44 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



spark at the moment of disjunction than the latter. Again, twenty-eight feet of cop- 

 per wire were made up into a helix, and being used gave a good spark on disjunction 

 with the electromotor ; being then suddenly pulled out and again employed, it gave a 

 much smaller spark than before, although nothing but its spiral arrangement had 

 been changed. 



1066. As the superiority of a helix over a wire is important to the philosophy of 

 the effect, I took particular pains to ascertain the fact with certainty. A wire of cop- 

 per sixty-seven feet long was bent in the middle so as to form a double termination 

 which could be communicated with the electromotor ; one of the halves of this wire 

 w^as made into a helix and the other remained in its extended condition. When these 

 were used alternately as the connecting wire, the helix gave by much the strongest 

 spark. It even gave a stronger spark than when it and the extended wire were used 

 conjointly as a double conductor. 



1067- When a shor^t wire is used, all these effects disappear. If it be only two or 

 three inches long, a spark can scarcely be perceived on breaking the junction. If it 

 be ten or twelve inches long and moderately thick, a small spark may be more easily 

 obtained. As the length is increased, the spark becomes proportionately brighter, until 

 from extreme length the resistance offered by the metal as a conductor begins to in- 

 terfere with the principal result. 



1068. The effect of elongation was well shown thus : 114 feet of copper wire, one 

 eighteenth of an inch in diameter, were extended on the floor and used as a conduc- 

 tor ; it remained cold, but gave a bright spark on breaking contact. Being crossed so 

 that the two terminations were in contact near the extremities, it was again used as 

 a conductor, only twelve inches now being included in the circuit : the wire became 

 very hot from the greater quantity of electricity passing through it, and yet the spark 

 on breaking contact was scarcely visible. The experiment was repeated with a wire 

 one ninth of an inch in diameter and thirty-six feet long with the same results. 



1069. That the effects, and also the action, in all these forms of the experiment are 

 identical; is evident from the manner in which the former can be gradually raised from 

 that produced by the shortest wire to that of the most powerful electro-magnet : and 

 this capability of examining what will happen by the most powerful apparatus, and 

 then experimenting for the same results, or reasoning from them, with the weaker ar- 

 rangements, is of great advantage in making out the true principles of the phenomena. 



1070. The action is evidently dependent upon the wire which serves as a con- 

 ductor ; for it varies as that wire varies in its length or arrangement. The shortest 

 wire may be considered as exhibiting the full effect of spark or shock which the 

 electromotor can produce by its own direct power; all the additional force w^hich 

 the arrangements described can excite being due to some affection of the current, 

 either permanent or momentary, in the wire itself That it is a momentarij e^Qct, pro- 

 duced only at the instant of breaking contact, will be fully proved (1089. 1100.). 



1071. No change takes place in the quantity or intensity of the current during the 



