266 Prof. Towosend and Mr. Morrell on Electric, 



is obtained when the current excites oscillations in a second 

 fork, if its frequency is an exact multiple of that of the first. 

 This principle has been used by Rayleigh * to determine the 

 pitch of a fork of high frequency from a fork of low 

 frequency. 



3. The oscillatory system consisting of two parallel wires, 

 as used by Lecher in his well-known experiments, is of 

 particular interest. If the wires be very long compared with 

 the distance between them, the frequencies of the free oscilla- 

 tions are almost exactly in simple proportions, as I he effects 

 of the ends and of neighbouring conductors are small in these 

 modes of oscillation where the currents are in opposite direc- 

 tions at opposite points of the two win s. 



A general theory of the propagation of waves along parallel 

 wires has been given by Rayleigh f, and a simple solution 

 of Maxwell's equations for the forces in the space around 

 the wires is obtained. The investigation indicates that the 

 waves are propagated along the wires with the velocity of light 

 in air, a result which is independent of the diameter of the 

 wires or of the distance between them, provided that the 

 resistance of the wires is negligible. 



When currents are induced in the wires by waves of a 

 given length \ emitted by a short-wave generator, resonance 

 is obtained when the length of the wires is adjusted so that 

 the frequency of one of the normal modes of oscillation is 

 equal to that of the generator. If wires of a fixed length 

 form part of an oscillatory system, this adjustment is made 

 by means of a bridge between two opposite joints on the 

 wires. If l x l 2 / 3 , be the lengths of the wires when the first,, 

 second, and third modes of oscillation are in resonance with 

 the short-wave generator, the wave-length X is 2(l 2 — li) or 

 2(? x — Z 2 ) if the effect of the ends is the same in all cases. 



A number of experiments have been made in order to find 

 to what degree of accuracy the lengths of short waves may 

 be obtained by parallel wires, as this method is much simpler 

 * than that depending on measurements of the capacity and 

 self-induction of a closed oscillatory circuit. The short waves 

 measured by means of the parallel wires may be tuned to 

 coincide with the harmonics of a long-wave generator, and 

 the wave-length of the fundamental oscillation may thus be 

 determined in terms of the lengths of short waves. 



The principle of comparing wave-lengths by harmonics is 

 used in the French multivibrateur, which is adapted for 



* Lord Rayleigh, 'Nature,' xvii. p. 12 (1877). 



t Lord R yleiyh. Phil. Mag. xliv. pp. 129-204 (1897). 



