HOLMAN, LAWRENCE, BARE. — MELTING POINTS. 



221 



nn^ sM 



Fig. 2. 



within a few degrees, no correction was needed. The raanganine coil 

 Figure 2, consisted of about 16 feet of No. 20 wire, had a total resistance 

 of about 8.8 ohms, and was divided into 

 nine sections by copper potential wires 

 leading into diflferent points along the coil. 

 These sections were so designed that by 

 suitably shifting the connections along a, b, c, 

 etc., any thermal emf. which was to be meas- 

 ured could be balanced by a current which 

 would deflect the ammeter to a point be- 

 tween 90 and 140 divisions (readable to 

 tenths), — corresponding to currents from 

 0.006 to 0.009 amperes roughly. The coil 

 was immersed directly in kerosene, and, as 

 its temperature coefficient was but 0.001 

 per 1° C, the correction became very small. 

 The relation and actual resistance (inter- 

 national 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 Wheat- 

 stone bridge arrangement. These data were reliable probably well 

 within 0.05 per cent throughout. 



In the thermo-couple circuit, the sensitiveness necessary in the gal- 

 vanometer to give the smallest emf. to 0.1 per cent was easily com- 

 puted to be only about 7. 7-10^ (mm. defl. at 1 m. per ampere or 

 d/c). The instrument as actually used exceeded this requirement, 

 averaging about 5-10^. Its resistance, all in series, was 14.3 ohms. 



The cold junction c of the thermo-couple was fused together in an 

 oxyhydrogen flame. The wires, insulated from 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 Figure 3. 



The intermediate junctions from which the copper leads went off to 

 galvanometer and key were soldered. They were kept at an equal 

 temperature by the device of enclosing them in a stoppered glass tube, 

 which was packed with hair felt into a one-inch hole in a five-inch 

 cube of cast iron. This arrangement was entirely satisfactory, but 

 seems to possess no material advantage over making the junction of 

 the copper leads with the Pt and Ft Rh serve as the cold junction, and 



