Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . oil is extremely light, the frictionsmall, and the instrument very sensitive to minute variations 206 PRACTICAL ELECTRICITY. of current. A current of about .015 ampere will give a fullscale deflection of the pointer. The instrument is carefullybalanced, so that it may be used in a horizontal or vertical position. An ammeter, however,should always be calibrated in theposition in which it is to be used; Amirror is located just below the scaleof the portable instruments. By-looking clow
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . oil is extremely light, the frictionsmall, and the instrument very sensitive to minute variations 206 PRACTICAL ELECTRICITY. of current. A current of about .015 ampere will give a fullscale deflection of the pointer. The instrument is carefullybalanced, so that it may be used in a horizontal or vertical position. An ammeter, however,should always be calibrated in theposition in which it is to be used; Amirror is located just below the scaleof the portable instruments. By-looking clown on the pointer so thatit is directly over its reflection in themirror, errors in reading the scaledivisions due to parallax * are thusavoided. In Weston instruments thepost marked -f is the one by whichthe current should enter the instru-ment so that the coil will be deflectedin the right Weston Ammeter Shunt.—Only a small portion of the total current to be measured issent through the movable coil, the remainder passing througha shunt, •[[ 162. The shunt is made of a special resistance. Fig. 197.—The Magnetic Cir cuit of the Weston Instruments. §- alloy, and contained either in the in-strument, in a separate portable case,or on an external block, Fig. 199, as inthe switchboard type of lead wires to the shunt, the instru-ment and the shunt, are all numberedto correspond, so that when usedtogether the indications agree with thecalibration. The shunt leads shouldnever be shortened, because the de-creased resistance in the shunt circuitwould permit more current to flowthrough it, so that the indicated read-ings would be higher than the actual Coil, Springs and Pointercurrent flowing. The resistance of the of Weston Instruments. *The apparent angular displacement of an object when seen from two differentpoints of view. AMMETERS. 207 instrument and its shunt is very low, little energy, there-fore, being lost when it remains continually in circu
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