Tom,
Same here! Spent many hours calibrating industrial 'controlling' meters; those with a small metal flag on the pointer that changed the capacitance of the circuit when it passed between two metal plates (the other poles of the capacitor). Of course they had to read the same on all three axes; this was done by adding or removing tiny blobs of rosin from selected points on the movement arm pivot assembly. Current standard was a platinum/platinum 10% rhodium thermocouple, traceable to NBS, in a 5 gallon distilled water ice bath. Needless to say, before doing the calibration the pivot jewels had to be cleaned or replaced, every minute speck of dust removed, and the pivot tension and movement springs adjusted properly. Not fun, but necessary to get FS accuracy of +/- .05mA. Today you just hook up a 20-cent op amp as a differentiator and you've got the same; no, BETTER!, thing.