178 THE CORRELATION OF FORCE. 



tance from the shore, where, in heavy weather, the sea 

 breaks over them with so much force that sometimes for 

 days, and even weeks, the keepers are cut off from all com- 

 munication with the laud. In such cases it is very plain 

 that the modes of producing and managing the light must 

 be of a very simple character. The apparatus must be 

 very little subject to accidents or derangement, and only 

 to such as can be easily remedied, when they do occur, by 

 persons of ordinary skill. 



The production of a current of electricity for the electric 

 light was at first, and for a long time, effected by a chem- 

 ical process involving a considerable degree of scientific 

 knowledge and skill in those directing it, if not in its ordi- 

 nary and successful working, at least in the emergencies 

 which in all such operations will sometimes occur. 



Within a somewhat recent period, however, a method 

 has been devised of developing the requisite current of 

 electricity by means of mechanical force through the me- 

 dium of magnetism. It is found that changes in the mag- 

 netic condition of an iron bar, for example, induce electric- 

 al movements in any conductors placed at right angles 

 near it. If an iron bar is wound round with a wire in a 

 certain way, and the magnetic state of the bar, while thus 

 wound, is made to change which may be easily done by 

 alternately bringing it near and drawing it away from a 

 permanent steel magnet a succession of electrical im- 

 pulses are induced in the wire. By combining many of 

 these wound bars or bobbins, as they are sometimes called 

 - in one machine, and causing a number of permanent mag- 

 nets to revolve in face of them in rapid succession, and then 

 combining the electrical impulses that are induced in a 

 proper manner, the effects, substantially, of a continued 

 flow are secured, so far, at least, as is essential for produc- 

 ing the electric light. 



