**What it shows**:

Determine the capacitance of the human body as follows. Charge a person of unkown capacitance to 1000 volts. The person is subsequently connected (in parallel) to an external capacitor of known capacitance. The voltage measured across the capacitor combination allows one to determine the unknown capacitance of the person (typically between 180 — 200 pF).

**How it works**:

A 1000 volt power supply (output is in the microamp range) is used to put charge on a person. We assume that the amount of charge transferred to the body is Q = C_{body}V_{i}, where V_{i} is the initial voltage = 1000 volts and C_{body} is the capacitance of the body. We next assume that charge is conserved when the person is connected in parallel to the external capacitor, e.g. Q = Q_{body}+Q_{ext}. With that assumption, Q = C_{body}V_{f} + C_{ext}V_{f}, where C_{ext} is known (= 0.1 μF, for example) and V_{f} is the final voltage across the parallel combination. Combining the two equations allows us to solve for the capacitance of the body, C_{body} = C_{ext} (V_{f}/(V_{i}-V_{f})).

**Setting it up**:

If available, the PASCO model ES-9077 electrostatics voltage source is convenient to use, but any high-voltage power supply will do. Just be sure to current limit the output to 0.1 mA or less with a 10 MΩ (or greater) resistor. For the external capacitor, use a value between 0.1 μF and 0.001μF. You'll end up with higher final voltages with the smaller caps, but the results tend to be variable. Using the larger cap, the final voltage will be around 1.5 V and is more reproducible.

**Comments**:

It's actually pretty impressive to be able to obtain a value for the body's capacitance considering the simplicity of the experiment and the assumptions that are made in the derivation. The fact that the obtained value is the correct order-of-magnitude is positive testimony for these assumptions. On the other hand, the accuracy is probably only good to one or two significant figures at best.