ANALOGY OF CONDUCTORS AND INSULATORS. 87 



1331. Again, to return to this beautiful experiment in the various forms which may 

 be given to it : the forces are not all in the wire (after they have left the Leyden jar) 

 during the whole time (1328.) occupied by the discharge; they are disposed in part 

 through the surrounding dielectric under the well-known form of induction ; and if 

 that dielectric be air, induction takes place from the wire through the air to sur- 

 rounding conductors, until the ends of the wire are electrically related through its 

 length and discharge has occurred, i. e. for the time during which the middle spark 

 is retarded beyond the others. This is well shown by the old experiment, in which 

 a long wire is so bent that two parts (Plate III. fig. 1. a, h.) near its extremities shall 

 approach within a short distance, as a quarter of an inch, of each other in the air. 

 If the discharge of a Leyden jar, charged to a sufficient degree, be sent through such 

 a wire, by far the largest portion of the electricity will pass as a spark across the air 

 at the interval, and not by the metal. Does not the middle part of the wire, there- 

 fore, act here as an insulating medium, though it be of metal? and is not the spark 

 through the air an indication of the tension (simultaneous with induction) of the elec- 

 tricity in the ends of this single wire r Why should not the wire and the air both be 

 regarded as dielectrics; and the action at its commencement, and whilst there is ten- 

 sion, as an inductive action ? If it acts through the contorted lines of the wire, so it 

 also does in curved lines through air (1219. 1224.), and other insulating dielectrics 

 (1228.) ; and we can apparently go so far in the analogy, whilst limiting the case to 

 the inductive action only, as to show that amongst insulating dielectrics some lead 

 away the lines of force from others (1229.), as the wire will do from worse conduct- 

 ors, though in it the principal effect is no doubt due to the ready discharge between 

 the particles whilst in a low state of tension. The retardation is for the time insula- 

 tion ; and it seems to me we may just as fairly compare the air at the interval a, b, 

 (fig. 1.) and the wire in the circuit, as two bodies of the same kind and acting upon 

 the same principles, as far as the first inductive phenomena are concerned, notwith- 

 standing the different forms of discharge which ultimately follow*, as we may com- 

 pare, according to Coulomb's investigations -}-, different lengths of different insulating 

 bodies required to produce the same amount of insulating effect. 



1332. This comparison is still more striking when we take into consideration the 

 experiment of Mr. Harris, in which he stretched a fine wire across a glass globe, the 

 air within being rarefied J. On sending a charge through the joint arrangement of 

 metal and rare air, as much, if not more, electricity passed by the latter as by the 

 former. In the air, rarefied as it was, there can be no doubt the discharge was pre- 

 ceded by induction (1284.) ; and to my mind all the circumstances indicate that the 

 same was the case with the metal ; that, in fact, both substances are dielectrics, ex- 



* These will be examined hereafter (1348, &c,). 



t M^moires de I'Acad^mie, 1785, p. 612. or Ency. Britann. First Supp. vol. i. p. 611. 



X Philosophical Transactions, 1834, p. 242. 



