io ELECTRIC LIGHTING. 



actions, that is to say, from deposits chemically produced on 

 the electrodes, and reacting in their turn on the electrified 

 medium. These deposits tend to produce an electrical 

 action in the direction inverse to that of the current; but 

 sometimes they proceed from an electro-static polarity com. 

 municated to the medium. A polarization of this kind is 

 often produced in the voltaic arc. 



We must also give some explanation of the word condensa- 

 tion, which we shall have occasion to apply more than once. 

 What is so called is the accumulation of charge obtained by 

 the inductive action which an electric charge produces on a 

 second conductor of large surface insulated from the elec- 

 trified one. In this action charges of opposite electricities 

 are retained by their mutual attraction, so that they can be 

 accumulated in quantities proportional to the extent of the 

 condensing surface. Under these conditions a slight leakage 

 of charge takes place by electrotonic conductivity, and as the 

 condensing plates then constitute true electrodes, the resist- 

 ance of the insulator or dielectric interposed between the two 

 plates or armatures is inversely proportional to the surface of 

 these plates ; or, what comes to the same thing, if it be the 

 effect on a submerged cable that is considered, inversely pro- 

 portional to the length of the cable. 



Electric Units. In order to find the values of the 

 electro-motive force, and of the resistance of a battery, as 

 well as the values of the other elements acting in an electric 

 circuit, it has been necessary to fix upon some units of 

 electric measurement ; and although scientific men are not 

 yet of one accord as to the units which should be adopted, 

 the tendency is to accept the "rational units" that, after 

 long investigations, were fixed upon by the British Associa- 

 tion.* These units have received different names, which 

 keep in remembrance the physicists who have in the highest 



* See the Report of the British Association for 1873, P a e 222. (TR.) 



