24 ELEMENTARY LESSONS ON [CHAP. I. 



in it is liable to oxidise and to attract moisture. To use 

 the electrophorus the resinous cake must be beaten or 

 rubbed with a warm piece of woollen cloth, or, better 

 still, with a cat's skin. The disc or " cover " is then 

 placed upon the cake, touched momentarily with the 

 finger, then removed by taking it up by the glass handle, 

 when it is found to be powerfully charged with positive 

 electricity, so much so indeed as to yield a spark when 

 the knuckle is presented to it. The " cover " may be 

 replaced, touched, and once more removed, and will 

 thus yield any number of sparks, the original charge on 

 the resinous plate meanwhile remaining practically as 

 strong as before. 



f 



Fig. 13. Fig. 14. 



The theory of the electrophorus is very simple, pro- 

 vided the student has clearly grasped the principle of 

 induction explained above. When the resinous cake 

 is first beaten with the cat's skin its surface is negatively 

 electrified, as indicated in Fig. 13. When the metal 

 disc is placed down upon it, it rests really only on three 

 or four points of the surface, and may be regarded as an 

 insulated conductor in the presence of an electrified 

 body. The negative electricity of the disc therefore 

 acts inductively on the metallic disc or " cover," attract- 

 ing positive electricity to its under side, and repelling 

 negative electricity to its upper -surface. This state 

 of things is shown in Fig. 14. If now, the cover be 

 touched for an instant with the finger, the negative 

 electricity of the upper surface (which is upon the upper 



