By inserting a large resistor in series with the probe and providing good electrical insulation, it is possible to create a probe that allows an ordinary voltmeter to measure very high voltages (up to about 50 kV). The value of the resistor must be chosen to form an appropriate voltage divider with the input resistance of the voltmeter. Because of the very high value of the resistor needed (several megohms), high voltage probes are mainly used for measuring DC and low frequency AC; the RC circuit that is formed with the parasitic capacitance of the voltmeter input will attenuate higher frequencies. http://en.wikipedia.org/wiki/Test_probe#High_voltage_probes
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