234 Electrification of Dielectrics by Mechanical Means, 



had not previously been electrically treated. The deflexion 

 of the electrometer was about 5 cms., but on joining P x to P 2 

 this disappeared. A 2-lb. weight was then dropped on P x 

 from a height of three inches. The spot of the electrometer 



moved very quickly off the scale to the right, but immediately 

 returned and settled down to a fairly steady deflexion to the 

 left. The electrometer thus appeared to receive two impulses, 

 of opposite sign, one quickly following the other. It appeared 

 probable that the first impulse might be due to the compression 

 of the sheet, and the second to the extension of the dielectric 

 when it recovers its original thickness. To test this, the sheet 

 was stretched transversely aud the effect on the electrometer 

 noted. A fresh sheet of rolled Para rubber was taken, 

 •022 inch in thickness, and placed on the plate P 2 . The top 

 plate P x was then gently placed on the sheet of dielectric. 

 The electrometer showed a deflexion of 5*0 cms. to the right, 

 and the deflexion was slowly increasing. This was due either to 

 the compression produced by the top plate, or to the mechanical 

 stress to which the dielectric had been subjected during manu- 

 facture. The two edges of the rubber sheet were then grasped 

 gently in the hands, and the sheet stretched until its length 

 increased about 30 per cent. This caused the electrometer- 

 spot to fly off the scale to the right, showing a difference of 

 potential of at least 7 volts established between the plates, the 

 top plate being negative. The condenser and electrometer 

 were then discharged for an instant, the sheet being then 

 released and allowed to return to its original dimensions. 

 This caused the electrometer-spot to fly oft the scale to the 



