Mr. F. S. Spiers on Contact Electricity. 11 



proved too fierce for the zinc plate, for it volatilized, covering 

 nearly the whole of the inside of the apparatus with a 

 fine layer of zinc dust. The tube therefore had to be dis- 

 mantled and opened, thoroughly cleaned, and a fresh plate 

 of zinc put in place of the old one. I thought it best this 

 time not to use the blowpipe at all, but merely to heat the 

 apparatus with a couple of bunsen-burners, continuing heating 

 and pumping until no more air should come away through 

 the pump. But here again, after about five hours'' heating, 

 the zinc again melted and the experiment proved useless. 

 I then decided to dispense with the zinc plate altogether and 

 to replace it by a plate of aluminium, whose melting-point 

 is 600° C, while that of zinc is 412° 0. 



In the course of the next series of experiments I unluckily 

 broke both the upper and lower parts of the tube just at the 

 middle joint C, (J in exercising a little too much vigour in 

 separating the two parts from one another after the usual 

 persuasive means of loosening obdurate ground-glass joints 

 had proved unsuccessful. As the apparatus had to be entirely 

 remade I introduced some radical changes in its construction 

 in order to remedy the inherent defects of the original design. 



§ 9. Fig. 3 represents the new apparatus as finally con- 

 structed. I here adopted the original Volta method of 

 procedure, with the application, of course, of Lord Kelvin's 

 compensation device. In this particular form of apparatus, 

 where great capacity can only be attained by diminishing the 

 distance between the condenser-plates, it is susceptible of 

 greater sensibility than the Ayrton and Perry modification, 

 and it is, moreover, somewhat simpler. 



In the fig. A, A is the insulated platinum plate supported 

 and insulated by the glass tube B. To insure rigidity, the 

 plate was fairly stout, too thick to be fused into the glass 

 support, so that it had welded to the top a stout piece of 

 platinum wu're which fitted tight into the glass tube B. C, C 

 forms the second plate of the contact couple (at first aluminium 

 and afterwards iron). It is suspended by two very fine 

 platinum wires D and E (front view of plates) that are sealed 

 into the upper part of the apparatus F, so that when C hangs 

 freely, it does so parallel and very close to A, A. The plates 

 are prevented from coming into contact by the three stops 

 G, H, and K. The stop Gr screws into the aluminium tube 

 L; H and K into the small cross-piece that is shown 

 in the figure (side view of plates). The tube L fits tightly 

 over the glass rod M that is fused into the head ot the tube. 

 A platinum wire which runs down M serves to earth the tube 

 L. The latter tube also serves another purpose, namely, to 

 carry on the small aluminium supports Y and Q an electro- 



