with the Bru*h Discharge. 41 



to what would be produced by the same speed of rotation 

 between knobs of one centimetre diameter. (See Joubert, 

 Foster and Atkinson, ' Electricity and Magnetism/ p. 103.) 



In all cases the results have been produced at ordinary 

 atmospheric temperatures and pressures, but, of course, the 

 brilliancy of the effects varies with the climatic conditions. 

 For this reason no attempts have been made to measure the 

 size of the brush, because it differs so much from day to day. 

 The experiments described can be reproduced under varying 

 conditions, and the effects may therefore be regarded as normal 

 accompaniments of this kind of discharge. 



1. Shape of the Brush-Discharge. 



As is well known, if a discharge of negative electricity takes 

 place from a pointed conductor, and if the point be examined 

 in a darkened room, it will be seen that it is surrounded by a 

 faint spot of light of a violet, or violet-blue colour. If, on the 

 contrary, a similar experiment be made with positive electricity, 

 the point will be seen to be surrounded by an innumerable 

 number of lines of light of a similar violet-blue colour, forming 

 what is called the brush. It is stated, on the authority of 

 Faraday, that the glow which surrounds the negative point is 

 separated from it by a dark space. Undoubtedly this is the 

 case when the discharge is taken in rarefied air, but at the 

 ordinary pressures I have been unable to detect it, although 

 the brush has been examined under the microscope. The 

 glow seems to be in contact with the point. The positive dis- 

 charge, however, behaves differently. When carefully observed 

 it is seen that the lines do not start from the exact end but at 

 some slight distance away (2 or 3 millims.). They appear to 

 keep in a bunch for a little distance and then to diverge. The 

 size of the positive brush is much increased by the proximity 

 of an earth-connected plate or sphere, and the outline of the 

 luminous portion is altered by the shape and nearness of this 

 " earth." Thus, when an u earth " is some distance away, the 

 emanation from a point may take the shape of a fan with the 

 side lines at right angles to the point as in the figure. If to 

 such a brush an " earth " be brought to within a few centi- 

 metres, the lines will curve themselves round and the angle 

 of the fan instead of being 180 degrees will become much 

 less. The glow at a point giving negative electricity becomes 

 brighter if an earth-connected body be brought near, but it 

 does not alter in size until the body is very close (less than a 

 centimetre), when small sparks pass between the point and 

 the body. 



