LET’S LOOK AT THE INSTRUMENTS THAT ENGINEERS USE TO MEASURE ELECTRICAL QUANTITIES. Some measuring devices work because electric and magnetic fields produce forces proportional to the field intensity. Some meters determine electric current by measuring the amount of heat produced as charge carriers move through a medium with a known resistance. Some meters have small motors whose speed depends on the measured quantity of charge carriers. Still other meters count electrical pulses or cycles.
Early experimenters noticed that an electric current produces a magnetic field. They could detect this field by placing a magnetic compass near a wire. The compass pointed toward magnetic north when the ...