Explain the principle of a device that can build up high voltages of the order of a few million volts. Draw a schematic diagram and explain the working of this device. Is there any restriction on the upper limit of the high voltage set up in this machine? Explain.
Solution
Principle: if a small charged conductor is placed inside and in contact with a large hollow conductor, all its charge flows to the outer surface of the hollow conductor, whatever the latter's potential. Working: a motor-driven insulating belt carries charge sprayed by a comb at the bottom up into a large metal sphere, where a second comb transfers it to the sphere's outer surface; repetition raises the sphere to millions of volts, used to accelerate ions/protons. Restriction: the maximum potential is limited by the breakdown (ionisation) of the surrounding air or gas — beyond a certain field, charge leaks away by corona discharge. Enclosing the sphere in a vessel of gas at high pressure raises this limit.
