420 
DR. HAROLD A. WILSON OX THE 
couple was standardised by determining the g’alvanoineter deflection corresponding 
to the melting point of K.SOj^, which melts, according to Heycock and Xeyille, at 
1066° C. Tlie K^SO, was introduced into the tube on a small platinum spatula, and 
the temperature gradually increased until it was seen to melt, and the corresponding- 
deflection noted. Then by maintaining the tube at a series of constant temperatures 
near the melting point, and finding when the K^SO^ melted, it was pos.sible to obtain 
two temperatures very near together, at one of which the melted and at the 
other remained solid. The mean of the two galvanometer deflections was taken as 
corresponding to 1066°, and the temperature corresponding to any other deflection 
was first calculated on the assumption tliat the deflection was j^roportional to the 
difterence betvreen tlie temperatures of the two junctions. The “ platinum tem¬ 
peratures” thus obtained have been corrected to the centigrade scale by means of 
the table of corrections given by Callendar Phil. Mag.,' Dec., 1899, p. 534). 
This method of getting the temperatures was quite sufficiently accurate for the 
purposes of the present investigation, for which it was useless to aim at a greater 
accuracy than 5° or 10°, and, according to Callexdae, the corrections are much 
more accurate than this near 1000°, whde even at 300° the error is not more 
than 10°. 
The Ft and Pt Rh wires dipped into mercury cups kept in a water bath at a known 
temperature, from which copper wires led to the galvanometer. 
The gas siqjplied to the furnace was kept at a constant pressure by means of a 
gasometer, and the air and oxygen supply tul)e was provided with a water mano¬ 
meter by means of which the pressure of the supply could he maintained constant 
if necessary, for any length of time. In this way the tidDe could be maintained 
constantly at any desired temperature within 5° without difficulty. 
The current through the air and salt between the electrode EE and the tube T, 
due to various ]jotential difterences between them, was measured by the galvano¬ 
meter used to measure the tem|)erature. By means of commutators the connections 
with the galvanometer could be immediately changed from the thermo-couple to 
the conductivity apparatus. The difterence of potential was supplied by a battery 
of small secondary cells. 
2. Variation of the Current v:ith t/u E.M.F. 
Before making observations on the variation of the current with the temperature 
at constant E. M.F.,some observations were made on the variation of the current 
with the E.M.F. at constant temperature. 
The results obtained at a temperature of 1080° C. for air alone, without salt and no 
air current passing along the tube, are shown graphically in diagram Xo. 1. 
len the outside tube was negatively charged tlie current soon attained an 
almost constant value as the E.M.E. was increased, but when the outside tube was 
