Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ady 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 terminals brought out to binding posts,Fig. 213. A double-scale Weston voltmeter is shown in , suitable for use with pressures ashigh as 150 volts. The 150 volt coilterminates in the lower right andleft-hand binding posts, and the cur-re
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . ady 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 terminals brought out to binding posts,Fig. 213. A double-scale Weston voltmeter is shown in , suitable for use with pressures ashigh as 150 volts. The 150 volt coilterminates in the lower right andleft-hand binding posts, and the cur-rent enters by the right-hand postmarked -{— A push-button abovethis post serves to close the resistance of the 150 volt coilis about 150,000 ohms, and there are 150 divisions 011 the Upper, or Fig. 213. —Connections ofblack ink scale, or one division per Weston Double Scale Volt-volt. The 15 volt coil terminates in the lower right-hand and upper left-hand posts, and has aresistance of about 1500 ohms. There are ten divisions onthis red ink scale per volt, so that one-tenth, five-hundredths. MEASUREMENT OF PRESSURE. 239 or even twenty-five thousandths of a volt may easily be readfrom this scale. In using a double scale voltmeter, if indoubt about the value of the voltage to be measured, always usethe higher reading scale first, then if the value is below thelimit of the low reading scale, the left-hand terminal maybe readily changed to the upper post marked 15. If 150 voltsare directly applied to the low reading scale the instrumentwill be seriously damaged and probably the insulation de-stroyed by a larger current flowing through it than the wind-ings will carry. This wire may be as small as .001 inch indiameter. For the same reason any voltmeter should not besubjected to a higher voltage than is indicated upon itsscale. Voltmeters are calibrated by means of standard cellsand are made up according to the range of the instrumentdesired, by means of the extra res
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