110 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XII.) 



secondary kind of action of the electricity, upon the particles which it finds in its 

 course and thrusts aside in its passage (1454.). 



1424. The spark maybe obtained in media which are far denser than air, as in oil 

 of turpentine, olive oil, resin, glass, &c. : it may also be obtained in bodies which 

 being denser likewise approximate to the condition of conductors, as spermaceti, 

 water, &c. But in these cases, nothing occurs which, as far as I can perceive, is at 

 all hostile to the general views I have endeavoured to advocate. 



The electrical brush. 



1425. The brush is the next form of disruptive discharge which I will consider. 

 There are many ways of obtaining it, or rather of exalting its characters ; and all 

 these ways illustrate the principles upon which it is produced. If an insulated con- 

 ductor, connected with the positive conductor of an electrical machine, have a metal 

 rod 03 of an inch in diameter projecting from it outwards from the machine, and 

 terminating by a rounded end or a small ball, it will generally give good brushes ; or, 

 if the machine be not in good action, then many ways of assisting the formation of 

 the brush can be resorted to ; thus, the hand or any large conducting surface may 

 be approached towards the termination to increase inductive force (1374.) : or the 

 termination may he smaller and of badly conducting matter, as wood ; or sparks may 

 be taken between the prime conductor of the machine and the secondary conductor 

 to which the termination giving brushes belongs : or, which gives to the brushes ex- 

 ceedingly fine characters and great magnitude, the air around the termination may 

 be rarefied more or less, either by heat or the air pump ; the former favourable cir- 

 cumstances being also continued. 



1 426. The brush when obtained by a powerful machine on a ball about 0*7 of an 

 inch in diameter, at the end of a long brass rod attached to the positive prime con- 

 ductor, had the general appearance as to form represented in fig. 3. : a short conical 

 bright part or root appeared at the middle part of the ball projecting directly from 

 it, which, at a little distance from the ball, broke out suddenly into a wide brush of 

 pale ramifications having a quivering motion, and being accompanied at the same 

 time with a low dull chattering sound. 



1427. At first the brush seems continuous, but Professor Wheatstone has shown 

 that the whole phenomenon consists of successive intermitting discharges*. If the eye 

 be passed rapidly, not by a motion of the head, but of the eyeball itself, across the 

 direction of the brush, by first looking steadfastly about 10° or 15° above, and then 

 instantly as much below it, the general brush will be resolved into a number of indi- 

 vidual brushes, standing in a row upon the line which the eye passed over ; each 

 elementary brush being the result of a single discharge, and the space between them 

 representing both the time during which the eye was passing over that space, and 

 that which elapsed between one discharge and another. 



* Philosophical Transactions, 1834, p. 586. 



