INDUCED ELECTEICITY. 21 



the magnet, the needle deviates from the magnetic meridian, 

 or the muscle contracts. Presently the needle returns to its 

 former position, or the muscle to repose. If a fresh movement be 

 given to the iron, so as to make it once more transverse to the 

 poles, a fresh deviation of the galvanometer needle will take place 

 in a direction opposite to the former ; or the muscle will again 

 contract as energetically as at first. 



These phenomena of induction are produced in the electro- 

 magnetic precisely as in the electro-dynamic instruments, by virtue 

 of a modification wrought in the state of tlie magnet and of its 

 soft iron, and in the state of the copper wire rolled round the soft 

 iron or the magnet, .^•' ' 



The theory is as follows : — Wben the soft iron is brought oppo- ^ 

 site to the poles of the magnet, the natural electricity of the iron 

 is decomposed by the magnet, is attracted to the opposite poles 

 and recomposed. There results a neutralisation of the artificial 

 magnet, and an induced modification of the electricity of the 

 copper wire, the spirals of wbicli also act upon one another. When 

 the soft iron is withdrawn from the magnet by rotation, the 

 magnetic fluid of the magnet regains its liberty and accumulates 

 at the poles. The soft iron, the polarisation of which ceases at the 

 same moment, returns to its normal state, and the coj^iper wire 

 sustains a fresh induction. 



If at the moment when these electrical modifications occur we 

 interrupt the circuit formed by the wire, the power of induction is 

 very considerably increased. Nearly all electro-magnetic instru- 

 ments are arranged so that these interruptions of the current only 

 take place when the soft iron arrives opposite the poles of the 

 magnet. It follows that they produce, by the rotation of the iron, 

 one indu(;tive action that is very feeble, inappreciable by man, and 

 one that is powerful : the first when the iron is transverse to the 

 poles of the magnet, the second when it is opposite to them. 



In the electro-magnetic instruments the interruptions of the 

 current are affected by a little bobbin (commutator) placed upon 

 the axis of the soft iron (armature). The latter is put in move- 

 ment by a mechanism of toothed wheels, so that the rotation of 

 the iron may be rendered extremely rapid. 



This rapidity of rotation is essential to the force and even to the 

 action of electro-magnetic instruments, for the reasons following : — 



When the current of an electro-dynamic instrument is inter- 

 rupted, the induction and magnetization of the soft iron cease 

 abruptly with the cessation of their producing cause; -and the 

 change from a great magnetic intensity to zero occurs suddenly, 

 and without transition in the central soft iron of the bobl)iii. 



