On the Differential Incluctometer. 131 



leaves approached one another. The leaves again separated 

 when the shellac was taken away. According to Faraday, 

 the attraction is due to the increased induction where the 

 shellac is, causing A to induce more negative electricity on 

 B. The side therefore of B (and its gold leaf) furthest 

 from A becomes positive. On the other outer plate C there 

 is less negative electricity induced; and the outer side of this 

 plate, together with the gold leaf connected with it, becomes 

 therefore negative. Hence the gold leaves attract one 

 another. 



In my experiments I employed three zinc plates of 30 millim. 

 diameter, and made use of, on some occasions, a glass jar 

 (described above) having the two insulated gold leaves. Some- 

 times, however, I used two electroscopes, one of which was 

 connected with the one and the other with the other outer 

 plate. Under these conditions the kind of electricity which 

 was to be found on the plate was easily determined. 



I should like to be allowed to mention a few details which 

 I think are of interest in electrostatic investigations. The 

 insulation of the wire to which the gold leaves were hung was 

 effected simply by passing the wire through a tube of ebonite 

 having an external diameter of 5 millim. and 1 millim. opening. 



In order to avoid binding-screws, the numerous edges of 

 which are troublesome in electrostatic experiments, connexions 

 were made by boring conical holes of diameters from J 

 tapering to -J millim., and simply thrusting the wires into the 

 holes. No electricity must be allowed to settle on the glass 

 of the electroscope. I therefore employed an ordinary cy- 

 lindrical glass 15 centim. high and 10 centim. wide, but led 

 round the glass, inside and outside, a continuous strip of tinfoil 

 3 centim. wide, parallel to the gold leaves and connected 

 with the earth. By this means the electricity of the leaves 

 induced electricity of the opposite kind in the tinfoil; and if 

 this charge became too strong, the leaves touched the tinfoil 

 and discharged themselves. The glass was provided with a 

 wooden cover, through which passed the little ebonite tube 

 enclosing the wire. Each zinc plate was fastened to a hori- 

 zontal Z made of shellac. The three Z's overlapped one 

 another, so that the plates could be brought to within 1*5 

 centim. of one another. 



Division of Induction. 



The three plates of the differential inductometer are to be 



looked on as two condensers which have one plate, namely 



the collecting-plate, in common. From this plate, as " induc- 



tric," the induction begins, and ends on the two side plates as 



K2 



