CURRENT ELECTRICITY 207 



cars, elevators, railway trains, and other machinery is gen- 

 erated in another way. When a wire moves through a 

 magnetic field, cutting the lines of force, a current is generated 

 in the wire. The amount of this current depends upon the 

 strength of the field, the speed of the wire, and the number of 

 turns in the latter. The electric dynamo is a machine for 

 revolving coils of wire in such a field. It is usually run by 

 steam or water power. Our houses are lighted by currents of 

 electricity induced by the powerful currents passing along 

 the service wire, the induction taking place in the transformer. 



173. Electroplating. When a current of electricity is passed 

 through a solution, it decomposes it by electrolysis. In this 

 way, water may be decomposed into hydrogen and oxygen, 

 and melted table salt decomposed into the metal sodium and 

 chlorine gas. Electroplating is based on the same principle. 

 The articles to be plated are attached to the positive pole of 

 the battery and the piece of metal to be used in plating, either 

 serves as the other pole or is attached to it. The electrolyte 

 must also contain some of the metal. When the current is 

 passed through the solution, small particles of the metal are 

 carried across the solution and deposited on the articles to be 

 plated. In this way, iron is " galvanized" with zinc or 

 covered with tin, and brass and other metals plated with 

 nickel, silver, or gold. In electrotyping, an impression of the 

 type is taken in wax and a thin plating of copper is deposited 

 on the wax, after which it is backed with type metal. Since 

 wax is not a good conductor of electricity, the mould must be 

 carefully dusted with powdered graphite before the work be- 

 gins in order to make a connected circuit. Gold, copper, and 

 silver are sometimes separated from their ores by electrolysis. 



174. Electromagnets. If a current of electricity be sent 

 along a coiled wire, the wire acts like a natural magnet, and, 

 if hung up and allowed to swing free, will assume a general 

 north and south direction. By slipping into the coil a piece 



