14 STATIC ELECTRICITY 



mutually inducing charges rise above a certain degree a Hash or 

 flashes may ensue between the clouds and conducting projections 

 from the earth, such as buildings, trees, or conducting rocks. But 

 if there be a lightning conductor with a sharp point directed 

 upwards and well connected below with the earth, the earth charge 

 may pass off' continuously by the point towards the cloud and may 

 neutralise the charge there, or at any rate prevent it from rising 

 to sparking intensity. We may illustrate this action of a lightning 

 conductor by placing a sharply pointed earth-connected metal rod 

 near the prime conductor of a machine. If the machine is now 

 turned the discharge from the point to the prime conductor 

 prevents the gathering of a charge in the latter, and this may be 

 shown by the fact that a gold-leaf electroscope placed near the 

 prime conductor is nearly unaffected. Here the prime conductor 

 may be taken to represent a cloud and the sharp point a lightning 

 conductor. If the point is now covered by a round metal knob 

 the continuous discharge from it ceases and the charge on the 

 prime conductor gathers till there is a spark from it to the knob. 

 The electroscope indicates the gathering of the charge and it^ 

 dispersal in the spark.* 



Induction machines. The electrophorus. TheeKrtm- 

 phorus is exceedingly useful for the production of a aeries of small 

 charges. It consists of a flat cake (KR, Fig. 1^) of some resinous 

 material, say of vulcanite, resting on a metal plate SS. called the 



St 



FJG. 12. 



sole plate, and a movable metal cover CC, furnished with an 

 insulating handle H. The surface of the ebonite is first electrified 

 by friction, say by beating with cat-skin. The negative charge 

 thus developed induces a positive charge on the upper surface 

 of the sole, which, lying on the table, is earth-connected. The 

 cover is now placed on the ebonite. Since the two surfaces are 

 not perfect planes they are not absolutely in contact except at 



* For an account of the phenomena of lijrhtnini; <li>cli;ir^c> and tin- u.-r of 

 lightning conductors^ see Sir Oliver Lodge's Li'jhtuii.y Cuitductorv and Liyktniiiy 

 Cf nurds. 



