 

 attain 100 miles, the problem of modulating the 

 necessary power becomes formidable, chiefly on 

 account of the limitations aftecting the micro- 

 phone. It is obvious that direct modulation by 

 means of ordinary microphones is impossible 

 except for small currents, say not exceeding 

 2 amperes ; in consequence, in the endeavour to 

 achieve long-range telephony, special microphones 

 — some of them employing liquid conductors — 

 have been devised, and sometimes many micro- 

 phones have been used in parallel. At this stage 

 the three-electrode vacuum valve comes to our 

 assistance in various ways, some of which 

 must now be explained. In the first place the 

 triode may be employed as an amplifier of the 

 variable currents or electromotive forces leaving 

 the microphone, and these may be applied to the 



modulation of larger high-frequency currents than 

 was before possible; the tube is then the actual 

 modulator. An instance of this appears in Fig. 6. 

 In the second place the triode may be utilised as 

 a by-pass to deflect high-frequency current from 

 the antenna to itself or to other apparatus capable 

 of dissipating the necessary amount of energy — a 

 subtraction method that has proved very success- 

 ful. The tube may serve in either or both of 

 these functions as modulator of the high-frequency 

 current from arcs, alternators, or other sources 

 of oscillations, not omitting the triode oscillator 

 itself. Evidently the fact that these three-elec- 

 trode valves can perform the distinct offices of 

 generator of oscillations, magnifier of high and 

 low frequencies, and modulator, and in each 

 office may be associated with a remarkable variety 

 of circuits, may be expected to lead, and is lead- 

 ing, to endless permutations and combinations in 

 the circuits proposed for medium- and long-range 

 wireless telephony. 



There is space for brief descriptions of only 

 two examples of large power plants. In 1914 



