The London, Edinburgh and Dublin philosophical magazine and journal of science . h resistance seriescoil could be cut out, leaving its resistance about 117 of the Weston voltmeters with a special connexion madeto effect that resi It would answer equally well. The volt-meter was preferred to a mil-ammeter as probably morereliable. The instrument was carefully and repeatedly cali-brated throughout its scale by an application of the Poggen-dorff method, measuring by the Clark cell the drop of potentialin a known resistance through which a current was passing in 40 Messrs. Holman, Lawrenc
The London, Edinburgh and Dublin philosophical magazine and journal of science . h resistance seriescoil could be cut out, leaving its resistance about 117 of the Weston voltmeters with a special connexion madeto effect that resi It would answer equally well. The volt-meter was preferred to a mil-ammeter as probably morereliable. The instrument was carefully and repeatedly cali-brated throughout its scale by an application of the Poggen-dorff method, measuring by the Clark cell the drop of potentialin a known resistance through which a current was passing in 40 Messrs. Holman, Lawrence, and Barr on the series with the ammeter, and at the same instant reading theammeter. The calibrations at different times were checkedat the same point, with an average deviation of only a fewhundredths of one per cent. A test for temperature errorshowed a change of but Ol per cent, for a change of15° C. ; so that, as the temperature during the work wasconstant within a few degrees, no correction was needed. The manonnine coil, fio\ 2, consisted of about 16 feet of Fig. No. 20 wire, had a total resistanceof about 8*8 ohms, and was dividedinto nine sections by copper po-tential wires leading into differentpoints along the coil. These sec-tions were so designed that, bysuitably shifting the connexions along a, b, c, & any which was to be measuredcould be balanced by a currentwhich would deflect the ammeterto a point between 90 and 140 di-visions (readable to tenths)—cor-responding to currents from 0*006to 0 009 ampere roughly. Thecoil was immersed directly in kero-sene, and as its temperature-coefficient was but OOOl per 1° C,the correction became very small. The relation and actualresistance (international ohms) of the whole coil and its severalsections were repeatedly determined against a standard ohmby the differential galvanometer, and checked by a modifiedWheatstone-bridge arrangement. These data were reliableprobably well within 0 05 per cent, th
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Keywords: ., bookcentury1800, bookdecade1840, bookidlondon, booksubjectscience
