For years, I have experimented with flyback transformers and microwave oven transformers (MOTs).  High voltages, while potentially deadly if proper safety precautions are not taken, are incredibly fun to play with. Few things on this earth are as satisfying as drawing 6″ arcs with a MOT and witnessing the blinding, brilliant yellow emission spectrum of air followed by the intense smell of ozone. I needed a means to measure the output voltage of my high voltage power supplies, as my multimeter could only measure up to 600 volts. For a while, I would guesstimate the voltage by using a spark gap, and slowing decreasing the spark gap distance until the electric field exceeded the dielectric strength of air, which is approximately 3 kV/mm. Using this distance, and knowing the barometric pressure and humidity in your environment, it is possible to calculate the voltage at the spark gap. However, this process is tedious and is prone to much error, and I wanted something more precise and accurate. I decided the best course of action was to build a voltage divider. After I stumbled upon these high resistance voltage dividers on the Electronics Goldmine, this project came to reality.   The circuit is quite simple – it is nothing more than a restive voltage divider. R1 = 70 mega-ohms, R2 = 22 kilo-ohms   The relationship between input and output voltages are described by the equation: The resistor I bought was rated as 70 mega-ohms, but when measured with an LCR meter, it was truly 61 mega-ohms, so I can conclude that the tolerances of these surplus resistors were quite poor (which is OK in this application, however).  I recommend you measure the true value of your resistors rather than taking them at their rated value – your calculations will be much more accurate this way. The circuit was constructed in a plastic enclosure, and care was taken to space out the connectors as much as possible in order to prevent any chance of arcing or corona discharge. Proper high voltage connectors are quite expensive, however, so I used banana jacks out of convenience. I hope that the use of banana jacks does not give anyone a false sense of security when using this device – the thin plastic around the banana jacks will NOT provide adequate protection from voltages on the order of 10 – 20 kV. Do not put any part of your body anywhere near the device while high voltages are present. Wear proper, electrically insulative rubber gloves, and stand on an electrically non-conductive rubber mat, if possible. Do not touch the voltage divider or even your multi-meter while operating this circuit.   I rested the perfboard upon these plastic standoffs, so to keep all conductors as far as practically possible from the sides of the enclosure. The extra inch or so of air provides more insulation between the perfboard and the (semi-conductive) wood workbench.   Nice and easy build. I used ring terminals to make it easy to remove the board should I ever somehow burn out the resistor.   Here is the voltage divider in action, measuring the output of a microwave oven transformer. To calculate the output voltage, I used the formula above, and got a voltage of 1,980 RMS volts AC.