Thermo-electric Powers of Metals and Alloys. 97 



M a length of about 54 centimetres. The junctions of each 

 couple having been carefully soldered together, each wire 

 was covered over with paraffined silk ribbon so as to insu- 

 late it, leaving only one junction exposed. A number of 

 these thermo-couples were made up into a bundle, the sets 

 of bare and covered junctions being bound together compactly, 

 the bare in one group and the insulated junctions in another. 

 In all the experiments to be described, the set of junctions 

 which were insulated was kept at 0° C. by being immersed in 

 finely-crushed melting ice, kept well drained ; the other set 

 of junctions was exposed to temperatures ranging from 

 -200° C. to +100°C. Two measurements had then to be 

 made, viz., that of the temperature of the heated or cooled 

 junction and also that of the electromotive force set up in the 

 corresponding thermo-couple by the difference of temperature 

 of the two junctions. The chief experimental difficulty en- 

 countered w 7 as that of determining the low temperature and 

 determining it at the instant when the electromotive-force 

 measurement of the couple was made. After long pre- 

 liminary experiments necessary to overcome obstacles in 

 .the path, the successful method of dealing with the problem 

 was found. These difficulties arose almost entirely out of the 

 fact that for our purpose it was not sufficient to determine 

 the thermo-electromotive force of the M-lead couple at fixed 

 isolated temperatures, but we desired to determine it at any 

 of the intermediate points of temperature lying within the 

 range of three hundred degrees from —200° C. to +100° C. 

 over which we were working. The apparatus finally con- 

 structed was based upon the employment of a platinum 

 thermometer of particular form for measuring the temperature, 

 and a measuring-instrument which could be changed instantly 

 from a potentiometer into a resistance-balance. 



4. In our previous researches on electric conductivity at 

 low temperature, we fully satisfied ourselves that a wire of 

 carefully annealed pure soft platinum can be cooled as often 

 as necessary to the temperature of boiling liquid air or 

 oxygen and yet return always to the same electrical resistance 

 at a normal standard temperature. 



Professor Callendar has demonstrated that this quality of 

 the constancy of resistance of pure annealed platinum at deter- 

 mined temperatures renders it admirably adapted for therino- 

 metric purposes ; and Callendar and Griffiths have shown 

 that a properly arranged platinum resistance affords the most 

 suitable means for determining temperatures, and our own 

 work at low temperatures has confirmed this fact. In our 

 investigations on the electrical resistance of metals and alloys, 



Phil. Map. 8. 5. Vol. 40. No. 242. July 1895. H 



