5 PHYSICS. 
wire to an iron stand. A conductor cut off from communication with the 
ground by being fixed on non-conductors, in this way, is said to be 
ansulated. 
Let us now return to our first experiment. We saw by it that positive 
electricity attracted negative, and negative, positive ; and also that both 
attracted bodies not electrified at all: it will now be shewn that the last 
part of this statement is the same as the first. If a brass cylinder be fixed 
on a glass stand, with a pair of pith-balls suspended by a cotton thread 
(not silk this time, because it is not desired to insulate them from the 
cylinder) at either end, and placed near another insulated conductor 
charged with positive electricity, this 1s what will happen—the balls will 
be repelled from each other, as in the figure, 
shewing that each pair has become charged with 
the same kind of electricity. But it is found . 
that the electricity at the two ends is of different 
kinds, that in the end next the ball being nega- 
tive, while that in the other end is positive. 
This is quite in accordance with the result of 
our first experiment, that positive electricity 
attracts negative, and repels positive, and vice 
versd. From this it is manifest, that in all conductors there exist (with- 
out friction) the two kinds of electricity, and that these are acted upon 
by an electrified conductor exactly in the same way as if the electricity 
had been produced by friction in both. It will now be seen that, 
when the rubbed glass attracts the pith-ball the first time, the attraction 
is not different from the subsequent attractions; the positive electri- 
city in the glass repels the positive in the ball to the opposite side, 
and attracts the negative. This attraction is great enough to overcome 
the slight weight of the ball; when contact takes place, the negative 
electricity in the side of the ball next the glass is replaced by positive 
electricity from the glass. The ball is then charged altogether with 
positive electricity, and of course is now repelled by the giass ; but at the 
other side, the sealing-wax which is charged with negative electricity 
attracts it; and so on. The influence which an electrified conductor thus 
exercises on a non-electrified one is called Induction. 




Lightning. 
In the case of the pith-ball, when induction takes place, owing to its 
lightness and the manner in which it is suspended, it comes in contact — 
with the glass, and electricity thus passes to it from the glass; but if, in 
fig. 33, there were a sufficient quantity of electricity in the ball, and the 
attraction were great enough, it would pass to the cylinder without contact ; 
and there would be a slight flash of fire and a slight crack. Now, this flash 
