Museum
SM002
Electrostatic Voltmeter
Lord Kelvin devised a very sensitive electrometer to make possible
accurate measurements of potential.
Two pairs of quadrants form a flat cylindrical box.
Alternate pairs of quadrants are connected together.
Each of the pairs is then connected to terminals on the outside
of the instrument.
In the middle of the quadrants is hung by means of a conducting
suspension wire what is termed a needle but is in fact a light vane.
This is also terminated on the outside on top of the instrument.
In use, the vane is charged to a high potential with respect to the quadrants. If a potential difference of a few volts is present between adjacent quadrants, the vane will tend to rotate to equalize the forces, being repelled by the quadrants of similar charge and attracted by those of opposite charge. A mirror attached to the vane will indicate the slightest movement due to this potential difference by means of a light beam projected on to a horizontal scale. The deflection of the needle is directly proportional to the potential difference between the quadrants.
