V f 



PHYSICS OF NINETEENTH CENT. ELECTRICITY. 583 



revolution, and in which part of the current is diverted JK> produce 

 electro-magnets for its own generation. Mr/Wjjlde found,' IDE ^ict, 

 that the slight magnetism retained by iron suffices to^e/e^ate a current f 

 which excites a greater magnetic power, and this in its turn gives rise 

 to a still greater current, and so on. A still later form of the magneto- 

 electric machine is the invention of M. Gramme, of Paris. A small table 

 model of a machine of his invention is shown in Fig. 295. Its great pe- 

 culiarity is the armature, which consists of a ring made of iron wires, 

 with a large number of separate coils of insulated copper wire disposed 

 radially, as seen in M A M'. The currents from these coils are collected 

 at the proper point, so that the result is a regular and continuous 

 current in one direction. N o s in the figure is a permanent steel 

 magnet, composed of several plates bolted together, and the space 

 between the poles and the annular armature is filled with blocks of 

 soft iron. For use on the large scale the Gramme machines are con- 

 structed with several electro-magnets, and are driven by steam power. 

 Magneto-electric machines, and not galvanic batteries, are the means 

 by which current electricity is now generated for the production of 

 light for lighthouses, for electro-plating, etc. The examples we have 

 given of such machines will, it is hoped, suffice to enable the reader 

 to observe for himself how all these later contrivances are essentially 

 nothing but modifications of the simple arrangements which were de- 

 vised in the laboratory of the Royal Institution in 1831 (see Figs. 291 

 and 292) by the illustrious discoverer to whose labours we once more 

 revert by stating another of the great laws of electricity which he was 

 the first to discover. 



As early as 1825 Faraday tried whether one electric current would 

 induce another current in an adjacent wire. He enters in his note- 

 book the outcome of the experiment "no result." In 1831 he finds 

 a result, and he finds why he had before overlooked it. When two 

 copper wires insulated from each other run for some distance side by 

 side, a current passed through one causes a current in the other, if 

 the extremities of this last are joined so as to form a complete circuit. 

 But this induced current lasts but a moment ; it occurs only at the 

 instant the battery current is established, and at the instant the battery 

 current is interrupted. In the first case the induced wave or current 

 is in the opposite direction to the battery current ; but in the second 

 case, that is, when the battery current is stopped, the wave of induced 

 electricity is in the same direction as the battery current. During the 

 continuance of the battery current no current or other electrical phe- 

 nomena are manifested by the wire forming the iriduced circuit. 

 Faraday, however, believed at first that the wire of the induced current 

 was, while the battery current continued, in a peculiar condition, to 

 which he gave the name of the " electro-tonic state." But as no evi- 

 dence could be obtained of any property other than that belonging 

 to the wire itself, the idea was soon abandoned. 



