Set the plate at about 2 cm separation and insert the electrostatic voltmeter probe as far as possible.  Charge the parallel plates with the high voltage source to a potential of 1 to 2000 volts as measured by the voltmeter.  Wait a few seconds to show that the voltage applied is not leaking off.  Then bring the americium or polonium source toward the separation of the plates.  When the source gets within 6 cm of the plate the voltage should start to decrease.  The closer the source is to the plates the faster the discharge rate.

NOTE:  This works best during the winter months when there is lower humidity.

A less intensive way to do this is to charge one of the electroscopes negatively with the cat's fur and PVC rod.  Bring the brush that has the polonium 210 source in it near to the ball of the electroscope and observe the discharge.

The brush with the Polonium 210 source can be purchased from the camera store.  It is used to neutralize the static from dust so that it can be wiped away.  The source will last for several years before it needs to be replaced.  It is a very convenient source of alpha particles that can be used for several demonstrations.