1897.] 



on Contact Electricity of Metals. 



525 



little mound of water, keeping it parallel to the copper disc so that 

 there is no metallic contact between them (Fig. 4) ; you see that the 



Fig. 4. 



spot of light moves to the left and settles at a point marked E (which 

 I call the electrolytic zero of our circumstances), a few scale divisions 

 to the left of the metallic zero. This motion and settlement is the 

 simplest modern exhibition of Volta's greatest discovery. 



(3) Now that the spot of light has settled, I lift the zinc disc 

 a millimetre till the water column is broken, and then two or three 

 centimetres farther (Fig. 5); the spot of light does not move, it 



remains at E. I lower the zinc disc again : still no motion of the 

 spot of light, not even when the zinc again touches the little mound 

 of water. 



(4) Now I tilt the zinc disc slightly till it makes a dry metallic 

 contact with the copper, as shown in Fig. 6 ; while the water arc still 



Fig. 6. 



