THE ELECTRIC CURRENT. 13 



entering the cell at the zinc electrode (the anode). This current 

 decomposes the sulphuric acid (H 2 SO 4 ), the hydrogen is liber- 

 ated at the copper or carbon cathode and escapes from the cell 

 as a gas, and the sulphuric acid radical (SO 4 ), which is set free 

 at the zinc anode, combines with the zinc and forms zinc sulphate 

 (ZnSO 4 ) which goes into solution. The combination of Zn and 

 SO 4 develops more energy than is required for the decomposi- 

 tion of the H 2 SO 4 so that the chemical action as a whole is a 

 source of energy. 



The available energy of the reaction above described may be 

 greatly increased by providing an oxidizing agent in the neighbor- 

 hood of the cathode so that the hydrogen may be oxidized and 

 form water (H 2 O) at the moment of its liberation by the current. 

 The energy of this oxidation is then added to the available energy 

 of the total chemical action in the cell.* 



7. Examples of voltaic cells. The ordinary " dry cell" One 

 of the most familiar types of voltaic cell is the cell in which 

 a plate of zinc and a plate of carbon are immersed in a solution 

 of ammonium chloride (NH 4 C1) with a mass of powdered black 

 oxide of manganese (MnO 2 ) packed around the carbon electrode. 

 When this cell delivers current, the NH 4 C1 is decomposed, and 

 chlorine is liberated at the zinc plate where it combines with the 

 zinc to form zinc chloride. As the NH 4 ioris are liberated at 

 the carbon electrode they break up into ammonia and hydrogen 

 (NH 4 = NH 3 + H), the ammonia goes into solution and the 

 hydrogen is oxidized at the expense of the oxygen in the black 

 oxide of manganese, forming water. The free ammonia in this 

 type of cell may be detected by the odor after the cell has been 

 delivering current for some time. 



This type of cell is exemplified by a great variety of commer- 

 cial forms of which the ordinary " dry cell " is the most familiar. 

 In this cell the electrolyte is soaked up in a porous material such 



*The student is referred to Professor H. S. Carhart's Primary Batteries, pub- 

 lished by Allyn & Bacon, Boston, Mass., for full information on primary batteries 

 (voltaic cells) and primary battery tests. 



