Moving coil meters are the most satisfactory for this type of work as the
measurements are rarely steady and this confuses DVMs. A pair of moving-coil
meters of 100 micro amps full scale are required therefore. One for the HRVM
output and one to show the output of the high voltage unit.
The HRVM meter is slightly complicated by the need to observe both positive and
negative readings. The arrangement is shown below.
The HRVM meter M1 has a double pole double throw reversing switch SW1 which enables the meter to be switched for either positive or negative input. Assuming 100 micro amp meters R1 is 100K.
It will be found that a centre zero is helpful when balancing the high voltage output against the plate. This is achieved by means of the 2 cell battery B1, SW2, R2 and VR1, which are 30K and 50K respectively. Switching on SW2 enables the meter to be set to mid scale (switch everything else off while adjusting VR1). In this way it enables plus or minus 5 volts to be read.
The second meter M2 is required to read 0 to 300 volts, consequently R3 must be 3M. Note M2 must not be more than 100uA FSD otherwise the required voltage will not be achieved. This is because the high voltage supply cannot supply enough current to be dangerous.