CHAP, in.] ELECTRICITY AND MAGNETISM. 145 



battery is composed of pure mercury, on which floats a 

 paste of mercurous sulphate, a plate of zinc resting on 

 the paste. Contact with the mercury, which acts as the 

 positive pole, is made with a platinum wire. The 

 E.M.F. is 1*457 volts. 



178. The following table gives the electromotive-forces 

 of the various batteries enumerated. 



179. Strength of Current. The student must not 

 mistake the figures given in the above table for the 

 strength of current which the various batteries will 

 yield; that depends, as was said in Lesson XIII., on 

 the internal resistance of the cells as well as on their 

 E.M.F. The E.M.F. of a cell is independent of its 

 size, and is determined solely by the materials chosen 

 and their condition. The resistance depends on the 

 L 



