Electricity and Magnetism > Electrostatics DCS# 5A40.70


charging by contact

Kelvin water dropper 202-18-E
container with plexiglas window 202-18-E
bilge pump 202-18-E
3 A DC power supply 202-08-D

Fill container with water until pump intake is submerged.  Adjust flow so that water breaks into droplets before reaching top cans.  Neon bulb should light periodically and spark should jump gap between balls.


This is ordinarily used in the display case.  Here is the text that goes with it:

Notice how the two cans into which the streams of water fall are cross-connected to the two tubes.  Assume that the can on the right becomes slightly charged when a stray ion in the air sticks to it.  The left tube becomes charged since it is connected to the can.  A charge of opposite sign is induced on the water stream above the tube.  When the water separates from the stream, it carries this charge down into the left-hand can below and charges it and the right-hand tube.  The result is that the two tubes acquire charges of opposite sign and induce opposite charges on the two streams of water, thus building up the charges on the tubes until the electric field between the two metal spheres is strong enough to cause breakdown of the air so that a spark jumps and the discharge current lights the neon bulb.  The entire process then begins again.  Potential differences of thousands of volts can easily be produced this way.  This is a beautiful example of the action of positive feedback.


Meiners, pp. 247-250.