Action of highly charged points
When a sharp pointed wire is connected to the conducting knob of a Wimshurst or other electric machine, and the machine set in motion, the surface density of charge on the point becomes exceedingly high and a current of air known as an electric wind streams away from the point. This electric wind may be demonstrated by placing a candle flame a short distance away from the point. The flame is deflected by the draught, and may even be blown out (Fig. 32.17 (a)).
The electric wind may also be used to work a simple type of jet-propelled motor known as Hamilton’s mill. This consists of several wires arranged as the spokes of a wheel on an insulated pivot and having their ends bent at right angles (Fig. 32.17 (b)). When connected to an electric machine an electric wind streams out from the ends of
the wires, and the resulting reaction on the wires causes the mill to rotate in the
opposite direction. In these experiments the electric wind is caused by point action, and may be explained as follows (Fig. 32.17 (c)).
We have already seen that ordinary air contains a certain number of positive and negative ions. Therefore, in the neighbourhood of a highly charged point conductor ions of the same sign will be strongly repelled. When these fast-moving ions collide with air molecules they often knock electrons out of them, thus creating more ions. This process is cumulative, so that, in a very short time, an avalanche of ions will be moving rapidly away from this point. The surrounding air molecules get caught up
in this stream and carried along with it, thus setting up the electric wind. At the same time ions of opposite sign are attracted towards the point, where they neutralize the charge on it. It has become customary to speak of charge as being “sprayed off” a
point, but this is merely a figure of speech. It cannot be compared with, say, water sprayed out of a hosepipe.