352 Mr. Faraday on the Magneto-electric Spark and Shock, 



magnet,) the spark, upon breaking contact, is much brighter 

 than if the soft iron were away; and because this effect occurs 

 at the same moment with the shock in Mr. Jenkins's experi- 

 ment, it might at first be supposed that the electricity pro- 

 ducing both the spark and the shock was the same, and that 

 the effects of both were increased, because of the increase in 

 power of this their common cause. But the fact is not so, for 

 the electricity producing the spark is passing in one direction, 

 being that which the zinc plate and acid determine, whilst the 

 electricity producing the shock is circulating in the contrary 

 way. 



From the appearance of the spark, which is always in this 

 form of the experiment due to the electricity which is passing 

 at the moment when contact is broken, it might seem that a 

 greater current of electricity is circulating during the time that 

 the contact is preserved, whilst the iron is present in the helix, 

 than when it is away. But this is not the case; for when the 

 quantity is measured by a very delicate galvanometer, it is 

 found to remain unchanged after the removal or replacement 

 of the iron, and to depend entirely upon the action at the zinc 

 plate. Still the appearance of the spark is an evident and 

 decisive proof, that the electricity which is passing at the mo- 

 ment of disjunction is of greater intensity when the iron is in 

 the helix than when it is away, and this increased effect is evi- 

 dently dependent, not upon any change in the state of things 

 at the source of the electricity, but in a change of the powers 

 of the conducting wire caused by the presence of the soft iron. 

 I do not suppose that this change is directly connected with 

 the magnetizing power of the current over the iron, but is due 

 rather to the power of the iron after it becomes a magnet, to 

 react upon the wire ; and I have no doubt, though I have 

 not had time to make the experiment, that a magnet of very 

 hard steel, of equal force with the soft iron magnet, if put into 

 the helix in the same direction, would exert an equal influence 

 over the wire. 



I will now notice another circumstance, which has a similar 

 influence in increasing the intensity of the spark which occurs 

 when the junction of the circuit is broken. If a pair of zinc 

 and copper plates immersed in acid are connected by a short 

 wire, and all precautions are taken to avoid sources of inaccu- 

 racy, then, as I have already shown, the spark, upon breaking 

 contact, is not greater than that upon making contact. But 

 if the connecting wire be much lengthened, then the spark 

 upon breaking contact is much increased. Thus, a connecting 

 copper wire of y^th of an inch in diameter when 12 inches 

 long, produced but a small spark with the same pair of plates 



