60 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



1092. All these effects, except those of decomposition, were reproduced by two ex- 

 tended long wires, not having the form of helices, but placed close to each other ; and 

 thus it was proved that the extra current could be removed from the wire carrying 

 the original current to a neighbouring wire, and was at the same time identified, in 

 direction and every other respect, with the currents producible by induction (1089.). 

 The case, therefore, of the bright spark and shock on disj unction may now be stated 

 thus : If a current be established in a wire, and another wire, forming a complete cir- 

 cuit, be placed parallel to the first, at the moment the current in the first is stopped it 

 induces a current in the same direction in the second, the first exhibiting then but a 

 feeble spark ; but if the second wire be away, disjunction of the first wire induces a 

 current in itself in the same direction, producing a strong spark. The strong spark 

 in the single long wire or helix, at the moment of disjunction, is therefore the equiva- 

 lent of the current which would be produced in a neighbouring wire if such second 

 current were permitted. 



1093. Viewing the phenomena as the results of the induction of electrical currents, 

 many of the principles of action, in the former experiments, become far more evident 

 and precise. Thus the different effects of short wires, long wires, helices, and elec- 

 tro-magnets (1069.) may be comprehended. If the inductive action of a wire a foot 

 long upon a collateral wire also a foot in length, be observed, it will be found very 

 small ; but if the same current be sent through a wire fifty feet long, it will induce in 

 a neighbouring wire of fifty feet a far more powerful current at the moment of making 

 or breaking contact, each successive foot of wire adding to the sum of action ; and 

 by parity of reasoning, a similar effect should take place when the conducting wire 

 is also that in which the induced current is formed : hence the reason why a long 

 wire gives a brighter spark on breaking contact than a short one (1068.), although 

 it carries much less electricity. 



1094. If the long wire be made into a helix, it will then be still more effective in 

 producing sparks and shocks on breaking contact ; for by the mutual inductive action 

 of the convolutions each aids its neighbour, and will be aided in turn, and the sum 

 of effect will be very greatly increased. 



1095. If an electro-magnet be employed, the effect will be still more highly ex- 

 alted ; because the iron, magnetized by the power of the continuing current, will 

 lose its magnetism at the moment the current ceases to pass, and in so doing will 

 tend to produce an electric current in the wire around it (37. 38.), in conformity with 

 that which the cessation of current in the helix itself also tends to produce. 



1096. By applying the laws of the induction of electric currents formerly deve- 

 loped (6. &c.), various new conditions of the experiments could be devised, which by 

 their results should serve as tests of the accuracy of the view just given. Thus, if a 

 long wire be doubled, so that the current in the two halves shall have opposite actions, 

 it ought not to give a sensible spark at the moment of disjunction : and this proved 

 to be the case, for a wire forty feet long, covered with silk, being doubled and tied 

 closely together to within four inches of the extremities, when used in that state, 



