82 



KNOWLEDGE. 



March, 1913. 



of 



a copper 



wire bent nearly into a circle. 



It has been said already that the discharge of a 

 Leyden jar is oscillatory. Lodge devised a method 

 of obtaining a persistent oscillatory discharge, and of 

 tuning a receiver for it. The transmitter consists of 



Figure 74. 

 Transmitter at the Lyngby Station. 



a Leyden jar A. (Figure 71) with a bent wire nearly 

 connecting the outer and inner coatings, which 

 are themselves connected to the terminals of an 

 induction coil. When the coil is working, a per- 

 sistent series of oscillatory sparks crosses the air-gap 

 G. The receiver is another Leyden jar B, also with 

 a bent wire, with an air-gap H, the planes of the 

 two bent wires being parallel to each other and at 

 right angles to the line joining the centres of the jars. 

 The sliding piece C can be adjusted so that when 

 A is at work sparks pass across the gap H. A very 

 slight displacement of C causes the sparks to cease. 



Figure 75. 

 Receiver at the Lyngby Station. 



Now, although such a transmitter and receiver 

 can be made very perfectly /'/) tune with each other, 

 such a receiver is not sufficiently sensitive for long- 

 distance work. Electric waves produce various 



other effects which can be employed to detect their 

 presence. Branly discovered that the resistance 

 between loose metallic contacts is diminished when 

 electric waves fall on them, and the detectors made 

 on this principle are known as Coherers. Rutherford 

 found that a bundle of iron wires magnetized to 

 saturation was demagnetized by the alternating cur- 

 rents caused by electric waves. Marconi's magnetic 

 detector employs this discovery in the form of an 

 endless flexible iron wire, made to move round and 

 round by clockwork under permanent magnet poles ; 

 the changes in the magnetization caused by the 

 electric waves induce currents in a telephone receiver. 

 Such detectors can be tuned to show maxima effects 

 for waves of particular lengths, but the effects do 

 not cease completely for other wave lengths. 



It has been said that the electric waves generated 

 by the discharge of a Leyden jar may be compared 

 to the sound waves produced by a pistol-shot. Per- 

 haps it would be fairer to compare them to the 

 waves produced by a drum-tap. There is at least a 

 very rapid damping of the waves, i.e., a rapid falling 

 off in their intensity, although the wave length, /' e., 

 the distance between successive crests, may remain 

 the same. (See Figure 75.) The waves of a sound of 

 a drum would produce in the groove of a gramo- 

 phone record hills and valleys similar to those shown 

 in Figure 75. The record of a tuning fork, however, 

 would be more like Figure 72. So would the record 

 of an organ pipe, so long as a stream of air is being 

 driven across its embouchure. Analogies are some- 

 times misleading ; but in seeking analogies between 

 different kinds of waves we are not likely to go far 

 wrong if we bear in mind the main characteristics of 

 all wave motion and the main specific peculiarities 

 of each kind. 



In all wave motion we find the following charac- 

 teristics, among others :— 



(i) The disturbance takes time to travel from one 

 place to another. Electric waves travel 

 one hundred and eighty-six thousand miles 

 per second, waves of sound in air about 

 one thousand one hundred feet per second. 



(ii) A medium to transmit the disturbance is 

 necessary. The medium for electric waves 

 is generally spoken of as the aether ; sound 

 usually travels to our ears in air. 



(iii) When waves follow each other at regular 

 intervals and fall on some system capable of 

 being disturbed by them, then if that system 

 has a natural period of vibration correspond- 

 ing to the period of the waves, it will be 

 caused to vibrate in sympathy with them. 

 This phenomenon is known as resonance, 

 and numerous familiar instances of it might 

 be recalled. 



(iv) The velocity of the waves, the wave length, 

 and the frequency are connected by the 

 equation V=NL. For instance, let us take 

 a tuning fork giving two hundred andseventy- 

 five vibrations per second ; the waves travel 



