THE INDUCTION OR ELECTRIC STRAIN 49 



the compressed state of the water which connects it with the plunger. 

 When Faraday first gave this account of electrical action it was 

 on I v supported hv the variation of the effect of a given charge 

 with variation of the medium, and no independent evidence of the 

 state of k< electrical strain" was in existence. But it has been dis- 

 covered that some insulating media when between charged surfaces 

 are affected in regard to the transmission of light. Unless we 

 make the further and very far-fetched hypothesis that the light 

 itself is acted on at a distance by the charged surfaces, Faraday ""s 

 supposition becomes for these media a proved fact. We have also 

 gooa evidence of electrical strain in the electro-magnetic waves dis- 

 covered by Hertz and used now in wireless telegraphy. We know 

 only one way of explaining the phenomena discovered by Hertz, 

 vi/. hv the supposition that the disturbances are waves travelling 

 through the air with a definite velocity. This implies the existence 

 of both electric and magnetic energy in the air. The electric energy 

 implies the existence of that which we have called electric strain. 

 It i> aKo called electric polarisation, electric displacement 

 M -\well) and electric induction (Faraday). 



No strain within conductors when the charges are at 

 rest. When the charges and the charged bodies in a system are 

 iiis condition of electric strain is confined to the air or 

 other insulator bet \\ccn the charged surfaces and does not occur 

 within the substance of the conductors. This is evident if we 

 the experiments already described with closed conductors. 

 These show that a hollow closed conductor may be regarded as 

 cntirelv MT. cuini; the -pace within from the space without. No 

 electrification is induced on bodies within by external charges ; no 

 mutual null is exerted on an electrified surface inside by an 

 electrified surface outride. Interpreting these facts by Faraday's 

 hypothcsk we must Mippose that the substance of the conductor 

 is not its, If in ;l state of electric strain. If it can acquire that 

 iiti >n it rapidly loses if, he in or incapable of permanently 

 n<j electric energy. The electric strain which exists in the 

 insulator and which is r manifested by a pull on the charged surface 

 of a conductor ceases at that surface. The pull outwards on the 

 conductor, of course, trains it, and corresponding to this strain 

 thei ress between the external layer and the layer beneath 



it which neutralises the outward electrical pull. But the strain in 

 the conductor is an ordinary elastic strain, not an " electric strain," 

 a change of shape of a visible kind which can be calculated 

 from the elastic properties of the conductor, though it is too small 

 to IKJ seen and there are no recognised electric phenomena con- 

 nected with it. The nature of the electric strain in the insulating 

 medium is at present unknown, though we may make guesses as to 

 its nature, and \\ e omv term it strain "* from analogy, since there 

 M a<-. nmpaiiN mir it somewhat resembling those accompany - 

 inL r rain. Taking this \je\v of the nature and function of 



