TABLE OF CONTENTS Before the advent of high-input impedance meters, the moving coil meter was the basis of most small current measurements. But all current meters, whether of the contemporary or ancient variety, must use shunts to measure large (relative to the meter movement sensitivity) currents. A shunt is a low-resistance conductor of a precise resistance. Current is derived by the voltage dropped across the shunt (nowadays). With analog meters, the current would provide a parallel path for current flow, "shunting" a large (compared to the meter fsd current) current around the meter measuring circuit. Again, to measure currents larger than the full-scale deflection current (analog meter) or those currents beyond the range of the built-in shunts of a digital meter, external shunts must be employed. The purpose of this chapter is to give you a solid understanding of current-measuring devices and the precautions necessary when measuring currents.
In this section we will focus on permanent magnet-type moving coil meters (analog meters). However, many of the techniques used with these devices apply to digital meter applications as well. The main difference between analog and digital is in how the current is displayed, not in how it is measured. Permanent magnet moving coil meters indicate measurements by the amount of current flowing through the coil. This current causes the meter pointer to deflect when the coil's magnetic field interacts with the permanent magnet field. Moving coil type meters are generally specified by their full-scale deflection (fsd)...
