116 DR. FARADAY'S EXPERIMENTAl, RESEARCHES IN ELECTRICITY. (SERIES XII.) 



ures, the whole of the electricity has not been discharged, but only portions of it, 

 more or less according to circumstances: whereas, whenever the effect has been a 

 distinct spark throughout the whole of its course, the discharge has been perfect, 

 provided no interruption had been made to it elsewhere, in the discharging circuit, 

 than where the spark occurred. 



1449. When an electrical brush from an inch to six inches in length or more is 

 issuing into free air, it has the form given, fig. 3. But if the hand, a ball, or any 

 knobbed conductor be brought near, the extremities of the coruscations turn towards 

 it and each other, and the whole assumes various forms according to circumstances, 

 as in figs. 5, 6, and 7- The influence of the circumstances in each case is easily 

 traced, and I might describe it here, but that I should be ashamed to occupy the 

 time of the Society in things so evident. But how beautifully does the curvature of 

 the ramifications illustrate the curved form of the lines of inductive force existing 

 previous to the discharge ! for the former are consequences of the latter, and take 

 their course, in each discharge, where the previous inductive tension had been raised 

 to the proper degree. They represent these curves just as well as iron filings repre- 

 sent magnetic curves, the visible effects in both cases being the consequences of the 

 action of the forces in the places where the effects appear. The phenomena, there- 

 fore, constitute additional and powerful testimony (1216. 1230.) to that already given 

 in favour both of induction through dielectrics in curved lines (1231.), and of the 

 lateral relation of these lines, by an effect equivalent to a repulsion producing di- 

 vergence, or, as in the cases figured, the bulging form. 



1450. In reference to the theory of molecular inductive action, I may also add here, 

 the proof deducible from the long brushy ramifying spark which may be obtained 

 between a small ball on the positive conductor of an electrical machine, and a larger 

 one at a distance (1448.). What a fine illustration that spark affords of the pre- 

 vious condition of all the particles of the dielectric between the surfaces of dis- 

 charge, and how unlike the appearances are to any which would be deduced from 

 the theory which assumes inductive action to be action at a distance, in straight 

 lines only; and charge, as being electricity retained upon the surface of conductors 

 by the mere pressure of the atmosphere ! 



1451. When the brush is obtained in rarefied air, the appearances vary greatly, 

 according to circumstances, and are exceedingly beautiful. Sometimes a brush may 

 be formed of only six or seven branches, these being broad and highly luminous, of 

 a purple colour, and in some parts an inch or more apart : — by a spark discharge ^t 

 the prime conductor (1455.) single brushes may be obtained at pleasure. Discharge 

 in the form of a brush is favoured by rarefaction of the air, in the same manner and 

 for the same reason as discharge in the form of a spark (1375.) ; but in every case 



