. The street railway review . e product of the rateby the time, or the ampere-hours, as a water metermeasures the total number of cubic feet of water voltmeter connected to any two points, measures thedifference of potential (electromotive force, pressure orvoltage) between them as a manometer tube measuresthe difference of pressure between the ends of the measuring instruments may be either directreading or zero instruments. The former give thevalue of the measurements directly by the amount ofdeflection of a pointer. In the latter class some part ofthe instrument mus
. The street railway review . e product of the rateby the time, or the ampere-hours, as a water metermeasures the total number of cubic feet of water voltmeter connected to any two points, measures thedifference of potential (electromotive force, pressure orvoltage) between them as a manometer tube measuresthe difference of pressure between the ends of the measuring instruments may be either directreading or zero instruments. The former give thevalue of the measurements directly by the amount ofdeflection of a pointer. In the latter class some part ofthe instrument must be adjusted until the pointer comesback to its zero position, or the position it occupies whenno current is passing. The position of the adjustable partwhen the balance is obtained gives the proper reading. The two classes are well illustrated by weighing scales,the spring balance being direct reading. The lever bal-ance is a zero instrument, since the load is balanced byadding or moving weights until the beam swings freely. T H A. C. AND D. C. AMMETER. and the pointer stands at zero, or midway between thestops, the load being measured by the position or numberof counter-weights on the beam. The advantage of thedirect reading instruments is that they indicate anychanges of the quantity being measured and do notrequire any handling. The advantage of zero instru-ments is that they ma} be adjusted to closer measure-ments. Since zero instruments may be set to read zeroat normal current or voltage, and may be so sensitve that a small variation will give a large deflection whichmay be seen across the room, they are of special value inthe entwine room or dynamo room, where the machinesmust be regulated to give constant current or constantpotential. What is commonly called an electric current is materi-ally diiferent from a current of water in that it cannot beobserved by the senses directly. It can only be detectedand measured indirectly by its effects. An electric cur-
Size: 1566px × 1595px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., book, bookcentury1800, bookdecade1890, booksubjectstreetrailroads
