Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . d to minute variations of cur-rent ; the scale divisions not cramped at either end of thescale, but even throughout; and the accuracy of the instru-ment should not be impaired when in close proximity topowerful magnetic fields, as switchboard conductors ordynamos. Ammeters are divided according to their use, intotwo classes : (1) a portable type, generally of a high class ofconstruction and accuracy, used for measurements of pre-cision, and (2) the switchboard type, in the constructi
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . d to minute variations of cur-rent ; the scale divisions not cramped at either end of thescale, but even throughout; and the accuracy of the instru-ment should not be impaired when in close proximity topowerful magnetic fields, as switchboard conductors ordynamos. Ammeters are divided according to their use, intotwo classes : (1) a portable type, generally of a high class ofconstruction and accuracy, used for measurements of pre-cision, and (2) the switchboard type, in the construction ofwhich such refinement of precision is not required. In somemakes of both types the whole current passes through the am-meter, while in others the ammeter is shunted, % 209. Theshunt may either be contained within the instrument case, orit may be external to the instrument. In the latter casespecial leads used in the calibration are furnished to connectthe shunt with its instrument. Milliammeters are ammetersin which the scale is graduated to read directly in thou-sandths of an AMMETERS. 201. Fig. 188.—SolenoidGravity Ammeter. 204. Gravity Ammeter.—In this simple type of amme-ter, the magnetising current overcomes the attraction ofgravity for a piece of suspended iron. The current passesaround a helix of heavy ivire, which is bent in the arc of acircle, Fig. 188. A soft iron core bent to the same arc is sus-pended, so that one end is free to be sucked up into the helixby the field of the magnetising current. A pointer attachedto the movable iron core swings over thescale, and will assume a different positionfor each value of the magnetising current. The instrument is calibrated by insert-ing it in series with a laboratory standardand marking the value of each currentpassed through it on the scale correspond-ing to each position of the pointer. Sup-pose that when 10 amperes were sentthrough the helix the core was in such aposition as to accommodate through its
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishernewyo, bookyear1901
