This demonstration is currently on display in Strobe Alley (fourth floor of Building 4).
There is "capacitance" between every electrically conducting object and any other conducting object. It is a measure of the amount of electrical energy that can be stored. The units are "farads" and "picofarads" (1 pF= 10^-12 F). Human beings are conducting objects and get charged up by shuffling across rugs, sliding out of cars, combing hair, etc. This apparatus will tell you your capacitance in picofarads relative to the surroundings ó it correlates with height and weight ó tall and/or heavy people have larger capacitance.
Stand on the plastic box about a foot away from the apparatus. Proceeding from left to right, moisten your finger, zero the instrument, charge yourself and measure.
The human body is an electrical conductor, full of salty fluids. Like any conductor, the human body has a capacitance, meaning that it stores electrical energy, with respect to its surroundings, such as the floor, the walls, or other people. Like a person's height or weight, a person's capacitance is one of his or her body's attributes. Of course, human attributes can be affected by the surroundings. For example, a person would weigh six times less on the moon than they would on the Earth. Similarly, a person's capacitance depends on many factors, including it's posture, its relative position, and its proximity to other electrically conducting things.
When the user touches the electrometer input, the electric charge he or she is carrying is shared with a 0.06 mF capacitor. The voltage that develops across this capacitor is measured with a high resistance voltmeter, which uses an op-amp with an input resistance greater than 100,000 MW, resulting in a time constant of approximately two hours. For purposes of this display, the gain has been set to one, so that the reading on the digital voltmeter equals the voltage across the 0.06 mF capacitor. In words, the user's capacitance multiplied by 600 volts equals 0.06 microfarads multiplied by the digital voltmeter reading; the digital meter reading, in millivolts, equals the user's capacitance, in picofarads.
The display uses an adjustable, regulated low voltage power supply (ranging from 1 to 12 volts at up to 1.5 amperes), which powers an emitter follower connected to the primary of a transformer made by eight turns on a 5 mH ferrite core inductor. The secondary of the transformer is tuned to about 220 kHz by a capacitative divider which provides positive feedback to the transistor base. A half wave voltage doubler then yields DC outputs between 100 volts and 1200 volts at less than one milliampere. One analog voltmeter indicates the output voltage on the charging plate (approximately 600 volts), while a second analog meter, connected in series to the charging plate, acts both as a current meter and as a limiting resistor. By touching the grounding switch and the charging plate simultaneously, both meters will read about the same, with a small difference originating from the user's non-zero resistance. The grounding switch also serves to activate a relay that drains the 0.06 mF capacitor at the electrometer input, thus resetting the meter to zero.