236 ELEMENTARY LESSONS ON [CHAP. iv. 



would rise, were it not that, by the action termed 

 induction, just enough negative electrification appears on 

 it to keep its potential still the same as that of the earth. 

 The presence of the induced negative electricity on the 

 plate will attract the + electricity of the ball downwards, 

 and alter the distribution of the electricity on the ball, 

 the surface - density becoming greater at the under 

 surface, and less on the upper. The capacity of the 

 ball will be increased, and therefore its potential will 

 fall correspondingly. The layer of air between the ball 

 and the plate is acting like the glass of a Leyden jar. 

 The more the ball is lowered down the greater is the 

 accumulation of the opposite kinds of electricity on each 

 side of the layer of air, and the stress across the layer 

 becomes greater and greater, until the limit of the 

 dielectric strength is reached; the air suddenly gives 

 way and the spark tears a path across. The greater 

 the difference of potential between the two bodies, the 

 thicker will be the layer which can thus be pierced, and 

 the longer will be the spark. 



282. Conductive Discharge. If the discharge 

 takes place by the passage of a continuous current, 

 as when electricity flows through a thin wire from the 

 collector of a machine back to the rubbers, or from the 

 positive pole of a battery to the negative pole, the opera- 

 tion is termed a " conductive " discharge. The laws 

 of the conductive discharge are explained in Lessons 

 XXIX. and XXX. 



283. Convective Discharge. A third kind of 

 discharge, differing from either of those above mentioned, 

 may take place, and occurs chiefly when electricity of a 

 high potential discharges itself at a pointed conductor 

 by accumulating there with so great a density as to 

 electrify the neighbouring particles of air ; these particles 

 then flying off by repulsion, conveying away part of the 

 charge with them. Such connective discharges may 

 occur either in gases or in liquids, but are best mani- 



