252 ENERGY OF CURRENTS. 



is absorbed by nitric acid, and forms nitro-compoimds of a lower 

 degree of oxidation. The zinc is placed in a solution of sulphuric 

 acid or of zinc sulphate. By substituting carbon for platinum, we 

 get Bunsen's element. 



The energy available in Grove's and Bunsen's cells represents a 

 quantity of heat of about 47 thermal units; they have therefore 

 almost twice as great an electromotive force as that of DanielPs cell ; 

 the liquids have, moreover, a far smaller resistance. Accordingly 

 they are usually employed whenever very powerful currents are 

 wanted ; but the liquids change rapidly, the resistance increases, the 

 electromotive force diminishes, and the strength of the current is 

 soon lessened. 



264. ELECTROSTATIC PHENOMENA IN PILES OR BATTERIES.* 

 The name of pile, frequently given to the association of several 

 couples in connection with each other, arises from the form originally 

 devised by Volta. Volta's pile consists of a series of double plates 

 of zinc and copper arranged one upon the other in the same order, 

 and separated from each other by discs of moistened cloth. 



A couple consists of the whole of the bodies which exist between 

 two zincs that is to say, zinc, copper, water, zinc. It may be sup- 

 posed that each of the zinc plates is the half of two successive couples. 



If the battery commences at the bottom by a copper and ends at 

 the top in a zinc, it will be seen that the first copper plate does not 

 come into play. The difference of potential being equal to e for 

 each couple, the potential will go on increasing from the bottom 

 upwards ; and if there are n couples, the electromotive force of the 

 battery is 



E = en. 



265. UNINSULATED BATTERY. If the bottom of the battery is 

 connected with the earth by conductors whose influence may be 

 neglected, the top disc A has a potential V a = E = ne which is propor- 

 tional to the number of couples. This is easily verified, either by 

 means of an electrometer, or by measuring charges given to a 

 condenser. 



266. INSULATED BATTERY. If the battery, which we will suppose 

 is formed of identical and equidistant couples, has not been connected 

 with the ground, or at any rate after such a length of time that it has 

 attained equilibrium, its total charge will be zero, and the distribution 

 of potentials will be symmetrical in reference to the middle. We 



* The term battery is more generally used in this country and will be here 

 adopted. TRANS. 



