VOLTAIC ELECTRICITY. 239 



of the practical difficulties to be overcome in working a Voltaic 

 element or battery. 



377. Electrodes. It will be readily understood by 

 keeping in mind the direction, of the two currents, that, 

 if the circuit be broken, negative electricity will accumu- 

 late at the end of the wire attached to the positive plate, 

 and positive electricity at the end of the wire attached to 

 the negative plate. These ends of the wires are then 

 called poles or electrodes. The negative pole is 

 attached to the positive plate and vice versa. For 

 many experimental purposes, strips of platinum are fastened 

 to the ends of the wires ; these platinum strips then con- 

 stitute the electrodes. 



378. Resistance. Even a good conductor ( 333) offers a 

 sensible resistance to the passage of an electric current ; the poorer 

 the conductor, the more resistance it offers to the passage of the 

 current. Experiments show that the quantity of electricity passing 

 in a unit of time, over a given conductor, is directly proportional to 

 the electromotive force. (This electromotive force, " K M. F." is 

 the supposed force that causes or tends to cause a transfer of elec- 

 tricity from one point to another.) When the E. M. F., the sec- 

 tional area and the material are constant, the resistance is propor- 

 tional to the length of the conducting wire ; doubling the length 

 doubles the resistance and halves the current. When the E. M. F., 

 the length and material are constant, the resistance is inversely 

 proportional to the area of the cross-section of the wire ; halving 

 that area doubles the resistance and halves the current. Hince the 

 resistance is inversely proportional to the area of the cross-section, 

 it will also be proportional to the weight of the wire per unit of 

 length. The difference between the resistance of a good conductor 

 and that of an insulator is very great. The resistance of a silver 

 wire being taken as unity, the resistance of a similar wire of Ger- 

 man silver would be 12.82, while that of a similar rod of gutta- 

 percha would be 8.5 x 10-. Hence, insulators are often spoken of 

 as bodies of great resistance. 



379. A Voltaic Battery. A number of Vbltaic 

 elements connected in such a manner that the 



