# Electric OU: Supplement: Calibration of Current Measuring Instruments

The Hickok moving coil meter is calibrated against Ohm’s Law using a known resistance and a regulated power supply. The measurement is compared with that made using the cheapo CenTech DMM (on sale at Harbor Freight for 3 dollars — including battery!) Next, I show the current reading on the oscilloscope, made by eyeballing the voltage drop across a 0.3 Ohm resistance in the circuit, using Ohm’s Law and the scope amplifier settings to arrive at a current figure with less than 5 percent error compared to the actual metered and computed Ohm’s Law value. NOTE WELL you NERDs: You have to take into account the value of the current viewing resistor when you compute the current value. You didn’t do this in your scope math in the demo video from last March… you used the entire voltage drop value, as if you had a 1 Ohm CVR…. but your CVR was 0.25 Ohms nominal.