Non-conducting Bodies in Electric Induction. 7 



the flame plays the inducteous part as before, and by its well- 

 known powers of convection transfers its state of charge to that 

 surface of the shell-lac (formerly the anterior) now opposed to 

 it; whereas on your view it ought to be simply discharged. 

 The shell-lac plate was therefore placed before the charged body 

 V, and the flame moved about before its posterior surface ; then 

 the plate was taken out of the inductive position and the flame 

 moved before its anterior surface ; after that it was examined by 

 the electrometer. When the anterior face was on the cap plate 

 of the instrumentj the latter indicated a positive charge ; when 

 the posterior face was in contact with the cap, the instrument 

 indicated a negative charge, being the same kind of electric 

 charge for that face as before, but much weaker. The apparent 

 weakness ought to occur, for now the negative charge of the 

 posterior face is exercised inductively through the shell- lac 

 towards the positive chai'ge of the anterior face, and vice versa ; 

 and this was proved to be the case by bringing the hand or an un- 

 insulated metallic plate towards the upper anterior surface, whilst 

 the posterior surface was in contact with the electrometer cap ; 

 for the negative divergence of the gold leaves then increased 

 very greatly, the negative electricity being set free to a large 

 extent from the induction of the positive anterior surface. And 

 when the positive antei-ior sui'face was in contact with the cap of 

 the electrometer, its highly charged condition could be exhibited 

 in like manner. So the flame, carefully approached, can only 

 discharge the side which has received a charge, and that only if 

 approached on that side ; if brought opposite the other side, it 

 conveys to it the opposite electricity and leaves the plate doubly 

 charged. 



These experiments are by no means difficult or delicate, and 

 are easily made in the most convincing and varied manner (a 

 few simple precautions being taken), but always with the same 

 results. P or the inductric body is best if of large surface. An 

 excited glass rod, or, better still, an excited plate of gutta-percha 

 (a shoe sole), are very good for the purpose; either may be brought 

 to within an inch of the shell-lac or sulphur S, and still com- 

 municate no charge to it if discharging conductors be not near. 

 A fine metal point may be used at N instead of the flame ; or 

 even conductors and contact be employed, as in the following 

 manner. A sheet of gold-leaf was laid on the cap plate of the 

 electrometer ; P was put into place, and also tlie shell-lac or 

 sulphur plate S, and the flame applied for a moment at N ; then 

 the plate S was removed and placed with its negatively charged 

 posterior surface in contact with the gold-leaf on the electro- 

 meter ; immediately the latter showed a strong divergence (often 

 more than the instrument, though very large, could bear) ; but 



