ELECTRICAL RESISTANCE OF NICKEL AT HIGH TEMPERATURES. 197 



and iron occur. In no other metals have any similar peculiarities been 

 observed. Hence we may regard it as an experimental truth that the interesting 

 changes in the sign of the Thomson effect in metals in which such changes do 

 occur are accompanied by peculiar changes in the manner of growth of resistance 

 with temperature. 



In the case of the nickel, the simultaneousness of the two peculiarities was 

 demonstrated by direct experiment. In effecting this direct comparison, we 

 tried many modifications; but the essential characteristic of the experiment 

 was to obtain, alternating with the resistance measurements, accurate deter- 

 minations of the electromotive force of a nickel-palladium pair. In some 

 cases the measurements of the platinum resistance were used as the tempera- 

 ture scale in which to express this electromotive force ; in other cases the 

 platinum wire was dispensed with, so far as resistance measurement was con- 

 cerned, but was introduced as a third element in the thermoelectric junction, 

 after the convenient manner invented by Professor Tait. That is, the three 

 wires — nickel, palladium, and platinum — were bound together as a triple junc- 

 tion, and the free extremities led off in such a way that the nickel-palladium 

 circuit and palladium-platinum circuit could be thrown on to the galvanometer 

 in rapid alternation. In this form of experiment the palladium-platinum 

 circuit played the rdle of a thermometer. The platinum was very similar 

 in its thermoelectric properties to the kind named " Soft Pt " in Professor 

 Tait's first approximation to a thermoelectric diagram. Its thermoelectric 

 line was but slightly inclined to the palladium line, and the electromotive force 

 of the palladium-platinum circuit increased at a somewhat quicker rate than 

 the temperature as estimated in centigrade degrees. 



In whatever way the temperature was virtually measured, whether by the 

 resistance of platinum or the electromotive force of the palladium-platinum 

 circuit, the experiment gave us the means of comparing directly the two 

 peculiar effects of nickel. Two curves could be drawn, the one showing the 

 march of the electromotive force of nickel-palladium with temperature, the 

 other giving the same thing for nickel resistance. The resistance curve was 

 similar in all respects to those already shown in Plate XII. (II.) ; the electro- 

 motive force curve reproduced with wonderful fidelity the old result of Professor 

 Tait. Beginning nearly straight at low temperatures, or if anything slightly 

 concave upwards, it became, as the temperature approached 250° C, distinctly 

 convex upwards. About 300° C. the neutral point was reached, and shortly 

 after passing the vertex the curve became accurately straight, and continued 

 so to the highest temperatures. In fact, it consisted practically of two straight 

 portions, oppositely inclined to the line of temperatures, and connected by a 

 parabolic arc with vertex at 300° C. This, of course, shows that the nickel 

 line on the thermoelectric diagram, lying at low temperatures below the 



