School of Physics Lecture Demonstration Manual

El-7 Kelvin Water Dropper Induction Device

Published: Tuesday 14 March 2006 - Updated: Thursday 31 March 2011


To demonstate a simple charge generating device.



Click on pictures to enlarge.

Kelvin's Water Dropper Induction Device

Kelvin Dropper Diagram 2

Kelvin Dropper Diagram


Discharge of Water Dropper Across Spark Gap

Kelvin's Water Dropper Induction Device


The Kelvin water dropper is an electrostatic generator capable of producing voltages in excess of 6000V.

The upper cannister contains water and two fine jets are produced by the droppers. Valves A and B are adjusted so that the water jets break into drops just before passing through the rings C and D. The drops then fall into the cans E and F. The cans are connected to an adjustable spark gap G. A Neon globe can be attached to one of the electrodes of the spark gap.

When adjusted correctly the neon flashes at a rate of once every 10 seconds. The distance between the spark gap electrodes can be enlarged and an electrostatic voltmeter connected into the circuit. The voltmeter will read full scale in a short time.

If the spark gap is increased and the voltmeter removed the electric field becomes so great that the smaller water droplets will go into orbit around the rings C and D. This is quite clearly visible with the aid of an appropriately placed light source.


It is necessary to assume that by some method (differing effects of ionised air drifting past the cans for example) the cans will drift apart in potential. Suppose the can at the right becomes positive; then its collar is also positive. The elctrical charges on the collar will induce opposite charges on the jets. A negative collar induces positive charge on the jet; positive charged drops break off and fall into the positive can, therby making it still more positive. At the other side the positive collar induces negative drops to make the negative can still more negative. Once started charges build up very rapidly.

Safety notes

Electrical safety 


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