3-7
Figure 3-4.—Dc ammeter using the D'Arsonval movement with external shunts.
SHUNT CONSTRUCTION.—The shunt strips (view B of figure 3-4) are usually made of the alloy
Manganin. Manganin has a temperature coefficient of almost zero. The zero-temperature coefficient
property is desirable because of the heavy currents that often flow through shunts producing heat. A zero-
temperature coefficient material is not affected by this heat; therefore, it remains stable in temperature.
Most other materials increase their resistance as they are heated. If shunts were made of these materials,
they would carry less current. More and more current would flow through the meter movement, and the
chances of damage would increase. Using shunts constructed with zero-temperature coefficient materials
eliminates this problem.
Q-9. What type of temperature coefficient material does not produce increased heat in response to
increased current flow?
The ends of the shunt strips are embedded in heavy copper blocks. The blocks are attached to the
meter coil leads and the line terminals. To ensure accurate readings, you should not interchangeably use
the meter leads for a particular ammeter with those for a meter of a different range. Slight changes in lead
length and size may vary the resistance of the meter circuit. If this happens, current will also change and
cause incorrect meter readings. External shunts are generally used where currents greater than 50 amperes
must be measured.
SHUNT SELECTION.—When using an external-shunt ammeter, you should select a suitable shunt
so that the scale deflection can be easily read. For example, if the scale has 150 divisions and the load
current you want to measure is known to be between 50 and 100 amperes, a 150-ampere shunt would be
the correct choice. Under these conditions, each division of the scale represents 1 ampere. In other words,
