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DR. J. T. BOTTOMLEY AND MR. F. A. KING ON THERMAL 
a circular scale 1 metre in radius ruled in half millimetre divisions, gave results which 
left nothing to be desired. The telescope, galvanometer, and each part of the 
radiation apparatus, were fastened firmly down, to prevent any alteration taking place 
after the thermojunctions had been calibrated. 
Calibration of the Thermojunctions. 
12. To calibrate the thermojunctions for low temperatures was not a simple matter. 
The difficulty was to get a number of reliable points between the temperatures 
0° C. and — 200° C. The method ultimately adopted is similar to that used, in J. T. 
Bottomley’s earlier experiments on radiation, for determining the resistance of a 
platinum wire at high temperatures; but it was suitably modified for the present 
requirements, and for dealing with low temperatures. 
13. A thick double-walled copper tube was taken, and one end immersed in a 
Dewar flask filled with liquid air, fig. 3. The end a projecting out of the flask was 
at atmospheric temperature, say 16° C. ; and the portion immersed, b, at — 194°C., 
the boiling point of liquid air. Between the points a and b, along the length of the 
tube above the surface of the liquid air, there was a fairly uniform temperature 
gradient, which was maintained with the greatest steadiness. 
One of the little copper cylinders o, into which the thermojunctions are soldered, 
was tied to the bulb of a pentane thermometer p, reading from +30°C. to — 200° C. 
This thermometer had previously been compared with a standardised Reichsanstalt 
pentane thermometer, and checked by a hydrogen thermometer, and by a platinum 
resistance thermometer. The other thermojunction was within the copper globe, and 
inside the enclosure, as shown in the diagram. In taking observations, the junction in 
the copper globe was kept at a known fixed temperature by surrounding the copper 
enclosure with ice, or water at the temperature of the laboratory, or with liquid air. 
The pentane thermometer with the junction attached was lowered into the copper tube, 
and, by placing it at different heights, series of temperatures were obtained which 
remained absolutely steady whilst the galvanometer readings were being taken. By 
raising or lowering the flask of liquid air, or by adjusting the height of the copper 
tube above the surface of the liquid air, or by raising or lowering the little copper 
cylinder from point to point in the copper tube, it was easy to find any required point 
of temperature throughout the whole range of the pentane thermometer. When the 
readings of the pentane thermometer and galvanometer showed that the thermo¬ 
junction in the copper tube was at a steady temperature, a stop-watch was started ; 
and if at the end of two minutes no alteration had taken place in the thermometer 
and galvanometer readings, the galvanometer deflections to the right and left of zero 
were observed, and then the zero was taken.* 
* This method of obtaining a series of known fixed temperatures, high or low, is most convenient; and 
is far superior to any other with which we are acquainted. 
