16. In Art. 1 reference was made to two methods of current generation, namely, by means of batteries and by dynamo-electric machinery. For electric lighting, the only practical method is the latter. Current is generated by revolving coils of insulated wire, mounted on an iron core and suitably connected together, in front of powerful electromagnets. The power for this purpose is supplied by a steam engine, or in some cases by turbines, when steady water-power is available. The current is led to the place of utilization, or the distributing center, by means of heavy copper cables covered with some insulating substance, such as gutta percha or rubber, to prevent electrical contact with the supports or other bodies which they may approach. These two materials are most commonly employed for the insulating covering of electric wires, being flexible and waterproof. Other insulators are paper, oils, porcelain, wood, silk, cotton, shellac, ebonite or hard rubber, paraffin, mica, glass, and dry air.