LESSON'S IN "ELECTRICITY. 



311 



FIG. 28. 



then try the effect of presenting the finger 

 ends, instead of the knuckle, to the con- 

 ductor. The spark falls exceedingly in 

 brilliancy. Substitute for the finger ends 

 a needle point : you fail to get a spark 

 at nil. To obtain a good spark the elec- 

 tricity upon the prime conductor must 

 reach a sufficient density (or tension as it 

 i* sometimes called) ; and to secure this 

 no points from which thu electricity can 

 stream out must exist on the conductor, 

 or be presented to it. All parts of tho 

 conductor are therefore carefully rounded 

 off, sharp points and edges being avoided. 

 It u usual to attach to the conductor 

 an electroscope consisting of an upright 

 metal stem, A c, fig. 28, to which a straw 

 with a pith ball, u, at its free end, is at- 

 tached. The straw turns loosely upon a 

 pivot at c. The electricity passing from 

 the conductor is diffused over the whole 

 electroscope, and the straw and stem be- 

 ing both positively electrified, repel each 

 other. The straw, being the movable 

 body, flies away. The amount of the 

 divergence is measured upon a graduated 

 arc. 



18, Further Experiments on the Action 

 of Points. The Electric Mill. The 

 Golden Fish. Lightning Conductors. 



If no point exist on the conductor, Q 

 single turn of the handle of the machine 

 usually suffices to cause the straw to stand 

 out at a large angle to the stem. If, on 

 the contrary, a point be attached to the 

 conductor, you cannot produce a large 

 divergence, because the electricity, as 



FIG. 29. 



fast as it is generated, is dispersed by the 

 point. The same effect is observed when 

 you present a point to the conductor. 

 The conductor acts by induction upon the 

 point, causing the negative electricity to 

 stream from it against the conductor, 

 which is thus neutralized almost as fast as 

 it is charged. Flames and glowing em- 

 bers act like points ; they also rapidly 

 discharge clectiicity. 



The electricity escaping from a point 

 or flame into the air renders the air self 

 repulsive. The consequence is that when 

 the hand is placed over a point mounted 

 on the prime conductor of a machine in 

 pood action, a cold blast is distinctly felt. 

 Dr. Watson noticed this blast from n 

 flame placed on an electrified conductor ; 

 while Wilson noticed the blast from a 

 point. Jallabert 'and the Abbe Nollet 

 also observed and described the influence 

 of points and flames. The blast is called 

 the " electric wind." Wilson moved 

 bodies by its action : Faraday caused it 

 to depress the surface of a liquid : Ham- 

 ilton employed the reaction of the electric 

 wind to make pointed wires rotate. The 

 " wind " was also found to promote 

 evaporation. 



Hamilton's apparatus is called the 

 " electric mill." Make one for yourself 

 thus : Place two straws s s', 88% fig. 29, 

 about eight inches long, across each other 

 ht a right angle. Stick them together at 

 their centres by a bit of sealing-wax. 

 Pass a fine wire through each straw, and 

 bend it where it issues from the straw, 

 so as to form a little pointed arm perpea- 



