40 



Messrs. Holman, Lawrence, and Barr on the 



series with the ammeter, and at the same instant reading the 

 ammeter. The calibrations at different times were checked 

 at the same point, with an average deviation of only a few 

 hundredths of one per cent. A test for temperature error 

 showed a change of but O'l per cent, for a change of 

 15° C. ; so that, as the temperature during the work was 

 constant within a few degrees, no correction was needed. 



The manonnine coil, fio\ 2, consisted of about 16 feet of 



Fig. 2. 



No. 20 wire, had a total resistance 

 of about 8*8 ohms, and was divided 

 into nine sections by copper po- 

 tential wires leading into different 

 points along the coil. These sec- 

 tions were so designed that, by 

 suitably shifting the connexions 



along a, b, c, &c. y any thermal 

 JE.M.F. which was to be measured 

 could be balanced by a current 

 which would deflect the ammeter 

 to a point between 90 and 140 di- 

 visions (readable to tenths) — cor- 

 responding to currents from 0*006 

 to 009 ampere roughly. The 

 coil was immersed directly in kero- 

 sene, and as its temperature-coefficient was but O'OOl per 1° C, 

 the correction became very small. The relation and actual 

 resistance (international ohms) of the whole coil and its several 

 sections were repeatedly determined against a standard ohm 

 by the differential galvanometer, and checked by a modified 

 "Wheatstone-bridge arrangement. These data were reliable 

 probably well within 05 per cent, throughout. 



In the thermo-couple circuit, the sensitiveness necessarv 

 in the galvanometer to give the smallest E.M.F. to 0*1 per- 

 cent, was easily computed to be only about 7*7 . 10 6 (mm. defl. 

 at 1 m. per ampere or d/c). The instrument as actually used 

 exceeded this requirement, averaging about 5 . 10 7 . Its resist- 

 ance, all in series, was 14*3 ohms. 



The cold junction c of the thermo-couple was fused too-ether 

 in an oxyhydrogen flame. The wires, insulated frorn each 

 other by having one strung through a very fine glass tube, 

 were run down another tube of about \ inch inside diameter 

 and 8 or 10 inches long. This tube was fused together at the 

 bottom and top, as well as at some intermediate points, and 

 when in use was always packed in a double vessel of cracked 

 ice, as shown in fig. 3. 



The intermediate junctions from which the copper leads 



