Non-conducting Bodies in Electric Induction. 3 



thoughts. In it the metallic piece (ball, hemisphere or plate) 

 referred to is uninsulated, not insulated (1218-1230.). It is also 

 the conductor upon which the induction terminates, and not a 

 conductor interposed in the course of the induction ; cases so 

 different, that much of the reasoning which belongs to one has 

 no relation to the other. The latter case is not specifically 

 referred to in the Experimental Researches, because I thought 

 it thoroughly well known, but it is given in my letter to Dr. Hare, 

 vol. ii. of collected papers, p. 263. 



Perhaps the following mode of putting the matter will make 

 my views en this point clear 



to you. Let P be an insu- ^~n y\ ^->. 



lated charged body, inducing \^) ""•■,, J/' \^J 

 upon N an uninsulated me- '"" 



tallic body, np being at first away. Then let np be introduced, 

 being a non-conductor equal to shell-lac or sulphur, but of the 

 same specific inductive capacity as air; no change of the dispo- 

 sition of the forces will take place, for the particles of np will be 

 polarized just as the particles of the air displaced by it were. 

 Then consider np to be endued with conducting power, as if it 

 were converted into a metal ; its particles will now discharge to 

 each other; the parts at n and jo will be more negative and posi- 

 tive than they were before, because the sum of induction distance 

 between P and N is shortened by the diameter of np, and so the 

 induction is stronger ; and instead of the lines of force from P 

 to N passing round np (as your paper makes me to say), mofe 

 will fall upon and pass through the space np now that it is a 

 conductor than before when it was an insulator (1326. 1337. 

 1338.). I am sure I need make no further reference to these 

 points, for I am satisfied that when you look at the paragraphs 

 1218. to 1230, and perhaps also to vol. ii. Exp. Res. p. 279-284, 

 you will at once see what my meaning was, and what my views 

 are. The results according to them are precisely such as you 

 describe at pp. 406, 407, Phil. Mag. 



In your paper (Phil. Mag. p. 410), you describe an experi- 

 ment which I know well, and consider as one of the strong 

 proofs of the truth of my views. A plate of shell-lac is placed 

 with its anterior face 12 inches from the positive knob of an 

 electrical machine, and its posterior face 1 inch from the flame 

 of a spirit-lamp, and then moved about before it ; when taken 

 away and examined, the anterior face is found by you charged 

 negative, and hence you conclude, that, prior to the discharge of 

 the posterior face by the flame, the induction had rendered the 

 anterior face of the shell-lac negative, and the posterior face posi- 

 tive, just as would have happened with a metallic plate, and as 

 far as I understand your paper, by a like act of conduction 



B2 



