GENERAL ACCOUNT OF COMMON PHENOMENA 13 



of the plate, the excellence of its surface and of the surface of the 

 rubbers. The advantage of the particular structure chosen for 

 the ring is not very clear. It certainly makes the sparks brighter, 

 and this is probably explained by the fact that it increases the 

 capacity of the prime conductor so that more electricity is stored 

 in it before a discharge occurs, and more, therefore, passes in the 

 discharge. 



The ease of discharge from points. If a very sharp 

 metal point is placed on the prime conductor it is found that dis- 

 clrirge takes place from it continuously, the air or dust in the air 

 acting to convey the charge to the surrounding and oppositely 

 charged conductors. We need not here discuss the mode of dis- 

 charge. It is sufficient at this stage to say that the electrification on 

 any conductor tends to accumulate at any projecting point on the 

 conductor and to a greater amount per unit area the sharper the 

 point. It appears then that even with comparatively weak charges 

 the crowding of the electrification on to a point may be so great 

 that the air is unable to insulate, and discharge occurs through it. 

 The point appears in the dark to glow. If the hand be held over 

 the point the impact of the electrified air streaming from it is dis- 

 tinctly felt. A sort of Barker"* mill may easily be made to work 

 by means of thi* point discharge. A light k * whirl " represented in 

 Fig. 11, con*i*ting of a number of spokes with ends pointed and 

 bent tangentially, is pivoted on a vertical ^ 



metal *upport on the prime conductor of a 

 machine. When the machine is turned the 

 uhirl rotate* uith the points directed back- 

 ward*. \Yc may explain the motion thus: 

 Were the point* covered with some efficient 

 insulator the whirl would be in equilibrium, 

 the pull on the strongly elect iifi<d points 

 being neutralised by the pulls over the more 



weakly electrified remainder of the surface. /s~ 



Hut as the points are not insulated the O( P-C. 



charge i* constant Iv streaming off them and ^ . 



*o the pull on them is diminished. The pull FIG. 11. 



on the rest of the surface is therefore not 



fully compensated and the whirl rotates. Or, without considering 



the arrangement of forces, it is enough to say that forward 



momentum is generated in the air streaming from the point, and 



that the equal backward momentum is manifested in the retreat 



of the point. 



Lightning conductors. The efficiency of lightning con- 

 ductors depends on the ease of discharge from points. A flash of 

 lightning is merely a spark discharge on a grand scale between 

 oppo*itr|y electrified clouds or between a cloud and the earth. 

 Suppose that a cloud is highly charged. The under surface induces 

 opposite electrification on the ground underneath it, and if the 



