The Flame Probe

I suspended a foil covered football from the ceiling and charged it up to +1000V. I used a flame probe to measure the electrical potential in the space around the ball. The flame probe is a hypodermic needle attached to a gas tap with a tiny flame on top. The tip of the needle is attached by wire to the plate of a gold leaf electroscope.  The large + voltage on the ball pulls free electrons up the wire from the plate of the gold leaf electroscope onto the tip of the needle. That leaves the electroscope positive so the gold leaf is repelled upwards. The angle is a measure of potential energy because work has to be done by the electric field of the ball against the attraction of the electrons to the nucleus. Energy is stored when work is done against an attractive force.

 There has to be a flame on the tip because the flame creates ions which discharge the free electrons. If they built up on the tip, they would cause a field that would distort the field being measured by the probe.

The closer the probe is to the ball, the stronger the field and hence the bigger the angle. We projected the gold leaf so that we could take a more accurate measurement of the angle.