VAN DE GAFF GENERATOR

A Van d Gaff generator is a device for generating very high voltage and collecting extremely high amount of charge. It is based on of discharging action of points and accumulation charges by a large hollow spherical conductor. In 1929, Robert J. Van d Gaff designed these machines, which was later named after him.

It consists of a large hollow metallic dome mounted on an insulating stand. There are two types pulley, one inside the dome and other is near the bottom as shown in the figure 32.17. A belt of insulating materials, e.g. Silk, goes over the pulley. The lower pulley is driven by an electric motor (or, by hand for a small demonstration machines). There are two metals brushes having a number of sharp needles near the two pulleys. The needles are pointed towards the belt without touching it. The upper brush is connected to the metallic dome from inside whereas the lower brush is connected to a source of high positive potential.

Because of the high electric field near the needles of the lower brush, the air molecules become charged due to discharging action of points and air in between begins to conduct. The positive charge forms those needles leak through those sharp ends and reaches the belt. The belt moves upwards and carries those sharp ends and reaches the belt. The belt moves upward and carries those charges up. When it reaches the upper brush needles, induction takes place and their interior of the dome develops negative charges and outer surface becomes positively charged to the same extent. Since the upper brush needles are connected to the interior of the dome, the positively charges on the belt are neutralized by the discharging action of those brushes. Thus, electricity neutral belt moves down on other side of the pulley. However, the outer part of the dome accumulates positive charges.

Thus, these machines continuously transfer positive charge of the sphere. Potential (we will study about it in the next chapter 34) of the dome increases continuously as the belt moves and its value may reach higher as 10^6 V until dielectric breakdown stars. After that, it can not be charged further as the air starts to conduct as this potential and charges leak to the earth or nearby metal through air. However, it is possible to increase further the amount of charge on the dome if we enclose it in a highly evacuated chamber.