ELECTRO-MOTIVE POWER. 



explained in our Tract upon the " Electric Telegraph," page 196, 

 that it will be sufficient briefly to recapitulate the general physical 

 principles from which this property arises. 



If a voltaic current be conducted spirally round a rod of soft 

 iron, the iron will become magnetic, and will continue magnetic 

 so long as the current passes round it. Its acquisition of the 

 magnetic virtue is simultaneous with the transmission of the 

 current. It is not gradual but instantaneous. The very instant 

 the current is transmitted, the magnetic virtue is imparted to the 

 iron, and does not afterwards increase in intensity. 



The loss of the magnetic virtue, upon the suspension of the 

 current, is equally instantaneous and complete. The very instant 

 the current is discontinued, the iron ceases to be magnetic. 



The subtlety of the electric fluid, and the celerity of its pro- 

 pagation, are such that it is capable of being transmitted and 

 suspended instantaneously, and, however short the interval may 

 be between the instants of its transmission and suspension, it will, 

 during that interval nevertheless, impart to the iron the magnetic 

 property. So true is this, that it is practically found that the 

 current may be alternately transmitted and suspended hundreds 

 or even thousands of time in a single second, and in these short 

 intervals the iron will alternately acquire and lose the magnetic 

 virtue. 



The manner in which the voltaic current is transmitted spirally 

 round the iron bar is as follows : The wire upon which the current 

 is transmitted is wrapped with silk or 

 cotton thread, which being a non-con- 

 ductor of electricity, will prevent the 

 lateral escape of the fluid, which will 

 therefore pass along the wire within the 

 coating of thread as water or air would 

 pass along a tube. The wire thus covered 

 is coiled spirally round the bar of soft 

 iron, which may or may not be bent into 

 the horse-shoe form, as shown in fig. 1. 

 One end of the wire being put in connection 

 with the positive, and the other with the negative pole of the voltaic 

 battery ; the current will be transmitted upon it, and will be pre- 

 vented from passing from one coil of the wire to the contiguous 

 one, by the interposition of the silk or cotton thread. So long as 

 the current is thus continued, the iron, whatever be its form, will 

 be magnetic, one end having the properties of the north and the 

 other of the south magnetic pole. 



5. By an expedient to which an infinite variety of forms may 

 be given, the current can be alternately transmitted and suspended 

 196 



