naPS wrote:Is that thing rated for the voltages it's going to see? Typically they are only rated to a few hundred volts, AC or DC.
The ground lead is at ground potential. If you put a series sense resistor in line with it the potential on the laser side of the resistor will be E=IR so for a 1R sense resistor the voltage will be 0.020A * 1R = 0.020V or 20mV. The voltmeter has some similar arrangement internally when in "amps" mode.
This is why I was curious how these were wired up: You could put a 250R resistor in there and have a voltmeter read 0..1.0V where 1.0 is 100% power. The power dissipated in the resistor would be I^2 R or 0.1W.