CHAP, in.] ELECTRICITY AND MAGNETISM. 171 



of them, with an intensity corresponding to the difference 

 of the strengths of the two currents. 



204. Ballistic Galvanometer. In order to measure 

 the strength of currents which last only a very short time, 

 galvanometers are employed in which the needle takes 

 a relatively long time to swing. This is the case with 

 long or heavy needles ; or the needles may be weighted 

 by enclosing them in leaden cases. As the needle swings 

 slowly round, it adds up, as it were, the varying impulses 

 received during the passage of a transient current. 

 The sine of half the angle of the first swing is proportional 

 to the quantity of electricity that has flowed through the 

 coil. The charge of a condenser may thus be measured 

 by discharging it through a ballistic galvanometer. 



LESSON XVIII. Chemical Actions of the Current: 

 Voltameters. 



205. In addition to the chemical actions inside the 

 cells of the battery, which always accompany the produc- 

 tion of a current, there are also chemical actions produced 

 outside the battery when the current is caused to pass 

 through certain liquids. Liquids may be divided into 

 three classes (i) those which do not conduct at all, such 

 as turpentine and many oils, particularly petroleum ; (2) 

 those which conduct without decomposition, viz. mercury 

 and other molten metals, which conduct just as solid 

 metals do ; (3) those which are decomposed when they 

 conduct a current, viz. the dilute acids, solutions of 

 metallic salts, and certain fused solid compounds. 



206. Decomposition of Water. In the year 1 800 

 Carlisle and Nicholson discovered that the voltaic current 

 could be passed through water, and that in passing through 

 it decomposed a portion of the liquid into its constituent 

 gases. These gases appeared in bubbles on the ends of 

 the wires which led the current into and out of the 

 liquid ; bubbles of oxygen gas appearing at the point 



