on a shaft that is pivoted a little off the center of a coil as shown in fig. 2,499, and carrying a pointer which moves over a scale.
Ques. How does it work?
Ans. Its principle of operation is that a piece of soft iron placed in a magnetic field and free to move, will move into such position as to conduct the maximum number of lines of force.
The current to be measured is passed around the coil, producing a magnetic field through the center of the coil. The magnetic field inside the coil is strongest near the inner edge, hence, the vane will move against the restraining force of a spring so that the distance between it and the inner edge of the coil will be as small as possible.
Fig. 2,501.—Solenoid and plunger illustrating the operation of moving iron instruments. When a current flows through the coil, a field is set up as indicated by the dotted lines of force. The current flowing in the direction indicated by the arrow induces a north pole at N, which in turn induces a south pole in the plunger at S, thus attracting the plunger. The effect of the field upon the plunger may also be stated by saying that it tends to cause the plunger to move in a direction so as to conduct the maximum number of lines of force, that is, toward the solenoid. Thus if ABCD be the initial position of the plunger only five lines of force pass through it: should it move to the position A´B´C´D´, the number of lines passing through it will then be 9, assuming the field to remain unchanged.
The operation of moving iron instruments of the plunger type may be explained by saying that the current flowing in the coil produces a pole at its end and induces an unlike pole at the end of the plunger nearest the coil, thus attracting the plunger, as illustrated in fig, 2,501 above.
Figs, 2,502 and 2,503.—Wagner series transformers. Fig. 2,502, wound primary series transformer; fig. 2,503, open primary transformer. Wagner series transformers are made in three general types: One for switchboard mounting with wound primary; one for switchboard mounting with open primary, and one with open primary suitable for slipping over bus bars or switch stud. These transformers have 5 ampere secondary winding, and are intended for use in connection with instrument of scale capacity 0-5, although the scale should be calibrated to indicate the primary current. The capacities are from 2 watts to 50 watts, being suitable for operation on circuits of 750 to 66,000 volts.