EECENT PROGRESS IN PHYSICS. 373 



§ 82. The electrical brush. — The most important facts which Fara- 

 day has obtained in reference to the brush are the following,) Poff. 

 Ann.,XLVII:) 



'' The brush and spark gradually pass into each other." (Faraday 

 calls the electrical brush " a spark to air.") " Making a small ball 

 positive by a good electrical machine with a large prime conductor, 

 and approaching a large uninsulated discharging ball towards it, very 

 beautiful variations from the spark to the brusli mav be obtained. 

 The drawings of long and powerful sparks, given by Van Marura, 

 (description of the large machine in Taylor's museum, Grerman trans- 

 lation of 1786, Tab._ III, fig. 1 ;) Harris, (Phila. Trans., 1834, p. 243,) 

 and others, also indicate the same phenomena," namely, a ramification 

 of the spark by which its transition to the brush is made. — (Fara- 

 day's Researches, § 1448.) 



"If an insulated conductor, connected with the positive conductor of 

 an electrical machine, have a metal rod 0.3 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 ;" 

 " or the termination may be smaller and of badly conducting matter, 

 as wood ; or sparks may be taken between the prime conductor and 

 the secondary conductor, to which the termination giving brushes 

 belongs;" ''or the air around the termination may be rarefied." — 

 (1425.) 



That the brush is not a continuous discharge is evinced in the 

 gradual transition of the spark to the brush. By proper proportion, 

 in the size of the small knob to the power of the machine, brushes are 

 obtained which show immediately that they consist of ramified sparks 

 rapidly following each other ; the machine being worked more rapidly, 

 or with the same working of the machine substituting a still smaller 

 discharging knob, the brush assumes a more uniform appearance, 

 which Faraday very well describes in the following words : "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." — (1426.) 



At first such a brush seems continuous, but Wheatstone has shown 

 that it consists of successive intermitting discharges, (Philos. Trans., 

 1834, p. 586,) which was to be expected from the gradual transition 

 of the spark to the brush. Faraday gives a very simple method for 

 decomposing the apparently continuous brush into its elementary 

 parts without the help of Wheatstone's rotating mirror ; he says : "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 stead- 

 fastly about 10° or 15° above, and then instantly as much below it, 

 the general brush will be resolved into a number of individual 

 brushes." — (1427.) This method of analyzing has not succeeded 

 perfectly in my trials. 



