276 Sir W. Thomson on the Time-integral of a 



seriously interfere with that from the front, but also no doubt 

 to reflexions from points situated in the substance of the glass. 

 A sheet of glass two centimetres thick gives fairly good 

 reflexion, while a sheet of paraffin of that thickness would 

 give almost no effect. 



Thus by using glass it was comparatively easy to obtain 

 stationary waves by reflexion from a small surface of a non- 

 conducting substance, in order to compare the effect thus 

 produced on the position of the stationary waves with that 

 produced by employing a small metallic reflector. The first 

 node was found to be shifted out nearly JX, as in the case of 

 metallic reflexion. That reflexion from glass is not of the 

 metallic sort was proved by obtaining polarized reflexion. 

 In this case the two opposite edges of the nonconductor may 

 be looked upon as undergoing variations of apparent electrifi- 

 cation. 



XXIX. On the Time-integral of a Transient Electromag- 

 netically Induced Current. By Sir William Thomson, 

 F.R.S* 



IT has hitherto been generally supposed that, in ordinary 

 apparatus for electromagnetic induction, with or without 

 soft iron, the oppositely directed transient currents, in the 

 secondary circuit, induced by startings and stoppings of 

 current in the primary circuit, have equal time-integrals. 



I have recently perceived [been wrongly led to imagine] 

 that this may be far from being practically the case by 

 the following considerations. The starting and stopping 

 of the current in the primary circuit was, in Faraday's 

 original discovery of this kind of electromagnetic-induction 

 (Exp. Res. Series I. Nov. 1831), and is generally in ele- 

 mentary illustrative experiments, produced by making and 

 breaking a circuit consisting of a voltaic element or battery 

 and the inductor-wire. In this arrangement the starting 

 of the inductor-current is generally much less sudden than 

 the stopping. Hence a thicker shell of the secondary wire 

 (or portion inwards from the outward boundary) is utilized 

 for conducting the secondary current, on the make, than 

 on the break f. Hence the effective ohmic resistance in 



* Communicated by the Author. 



t [In reality the whole cross sectional area of the secondary conductor 

 is utilized, equably in all its parts, in conducting the secondary current. 

 See Postscript of February 23.] 



