Fig. 2,497.—Plunger form of electromagnetic or moving iron type of ammeter.
There are several forms of moving iron ammeters, which may be classified as
Ques. Describe the plunger type.
Ans. This type of ammeter consists of a series coil and a soft iron plunger forming a solenoid, the plunger is so suspended that the magnetic pull due to the current flowing through the coil is balanced by gravity, as shown in fig. 2,497.
Ques. How should the plunger be constructed to adapt it to alternating current, and why?
Ans. It should be laminated to avoid eddy currents.
Fig. 2,497.—One form of plunger instrument as made by Siemens. It has gravity control, is dead beat, and is shielded from external magnetic influence. The moving system consists of a thin soft iron pear shaped plate I pivoted on a horizontal spindle S running in jewelled centers. To this spindle S is also attached a light pointer P and a light wire W, bent as shown, and carrying a light piston D, which works in a curved air tube T. This tube T is closed at the end B but fully open at the other A, and constitutes the air damping device for making the instrument dead beat.