24 Electric Light 



place in the lead plates and acid in the act of charging and dis- 

 charging, is as follows, according to Dr. Frankland (see the 

 report published in " The Electrician" of 31st March, 1883): 

 " Occluded gases play no part, practically. The active material 

 on lead plates is lead sulphate. The initial action in charging 

 the battery is the electrolysis of sulphuric acid into hydrogen, 

 sulphuric anhydride, and oxygen. The hydrogen decomposes 

 the lead sulphate on the negative plate into spongy lead and 

 sulphuric acid, whilst the oxygen decomposes the lead sulphate 

 on the positive plate into lead peroxide and sulphuric anhydride. 

 All sulphuric anhydride is at once converted into sulphuric acid. 

 In discharging, the initial action is again the electrolysis of 

 sulphuric acid, which restores the coating of the two plates to 

 the original condition of lead sulphate. As the charging of a 

 cell is attended with the liberation of sulphuric acid, and its 

 discharge with the abstraction of this acid from the liquid con- 

 tents of the cell, it is only necessary to ascertain the specific 

 gravity and consequent strength of the acid, to determine the 

 amount of charge in a cell at any given moment, provided that 

 the specific gravity of the acid in the charged and uncharged 

 conditions of the cell be previously known. In the case of a 

 cell with which Frankland experimented, each increase of 0-005 

 in the specific gravity of the dilute acid, meant a 'storage' of 

 available energy equal to 20 amperes for one hour." 



11 Thermo-electricity," by reason of its low tension, has only 

 been used for electro plating, as in this process high tension is 

 not admissible. " Voltaic electricity" has been employed to a 

 limited extent for electric lighting ; but one serious drawback 

 to its general adoption for this purpose is the great expense 

 entailed, as electricity produced by the consumption of zinc and 

 acid in a battery costs, in round numbers, about ten times more 

 than the same amount of electricity obtained by the use of a 

 dynamo machine. 



Soft iron, after being magnetised, loses nearly all its magnetism 

 as soon as the exciting agent is removed ; but it retains a 

 very minute trace, although perhaps not sufficient to indicate 

 its presence to a marked degree, and it is this residual trace 

 which plays so important a part in dynamo machines. 



