What is it?
It is used to measure high voltage. They are used to measure high ac voltage. As generating voltmeter have very high impedance, they do not load the source. As it does not make contact with the high ac voltage device, it is useful. It is a electrostatic generator which generates current proportional to external applied voltage
I am refering to Generating voltmeter figure number 7.4 of MS Naidu book.
This figure closely resembles like this
It can be used to measure voltage if the frequency of the motor is half the frequency of external voltage.
For example, if we have to measure 50Hz external voltage, then the motor should have the frequeny of 25Hz. A 4 pole motor running at 1500 rpm has freqency of (NP/60 = 1500 * 4 / 60 = 25 Hz)
Here the S3 is external high voltage which has to be measured. The S0, S1 and S3 are low voltage electrodes.
How does it work?
When the motor is rotated at suitable speed, there is capacitance between S4 and S3. The S1 and S2 electrodoes cause sinosuidal variation of capacitance. Due to this, the ac current is measured by the a current measuring device.
The scale of current and voltage is linear. So by extrapolating the current voltage graph, we can know the value of current at high voltages.