Messrs. Lodge and Howard on Electric Radiation. 63 



point was found at which the amount of rotation required to 

 make the sparks disappear was a minimum. In this position 

 the centre of the resonator was 101 centim. from the reflect- 

 ing plate. The observations agree with the previously calcu- 

 lated value of the wave-length, viz. 100 centim. 



In the above experiments the oscillator was always placed 

 in the focal line of the first lens, that is, vertically. Some 

 observations were made later, after turning the oscillator 

 through an angle so as to leave its centre in the axial plane, 

 but its direction inclined to this plane. The effects were 

 always of the same nature as those already described, even 

 when both oscillator and receiver had been turned through a 

 right angle, but the intensity of the radiation was not so 

 great beyond the first lens. The focussing of the rays by 

 the second lens could not be observed in this case, even when 

 they were rotated only ten or twenty degrees, as the intensity 

 was too small. 



The above results all go to confirm the identity of elec- 

 trical radiation and light ; and are merely a slight extension 

 of the famous researches of Hertz. 



University College, Liverpool, 

 May 1889. 



Addendum dated June 20. 



An expression for the self-induction of a straight copper 

 rod, of length I and thickness d, we do not see how to calcu- 

 late on Maxwellian principles without some sort of a return 

 circuit somewhere. On action-at-a-distance principles it can 

 be done thus : — 



Consider two parallel filaments or thin straight wires at a 

 distance c apart ; call an element of one, at a distance a from 

 some plane of reference, da, and an element of the other, at a 

 distance b from the same plane, db. The mutual induction or 

 potential of two elements on each other is 



da db cos e 

 r 



where e is the angle, and r the distance, between them. Hence 

 the mutual induction of the two parallel filaments, each of 

 length I, is 



C>C> dadb 



