ELECTROSTATIC METER MOVEMENT

The final meter movement covered in this chapter is the ELECTROSTATIC

METER MOVEMENT. The other meter movements you have studied all react to current, the electrostatic meter movement reacts to voltage.

The mechanism is based on the repulsion of like charges on the plates of a capacitor. The electrostatic meter movement is actually a large variable capacitor in which one set of plates is allowed to move. The movement of the plates is opposed by a spring attached to the plates. A pointer that indicates the value of the voltage is attached to these movable plates. As the voltage increases, the plates develop more torque. To develop sufficient torque, the plates must be large and closely spaced. A very high voltage is necessary to provide movement, therefore, electrostatic voltmeters are used only for HIGH VOLTAGE measurement.

VOLTMETER SAFETY PRECAUTIONS

Just as with ammeters, voltmeters require safety precautions to prevent injury to personnel and damage to the voltmeter or equipment. The following is a list of the MINIMUM safety precautions for using a voltmeter.

• Always connect voltmeters in parallel.
• Always start with the highest range of a voltmeter.
• Deenergize and discharge the circuit completely before connecting or disconnecting the voltmeter.
• In dc voltmeters, observe the proper circuit polarity to prevent damage to the meter.
• Never use a dc voltmeter to measure ac voltage.
• Observe the general safety precautions of electrical and electronic devices.

Q.38 What type of meter movement reacts to voltage rather than current?
Q.39 What is the only use for the voltage sensitive meter movement?
Q.40 List the six safety precautions for the use of voltmeters.