POLE CONSTRUCTION.Note that the pole pieces in figures 3-2 and 3-3 have curved faces.
You can see the advantage of this type of construction if you remember that lines of force enter and leave
a magnetic field in the air gap at right angles to the coil, regardless of the angular position of the coil.
Because of this type of construction, a more linear scale is possible than if the pole faces were flat.
Q-7. What advantage is gained by using pole pieces with curved faces in the DArsonval meter
The movable coil of the D'Arsonval meter movement we have been discussing up to now uses small-
size wire in its windings. This small-size wire places limits on the amount of current that can be safely
passed through the coil. Therefore, the basic D'Arsonval movement discussed can be used to indicate or
measure only very small currents. Certain circuit changes must be made to the basic D'Arsonval meter
movement for it to be practical in everyday use. To measure large currents, you must use a SHUNT with
A shunt is a physically large, low-resistance conductor connected in parallel (shunt) with the meter
terminals. It is used to carry the majority of the load current. Such a shunt is designed with the correct
amount of resistance so that only a small portion of the total current flows through the meter coil. The
meter current is proportional to the total load current. If the shunt is of such a value that the meter is
calibrated in milliamperes, the instrument is called a MILLIAMMETER. If the shunt has such a value that
the meter must be calibrated in terms of amperes, it is called an AMMETER.
Q-8. What structurally large, low-resistance conductor is connected in parallel with the meter
movement to prevent damage?
SHUNT RESISTANCE.A single, standardized meter movement is normally used in all
ammeters, no matter what the range is for a particular meter. For example, meters with working ranges of
0 to 10 amperes, 0 to 5 amperes, or 0 to 1 ampere all use the same meter movement. The various ranges
are achieved through the use of different values of shunt resistance with the same meter movement. The
designer of the ammeter simply calculates the correct shunt resistance required to extend the range of the
meter movement to measure any desired value of current. This shunt is then connected across the meter
terminals. Shunts may be located inside the meter case (internal shunts) with the proper switching
arrangements for changing them. They may also be located outside the meter case (external shunts) with
the necessary leads to connect them to the meter.
EXTERNAL SHUNTS.An external-shunt circuit is shown in figure 3-4, view A. Typical external
shunts are shown in view B. View C shows a meter movement mounted within the case. The case
provides protection against breakage, magnetic shielding in some cases, and portability.