The drawing below shows the interconnections between the various system components. There are three controls, an on/off switch and a reversing switch to change the potentials of the high voltage supply, plus a rotary control to adjust the voltage.
There are also two moving coil meters associated with this equipment and I
mention this here because the constructor may prefer to build the dual supply,
its controls and these meters all into one case.
The switch SW1 is a double pole double throw rated at least at 250 VAC. The current involved is tiny (microamps). This is the switch which reverses the high voltage suppy. SW2 is single pole double throw also rated at 250 VAC. Which serves to switch the high voltage supply in or out of circuit. In effect its on/off switch.
The actual voltage of this supply is adjusted by means of VR1 a 5K linear potentiometer.
The panel layout is open to the constructors preference - I prefer the connectors at the top of the panel and the controls below.
A 6 way cable (low voltage) with a 7 pin Din plug at either end connects this unit with the HRVM and needs to be long enough to enable the user to be well away from the plate if it is being used for atmospheric measurments. Something like 15 metres.
The final point is most important. The 0v line of the supply and the case MUST BE CONNECTED TO GROUND. If a mains power supply is used for the 12 volts then the mains earth will be sufficient but check that there is no significant voltage between this and true ground. If a battery is being used for the 12 volts then an earth connection must be made to the 0v line.