Differential scope probes?

[Dave Cigna]

Differential probes are handy for measuring current across a small sense resistor. If there's anything interesting to look at then there's probably also a relatively large AC voltage present. The two-probe method doesn't always work in that case because most of the signal is common mode which chews up the dynamic range of the vertical deflection amplifiers. Differential probes perform the subtraction before sending the signal to the vertical deflection amp thus removing the large common mode component.

Really, that's what differential probes are for; looking at a small differential signal on top of a large common mode signal.

Good ones are expensive. We just ordered one from Tektronix at work. I don't recall exactly what it cost, but it was more than most people pay for a scope. At home I have an old Tektronix scope and a differential plugin that I use from time to time, often for current measurements, but I have used it to look at balanced circuits when the two-probe method would probably also work.