178 



APPLIED SCIENCE 



connected to the carbon of the second, etc. (Fig. 77), or the 

 wire from the zinc of the first may be connected to the zinc 

 of the second, and the wire from the carbon of the first to 

 the carbon of the second, and so on. (Fig. 78.) 



A battery is rated commercially by the resistance, and by 

 the electromotive force of a single cell. There are two re- 

 sistances to be con- 

 sidered in the calcula- 

 tions of the capacity 

 of batteries: the re- 

 sistance of the bat- 

 tery, due to polariza- 

 tion, etc., which is 

 represented by R, and the resistance of the external circuit, 

 such as the wire, which is designated by r. The current given 

 by a battery according to Ohm's Law is equal to the elec- 

 tromotive force divided by the resistance, which in this case 

 is divided into two parts. 



FIG. 77. Cells Ar- 

 ranged in Scries. 



FIG. 78. Cells Ar- 

 ranged in Parallel. 



Current 



Electromotive force 



Resistance (internal) + resistance (external) 

 E 



C = 



R 



212. Galvanometer. One of the instruments used to 

 measure electricity is called a galvanometer. It depends 

 for its usefulness on the principle of magnetism. There are 

 many varieties of this device. The D'Arsonval galvan- 

 ometer (Fig. 79) consists of a horseshoe magnet placed 

 vertically. Between the poles of the magnet there is an iron 

 cylinder; above the cylinder is suspended a fine wire wound 

 on a thin copper frame so that it will swing freely between 



