Contact Electricity of Metals. 87 



between them (fig. 4) ; you see that the 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 



Fiar. 4. 



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

 is the simplest modern exhibition of Volta's greatest dis- 

 covery. 



(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 



Figr. 6. 



metallic contact with the copper, as shown in fig. 6 ; while 



