48 STATIC ELECTRICITY 



cannot be charged absolutely," i.e. with one kind of electrification 

 alone in existence, " but only relatively " to other bodies some- 

 where in their presence which take up the opposite charge " and 

 by a principle which is the same with that of induction. All 

 charge is sustained by induction.*" 



We may arrange our experimental knowledge of induction 

 more clearly by the aid of a hypothesis, originally due to Faraday 

 and subsequently developed and extended by Maxwell. UV shall 

 here give a gene/al account of this hypothesis, filling in the 

 details as they become necessary for the explanation of the various 

 points in the theory of electric action. 



Electric action in the medium. Let us suppose that a 

 charged insulated body is placed within a hollow conductor. We. 

 may think, for instance, of a charged brass sphere hanging bv a -ilk 

 thread in the middle of a room. Then there will be an opposite 

 charge on the walls. In describing the two opposing surfaces as 

 electrified the most important experimental fact which we connote 

 is that the two surfaces are being pulled each towards the other. 

 This action was formerly supposed to be direct, each little bit of 

 wall surface, for instance, pulling at each bit of brass surface at a 

 distance, .the intervening matter playing no part in the action and 

 being unaffected by the existence of the two electrified M 

 between which it lay. But Faraday succeeded in showing that 

 the action varies with the nature of the intcrxciiiiig iiiMilator. 

 and we may describe the variation by saving that the pull on the 

 electrified surfaces with given charge- i : with 



insulators than with others. This fact is entirely unexplained, 

 unless we adopt the view taken by Faraday that the insulator 

 plays an essential part in the action. He supposed that it i- 

 altered in some way during the process of electrification, so that it 

 exerts pulls on the conducting surfaces with which it i- in contact. 

 In fact, he regarded electrical actions as similar to the drawing 

 towards each other of two masse- connected by a stretched india- 

 rubber cord. In the case of a cord the pulls on the mussel depend 

 on the nature, quality, and strain of the cord, and can be expressed 

 in terms of the strain. The energy which appears in the motion 

 of the masses is regarded as having been previously stored up as 

 strain energy in the cord during its stretching. So the electrical 

 pulls are supposed to be accompanied by a condition in the in- 

 sulating medium analogous to strain and to be expressible in terms 

 of this condition, which we may call electrical strain. Th< 

 trical energy is supposed to be stored in the medium during the 

 electrification and to pass out of it again on the motion of the 

 charged bodies towards each other or on the neutralisation of their 

 charges. The electrification of a conducting surface may therefore 

 be regarded as a surface manifestation of an alteration in the state 

 of the contiguous insulator, somewhat as the pressure on th< 

 of a hydraulic press may be regarded as a surface manifestation of 



