The correct answer is: D. electrodynamic voltmeter.
An electrodynamic voltmeter is a type of voltmeter that uses the principle of electromagnetic induction to measure voltage. It consists of a coil of wire that is placed in a magnetic field. When a voltage is applied to the coil, a current flows through it, which creates a magnetic field. This magnetic field interacts with the magnetic field of the permanent magnet, which causes the coil to move. The amount of movement of the coil is proportional to the voltage applied to it.
Electrodynamic voltmeters are very accurate and can be used to measure voltages over a wide range. They are also very stable and have a long life. However, they are also relatively expensive and bulky.
The other options are incorrect for the following reasons:
- A. Electrostatic voltmeters are not very accurate and are not suitable for measuring high voltages.
- B. Hot wire voltmeters are not very accurate and are not suitable for measuring low voltages.
- C. Isothermal voltmeters are not very accurate and are not suitable for measuring DC voltages.