Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . equired totransmit the voltmeter current to the instrument. The less thiscurrent the more accurate the indication, consequently the bestvoltmeters have a very high resistance and their current is prac-tically negligible. When a voltmeter is placed in parallelwith any part of a circuit the resistance of the circuit ispractically the same as before, since the voltmeter resistanceis so very high the current in the circuit is not materiallychanged, and the calibrated indication records,
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . equired totransmit the voltmeter current to the instrument. The less thiscurrent the more accurate the indication, consequently the bestvoltmeters have a very high resistance and their current is prac-tically negligible. When a voltmeter is placed in parallelwith any part of a circuit the resistance of the circuit ispractically the same as before, since the voltmeter resistanceis so very high the current in the circuit is not materiallychanged, and the calibrated indication records, not the currentin the circuit, or the current through the voltmeter, but thedifference in pressure between the voltmeter terminals. Themovement of its magnetic system, of course, depends uponthe current flowing through the voltmeter, but the scale iscalibrated by applying known E. M. to its terminals andmarking the position of the needle with reference to the scalefor each particular pressure applied, H 180. In an ammeterthe whole current passes through the instrument, or its 238 PRACTICAL shunt, and the instrument measures the current. A voltmetermeasures the current flowing through it, but the calibration isin terms of the pressure causing this current to flow. See Exp. 57,1[182. 234. Construction of Voltmeters.—The same principlesemployed in the construction of ammeters, 1[ 204, etc., are employed in construct-ing voltmeters, the onlydifference being that thewindings are of very finewire, suitable to thesmall current that is tobe carried, and that ex-tra resistance coils aregenerally added in serieswith the voltmeter coilsto produce an instru-ment of very high re- ^. . sistance, for the reasons already given. lhemethod of calibration is given in H 182. 235. Weston Voltmeter.—The same construction is em-ployed in this make of voltmeter as in the Weston ammeter,11 208, extra resistance being connected in series with themovable coil and the term
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