Measuring high voltage with an electrostatic voltmeter

    Just like the N and S poles of a magnet have an attractive force, when a voltage is applied to the electrode plate, an electrostatic force is generated. The measuring device made by using the electrostatic force is called an electrostatic voltmeter.
    Figure 5.133 (a) is the principle diagram of the electrostatic voltmeter. The measured voltage V is applied to the fixed plate and the movable plate. The two plates generate an attractive force because they carry charges ① and O respectively, causing the movable plate to move toward the fixed plate until the attractive force F is balanced with the spring force. The movement of the movable plate is converted into the deflection of the pointer, and the voltage is measured. The relationship between the measured voltage V and the attractive force F is
    F=k Vz (N)
, that is, the attractive force between the plates is proportional to the square of the measured voltage. The electrostatic voltmeter can be used to measure DC voltage or AC voltage (effective value). The electrostatic voltmeter shown in Figure 5.133 (b) can measure voltages below 50kV. When it is higher than 50kV, a high voltage multiplier should be used.

    Measurement of large DC current. The maximum measurement current of a portable DC ammeter is 100A. When measuring large currents of 100A to several thousand A, an external shunt is generally used due to the increased heat generation of the shunt. When measuring, a millivoltmeter is used to convert the voltage drop of the shunt into a current value.
    Figure 5.134 (a) shows a measurement circuit for measuring large currents by connecting a millivoltmeter to a shunt made of a plate resistor. It is difficult to manufacture a shunt above 10kA. For large currents of 1kA to tens of kA, a DC transformer can be used for measurement. Compared with the shunt, the measurement accuracy of the DC transformer method is worse, but the measurement instrument is isolated from the high voltage, and the ability to achieve safe measurement is the advantage of this method.