Right. Try measuring the resistance or should I say conductance, of a piece of paper. I could.
There are different techniques for measuring pA, amps and hundreds of amps. Between 1 nd 100 mA and low voltages, there isn't much out there to use besides Agilent's and Keithley's Source Measure Units. There is nothing that will measure in an analog way 100 mA to 0.01 mA in 4 ranges, so I had to build it. Biasing was +-10V, it was 4 and 2 terminal, 4 quadrant, +-10V out and a V(open circuit) mode. This was a front end to a Lock-in. 0 to -1V bias was it's usual operating parameters. It also sported a clipping indicator. Autoranging was incorporated in the high level program.
Zero Check and Zero correct were problematic and never fixed, but the DC performance was secondary to the application. Ther main issue was that the IEEE-488 bus D/A converter didn't output exactly zero volts. The IOTech version of the same Keithley product supposedly did. They used a relay to output exactly zero bearing in mind thermal EMF's. I also left out all of the offset trimmers. Offsets were below a few uV anyway, but they really matter.
For fun and giggles, I added a +-50 mA suppression, but only if the bias was between +-5V.