Mr. F. S. Spiers on Contact Electricity. 79 



shield. In other respects the apparatus is essentially the same 

 as the older one which it replaced. 



There is no need to discuss here the well-known method of 

 making an observation. In the actual apparatus (fig. 3), 0, 

 when not of iron, had a plate of that metal attached to its 

 back, so that by closing the circuit of the small electromagnet 

 W by means of a spring key it was drawn away from the 

 platinum plate A. This reduced the taking of a reading 

 to the exceedingly simple operations of making and then 

 breaking a single platinum contact to the potentiometer, and 

 then pressing a spring key. 



In its new form the apparatus proved entirely satisfactory 

 in every way. The adjustments were not at all troublesome 

 to make, in fact, it was eventually found possible to dispense 

 altogether with the stops L, H, and K (fig. 3), for the plates 

 could be prevented from touching and yet allowed to come 

 within y 1 - in. of one another by just giving the proper tilt to 

 the apparatus as a whole. The sensibility was all that was 

 desired. When that of the quadrant electrometer for directly 

 applied E.M.Fs. was 553 scale-divisions per volt, a P.D. be- 

 tween the condenser-plates of 1 volt gave on the average a 

 deflexion of about 70 on pulling away the movable plate. 

 It was quite possible, by taking great care in making the 

 adjustments, to attain a sensibility of 90 divisions per volt. 

 The readings in general are accurate to 0*02 of a volt, and 

 they are pretty reliable to 001 volt. 



§ 10. The experiments were continued with this improved 

 apparatus, using plates of aluminium and platinum as the 

 dissimilar metals. The P.D. between these metals measured 

 as quickly as possible after cleaning the surface of the alu- 

 minium with fine emery-paper was 



1-45 volts. 



This value slowly fell as time went on* ; thus after about 1-J 

 hours, it was 



1-20 volts; 



and it finally assumed, very slowly, a steady value of about 



1-02 volts. 



After this steady value had been arrived at, the experiment 

 was proceeded with much as described in §8. The tube was 

 exhausted to about jjfo m i~Uim. pressure, and then heat was 

 applied from two bunsen flames. The variations of pressure 



* Evskine-Murray on " Contact Electricity of Metals," Proc. Roy. Soc. 

 vol. Ixiii. p. 124. 



