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Non-conducting Bodies in Electric Induction. 5 



whether it is the anterior or posterior surface which is negatively- 

 charged, may be solved, though not by the indifferent approach 

 of either side of the plate to a gold-leaf electrometer ; for with 

 either side, indications of negative electricity will be obtained; 

 and if the excited surface be in both cases at equal distances from 

 the electrometer plate, the action will be greater (because of 

 specific inductive capacity) when the body of the shell-lac plate 

 intervenes between the cap and the excited surface than when 

 air only is interposed. 



To make these points clear, once for all I will describe the 

 plates I have used ; and for easy reference to position, will give 

 a diagram of the forms of experiment. One plate w as of shell- 

 lac, 4^ inches square 

 and 0-9 of an inch ^ 



thick; the other was ^ ^ 



of sulphur, 5 inches j| ^ 

 square and0"8 of an © 

 inch thick. A strong 

 white silk thread was made fast round the edges of each plate, 

 and then a long loop of the like silk being fastened at the two 

 corners of one side-edge, and a like loop at the two corners of 

 the opposite side-edge, the two loops served as handles by which 

 the plates could be insulated, and yet carried about or applied 

 in any position to the electrometer. In the figure, S is one of 

 these plates supposed to be seen edgeways ; P represents the in- 

 ductric or originally charged body, and N (whether flame, point, 

 hand, or ball) the inducteous body ; between these two, P and N, 

 that induction takes place to which the plate S is, as far as 

 regards the results of the experiment, subjected, and the effects 

 of which are to be examined. The results were precisely alike 

 with both sulphur and shell-lac plates. If P was made negative, 

 they were also the same, but with inverted signs. I will describe 

 those obtained with the shell-lac, and will always call that face 

 towards P the anterior, and the face towards N the posterior 

 faces of the plates. 



Making P the end of the positive conductor of an electrical 

 machine, and N an uninsulated metal ball or plate, then the 

 shell-lac plate was put into its position, retained there for thirty 

 seconds or more, was removed, examined by a gold-leaf electro- 

 meter, and found perfectly free from charge on either face or any 

 part. The shell-lac plate was then restored to its position in the 

 induction, and N made a spirit-lamp flame applied in the manner 

 you describe. The shcU-lac being taken away, was examined by 

 laying the plate without friction on the cap plate of the electro- 

 meter. The shell-lac was found to give strong negative charge 

 to the leaves, whichever face was on the cap ; but the signs were 



