* The Van de Graaff generator produces 50-150 kVDC. This is sufficient to throw a five inch spark on a dry day or deflect pithballs and the hair of student volunteers.
Further information on Theory, Apparatus, Procedure, and Helpful Hints is available. The equipment for this demo is stored in Rockefeller rooms 302 and 302B.
The purpose of this experiment is to show that when two conducting spheres come into internal contact, (one is charged and the other is not) the result is that all of the potential is transfered from the charged sphere to the uncharged sphere. The second sphere can be charged to any desired value of potential, with the max depending on the rate of leakage.
The Van de Graaff generator demonstrates that if a charged body is brought into internal contact with a hollow conductor, all of the charge will transfer to the outer conductor, regardless of its potential. Thus, if the insulation was flawless, the potential could be raised to any desired value by simply adding more charges through internal contact.
Our generator works as follows: supporting the hollow spherical conductor is an insulating hollow column in which we find a belt passing over two pulleys, the lower of which is driven by a motor. At the bottom by the motor is a terminal projecting sharp points. In an efficient VDG, this terminal would be at a negative potential of a few hundred volts. In our machine, the electron emission is driven by random processes having to do with atmospheric electricity. In any event, patches of electrons are emitted onto the belt. At the top of the belt (at the other pulley which is located inside the sphere) is a terminal like the lower one. The air around the points at the top and at the bottom becomes ionized, and a "spray" of ions is repelled from each set of points--negative from the bottom set, and positive from the top set. The ions attach themselves to the belt in such a way, that on the way up, the belt is negative, and on the way down it is positive. The electrons unload thenselves onto the points inside the upper sphere, because it is energetically favorable for them to do so. This leaves the top sphere negative. On a dry day, a current of about 5 microamperes can be produced. This charges the top terminal to about 150 kV before leakage through the atmosphere limits the charging process.
under construction