PHYSICS OF NINETEENTH CENT. ELECTRICITY. 567 



tended that of (Ersted and confirmed Ampere's theory of magnetism, 

 but it placed at the disposal of the physicist magnets of a power im- 

 mensely superior to any that could be constructed by other means. 

 The magnetic energy which can be developed in soft iron by voltaic 

 currents is almost without limit. This magnified power afterwards 

 brought into view new and unsuspected properties of magnets. 



The electro-magnet has therefore been the parent of other discoveries, 

 and in numberless practical applications of electricity it is an essential 

 part of the apparatus. As represented in Fig. 283, its usual .construction 



FIG. 283. 



will be readily understood. The wire, instead of forming but one helix, 

 as in Figs. 281 and 282, is wound many times upon itself. For con- 

 venience the wire, which is usually of copper covered with silk thread, 

 is wound upon a hollow wooden bobbin, A, capable of receiving in its 

 axis a cylindrical bar of soft iron. This bar of iron becomes a power- 

 ful magnet the moment the coil is traversed by a current of electricity ; 

 it continues a magnet while the current continues, and on the inter- 

 ruption of the current instantly reverts to its ordinary condition of un- 

 magnetic soft iron. Consequently, a piece of soft iron F, attached to 

 a spring E, will, the instant the contacts with the -battery are made, be 

 drawn to the electro-magnet, and there retained so long as the current 

 continues to flow ; but, when the current ceases, F will be drawn back 

 by the spring and resume its former position. As the battery which 

 supplies the current may be at any distance whatever, provided only 

 that proper conducting-wires connect it with the coil A, this com- 

 bination offers obvious advantages for instantly calling into action 

 mechanical forces at a distance. 



The applications of scientific principles to practical arts, however 

 useful these applications may be, are not in general subjects coming 

 within the range of the present work. The results which have been 

 attained by Electric Telegraphy have, however, impressed upon the 

 popular mind an idea of the wonder-working power of science. It is, 

 therefore, by no means unlikely that the purely scientific discoveries 

 of Volta, (Ersted, Ampere, and Arago, will, in the eyes of many, derive 

 additional interest from a glance at the manner in which the discoveries 

 of these philosophers have been made available for every-day purposes. 

 Proposals to employ electricity as the means of telegraphy were made 

 as long ago as the middle of the eighteenth century. But electricity 



