Fig. 632.--Weston portable voltmeter, inspector's style. This instrument is provided with a reversing key. Instead of the regular binding posts, pins are used with which connections are made by means of contact cups attached to flexible cords. These contact cups are convenient in making connections, or in changing quickly from one range to the other, if the instrument have a double scale. Connections for the different ranges are made in precisely the same way as with the regular double scale voltmeters. For the upper scale values, the contact pin to the right and the front contact pin to the left being taken, and for the lower scale values, the left contact cup is changed to the rear contact pin.
Ques. Describe a solenoid or plunger ammeter.
Ans. This type consists of a "plunger" or soft iron core arranged to enter a solenoid. Current being passed through the wire of the solenoid causes the core to be more or less attracted against a restraining force of gravity or springs. A pivoted pointer attached to the core indicates the current value on a graduated dial as shown in [fig. 633].
Ques. What are the objections to plunger instruments?
Ans. They are not reliable for small readings, and are readily affected by magnetic fields.
Fig. 633.--Plunger type instrument. The current to be measured passes through the solenoid, producing a magnetic effect on the soft iron plunger which tends to draw it into the coil, and thus cause the pointer to move over the graduated scale. The distance the rod moves depends on the value of the restraining force (which may be springs or gravity), the coil winding, and strength of current. The winding consists of a few turns of heavy wire for an ammeter, and a large number of turns of fine wire when constructed as a voltmeter. Since the iron has a certain amount of residual magnetism, the deflection with smaller following large currents is more than would be produced by the same current following a smaller one. The instrument therefore is less reliable than the usual types.
Ques. Describe a magnetic vane instrument.
Ans. It consists of a small piece of soft iron or vane mounted on a shaft that is pivoted a little off the center of a coil as shown in [fig. 634]. The principle upon which the instrument works 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. A pointer, attached to the vane shaft moves over a graduated dial.