. The street railway review . rnal resistance beingsometimes added), so that only a small current will passthrough the instrument when it is connected to the twopoints whose difference of potential is to be Ohms well known law the current through theinstrument will be the difference of potential divided bythe resistance of the voltmeter; but since the resistance ofthe voltmeter is practically constant, the current throughit is proportional to the difference of potential at its ter-minals, so that the instrument may be calibrated to read volts. P^lectro-magnetic measurements are bas
. The street railway review . rnal resistance beingsometimes added), so that only a small current will passthrough the instrument when it is connected to the twopoints whose difference of potential is to be Ohms well known law the current through theinstrument will be the difference of potential divided bythe resistance of the voltmeter; but since the resistance ofthe voltmeter is practically constant, the current throughit is proportional to the difference of potential at its ter-minals, so that the instrument may be calibrated to read volts. P^lectro-magnetic measurements are based upon thefact that a current of electricity is surrounded by mag-netic lines of force which follow the path of least resist-ance. Since iron gives a better path for magnetic linesthan air the lines will be-attracted to the iron, and (bytheir tendency to shorten) will tend to draw the iron intosuch a position as to furnish the shortest possible path forthe lines of force. This tendency to move the iron may ^@^^fa!^%yic»^ 89. T!IOMSOX-R1CE VOLTMETtK. be measured by the force required to balance it, the forcebeing exerted bj- a spring, gravitj, magnetic field or anycombination of them. The simplest and cheapest instruments for measuringcurrent are based upon the tendency of a solenoid or coilof wire carrying a current, to draw or suck an iron coreinto itself, this motion being opposed by a weight (some-times that of the iron core), or by a spring. This classof instruments is illustrated by the well known Edison orBrush ammeters. If the core is not the center of the coil ittends to move away frouithe center and get as closeas possible to the is the principle usedin the well known and voltmetersfor direct or alternatingcurrents. A thin strip ofsoft iron is bent so as toform three sides of a rect-angle and IS pivoted ec-centricall} inside a coilthrough which the currentpasses. As the iron striprotates it comes closer toone side of the coil, thero
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Keywords: ., book, bookcentury1800, bookdecade1890, booksubjectstreetrailroads
