July 7, 1923] 



NA TURE 



a very minute variation of the condenser of either 

 oscillator makes the deflexion increase enormously. 



There are several ways of applying this novel pheno- 

 menon to wireless telegraphy. Two of these may be 

 illustrated here. Suppose one of the two oscillators to 

 be a distant transmitter from which electric waves are 

 proceeding, and that these waves are picked up by the 

 antenna at a receiving station. Let the antenna be 

 coupled to a local oscillator in the relationship of 

 master, and let a tuned detector circuit be acted upon 

 by both the antenna and the local oscillator. Then 

 suppose the local oscillator adjusted until it is in the 

 accordant state with the antenna oscillations, and, in 

 fact, adjusted until the detector current is at the mini- 

 mum value corresponding to the cusp of Vincent's 

 curve (Fig. 6). It then follows that a very minute 

 variation of the frequency of the oscillations emitted 

 by the distant station will give rise to a deflexion of 

 the galvanometer. It is suggested that signals could 

 be transmitted by up and down changes in frequency — 

 such changes would be far smaller than the changes of 

 frequency employed by the accepted methods of the 

 present day, and thus the interference between stations 



§ — FREQUENCY OF VARIABLE OSCILLATOR * 



Fig. 6.— Vincent's curve. 



would be minimised. There are many easy ways of 

 producing small changes of frequency at the trans- 

 mitting station. 



Another and very different method of signalling may 

 be illustrated by this same apparatus, after again 

 adjusting the receiving apparatus to the minimum 

 deflexion obtained in the accordant state. On trial it 

 is found possible to bring the spot of light to any 

 desired point of the scale — that is, to any desired point 

 on the vertical portion of the Vincent curve — by appro- 

 priate adjustments of the frequency of the transmitting 

 unit. These latter adjustments are for this purpose 

 conveniently effected by the motion of a short cir- 

 cuited coil of wire near the inductance coil of the 

 transmitting oscillator. Therefore, to every position 

 of the auxiliary movable coil at the transmitter there 

 corresponds a position of the spot of light actuated by 

 the receiving apparatus. It might even be possible to 

 mark the scales at each place with an alphabet and so 

 communicate intelligence without the aid of the Morse 

 code. 



The above-described methods of signalling are based 

 on the discovery of accordance between triode oscil- 

 lators. Another distinct series of methods can be 

 suggested and illustrated. These methods depend 

 on the fact that the combination of two high- 

 frequency electrical vibrations of slightly differing fre- 



NO. 2801, VOL. I 12] 



quencies yields a throbbing amplitude which may be 

 made of audible frequency and of any desired pitch 

 by adjusting the frequency of either of the original 

 vibrations. The formation of relatively slow throbbings 

 from two quicker oscillations is shown diagrammatically 

 in Fig. 7. The existing modern method of receiving 

 continuous waves known as the heterodyne method 

 utilises this principle in the following way : The trans- 

 mitting station emits long and short trains of waves 



B 



^=0-1 



mhimNwm 



71^ 200 



Fig. 7. — Illustrating the heterodyne method of reception. 



^/ = ^2 



71= n-TL^ 



50 



corresponding to Morse dashes and dots and of fre- 

 quency, say 200,000 per second. These waves produce 

 in the receiving antenna feeble oscillations which are 

 combined with locally generated oscillations of about 

 the same strength and of frequency, say, 200,500 per 

 second. The result is a compound high-frequency 

 current with 500 throbbings in it per second. ' These 

 when rectified can be heard in a suitably connected 

 telephone. The long and short trains of waves from 



-frequency- 



Fig. 8. — Diagrammatic representation of sounds heard in 

 heterodyne reception. 



the transmitting station thus give rise to sounds of long 

 and short duration and of constant pitch. The pitch 

 is adjustable by altering the local frequency from 

 200,500 to other values. 



By altering this frequency from, say, 199,300 per 

 second to 200,000 and then to 200,700 the sounds in the 

 telephone run through a continuous scale of notes as 

 represented in Fig. 8, This starts on the left with a 

 note of 700 which falls in pitch to about 40 and becomes 

 inaudible, passes through resonance, becomes audible 

 again, and ascends a scale in opposite order to the first 



A 2 



