Ans. To calibrate a measuring instrument is to determine the variations in its readings by making special measurements, or by comparison with a _standard_. [Illustration: Fig. 631.--Weston ammeter; view showing shunt enclosed within the instrument. Weston instruments are direct reading and dead beat. Although the scales have practically uniform divisions, it is not assumed in the calibration that they are uniform, and the scales are not printed or engraved. The method of calibration consists in laying out each large division of the scale by comparing the instrument with a standard, and then inking in the division lines so found. The smaller divisions between the large ones are then equally spaced and marked by a mechanical method.] [Illustration: 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. [Illustration: 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.