484 
MR. C. H. LEES ON THE THERMAL CONDUCTIVITIES 
used by Wiedemann and Franz of measuring the temperature by means of a thermo¬ 
junction brought into contact with the bar is free from this objection, and will produce 
no appreciable effect on the distribution of temperature in the bar if the mass of the 
junction is not large. 
The bars used by Wiedemann and Franz had a silvered surface, and it was suffi¬ 
cient to bring the thermo-couple into contact with the surface in order to get the 
temperature. In the later of the following experiments the bars were painted, so that 
contact could not be made in this way. Small conical holes about '5 mm. deep were, 
however, made at opposite extremities of horizontal diameters of the bars, and at 
regular distances along. Eight such diameters were taken, the two nearest the centre 
of the uncut bar being 1 cm. apart, and the rest 10*5 cms. apart. Each of these 
small holes was amalgamated, and sufficient mercury left in them to make good 
contact with the ends of the two wires which were used as a thermo-couple. The 
thermo-circuit would thus consist at the bars of—first wire of couple; mercury of 
first hole ; brass of bar; mercury of hole at opposite extremity of diameter; second 
wire of couple. The mercury in the holes was constantly cleaned by touching the 
surface with dilute nitric acid, washing and drying, and the ends of the thermo¬ 
couple wires were kept bright. Under these conditions the arrangement worked 
satisfactorily. 
The wires used originally for the thermo-couple were of brass and iron, but these 
gave unsatisfactory readings, mainly on account of the poor contact between iron and 
mercury. As there is an advantage in using copper as one of the elements, a few 
combinations of copper with other metals and alloys were tried, and eventually the 
platinum-silver alloy used for resistance coils was selected as the other element. This 
couple gives an E.M.F. of about ‘00016 volt for 70° difference of temperature, and its 
constant only increases about 7 per cent, for 100° C. It is, therefore, vrell suited for 
thermo-electric measurement of temperature. The wires used for the thermo-couple 
were about 20 cms. long and ‘2 mm. diameter. The ends which served to make 
contact with the galvanometer circuit dipped into mercury cups in blocks of wood 
which could be moved along guides parallel to the bars in order that the wdrole of the 
temperature observations could be made with one couple. The cup into which the 
platinum-silver wire dipped was provided with a thermometer graduated in -j 3 ^ 0 C. 
The galvanometer was a low resistance one of the Wiedemann type, with Siemens’ 
bell-magnet and copper damping sphere. The resistance of the whole circuit was only 
about 1*5 ohm, and as the resistances of the contacts at the holes in the bar might 
vary, it was necessary to take measurements of the resistance of the circuit when a 
galvanometer deflection was taken. For this purpose Thomson’s modification of the 
bridge method was used. The diagram (fig. 2) shows the arrangement of the circuit. 
G is the galvanometer in series with the thermo-element. C, D, E are three 
mercury cups, arranged so that when E and D are connected, the galvanometer and 
thermo-element are in circuit alone, and when C and D are connected the galvano- 
