1 62 POPULAR LECTURES AND ADDRESSES. 



electricity. I do not feel it, because the force is 

 very small, but you may do the thing in a measured 

 way ; you may place a little metallic knob or 

 projection of i/ioo,oooth of a centimetre, on one 

 of them, and lean the other against it. Let there 

 be three such little metal feet put on the copper ; 

 now touch the zinc plate with one of them, 

 and turn it gradually down till it comes to touch 

 the other two. In this position, with an air-space 

 of i/ioo,oooth of a centimetre between them, 

 there will be positive and negative electricity on 

 the zinc and copper surfaces respectively, of such 

 quantities as to cause a mutual attraction amount- 

 ing to 2 grammes weight per square centimetre. 

 The amount of work done by the electric attraction 

 upon the plates while they are being allowed to 

 approach one another with metallic connection 

 between them at the corner first touched, till 

 they come to the distance of i/ioo,oooth of a 

 centimetre, is 2/ioo,oooths of a centimetre-gramme, 

 supposing the area of each plate to be one square 

 centimetre. 



Let me read you this statement taken from an 



