It is a needle type voltmeter. It can be used to measure the potential difference of direct current. and can choose to measure the difference The electric potential can be 0-3 V, 0-15 V, 0-30 V and 0-300 V.
The machine is a moving coil.
There is a button for adjusting the needle to the center point.
There is a user manual. and the experimental manual in Thai language