in position by pivots at the top and bottom of the shaft. To the shaft is connected a pointer which moves over a graduated scale. Springs are so arranged that they hold the pointer to a zero position on the scale.
The armature of an ammeter is a small load, which is placed in series with the main load R in a circuit. A very small part of the energy passing through the circuit is converted into magnetic energy in the coil. The armature coil of an ammeter or milli-ammeter has very little resistance so it should not be connected across a source of e.m.f. without an extra load to limit the current to the safe carrying capacity of the size of wire used in the coil.
The small current flows through the springs and through the coil. You know what happens then--a magnetic field is set up about the armature coil. This field joins forces with the field
Fig. 22
Fig. 23
of the permanent magnet--the tendency being for the lines of force of each field to get into a straight line with each other. A force is exerted, which either pushes or pulls the armature, depending upon the direction of current flow, overcoming the pressure exerted in the opposite direction by the spiral springs to a degree depending on the current in the coil. The greater the current, the more lines of force about the armature and the nearer these lines of force will approach a parallel position in respect to the field of the permanent magnet. Of course, as the lines of force move, the armature moves along with them, causing the pointer attached to it, to move over the dial.
If larger currents are to be measured with a small current-carrying ammeter, several alloy strips of comparatively low resistance, placed between two copper lugs (the assembly is called a “shunt”) are connected across the terminals of the ammeter as shown in Fig. 22. The shunt divides the current so
