This machine is a useful way of using static electricity to produce very high voltages. The machine is used both in schools and in nuclear research where it can produce up to 10 million volts!
A rubber belt passes a series of points or a sharp metal bar and
charge is sprayed onto the belt from there. This charge is carried up by the belt to another
series of points where it is collected and then stored on a large metal dome. As the belt
rotates more and more charge is given to the dome.
If the small, earthed dome, or
any earthed object, is put near the big dome a spark will jump across to it - the charge is
trying to get to the earth. The bigger the voltage the further the spark will jump. In dry air, a
voltage of at least 35 000 V is needed to make a spark 1 cm long! If the air is damp the
charge will leak away through the moisture in the air and you will not be able to get such a
large spark.
As you know charge is concentrated at points and so if a pointed object
is brought near the dome the charge will leak away to it - this will work with your
hair!
You can let a charge leak away through you to the ground if you put your knuckle near the large dome - a spark
will jump to it. The voltage is very high but because the charge transferred is very small the
current is minute you won't get a bad shock and it is not really dangerous. However never
allow a spark to go from the dome to any point near your head or your
heart!
The size of the electrical potential on the dome of a Van de Graaff
generator may be found by measuring the mass of the largest piece of paper that is repelled
from the surface of the dome when the generator is switched on.
