CHAP, vi.] ELECTRICITY AND MAGNETISM. 311 



of carbon appears on the other hand to diminish on 

 heating. German-silver and other alloys do not show 

 so much change, hence they are used in making standard 

 resistance-coils. Those liquids which only conduct by 

 being electrolysed (Art. 205), conduct better as the 

 temperature rises. The effect of light in varying the 

 resistance of selenium is stated in Art. 389. 



35O. Typical Circuit. Let us consider the typical 

 case of the circuit shown 

 in Fig. 128, in which a 

 battery, ZC, is joined up 

 in circuit with a galvano- 

 meter by means of wires 

 whose resistance is R. 

 The total electromotive- 

 force of the battery we 

 will call E, and the total Fi - I28 - 



internal resistance of the liquids in the cells r. The 

 resistance of the galvanometer coils may be called G. 

 Then, by Ohm's law :-==, 



E - 



R + r + G 



The internal resistance r of the liquids ol the battery 

 bears a very important relation to the external resistance 

 of the circuit (including R and G), for on this relation 

 depends the best way of arranging the ^battery cells 

 for any particular purpose. Suppose, for example, 

 that we have a battery of 50 small Daniell's cells at 

 our disposal, of which we may reckon the electro- 

 motive-force as one volt (or more accurately, 1-079 volt) 

 each, and each having an internal resistance of two 

 ohms. If we have to use these cells on a circuit where 

 there is already of necessity a high resistance, we should 

 couple them up " in simple series " rather than in 

 parallel branches of a compound circuit. For, suppos- 

 ing we have to send our current through a line of 

 telegraph 100 miles long, the external resistance R will 



