1899.] on Measuring Extreme Temperatures. 103 



with Mr. Emnorfopoulos, in a direct comparison of methods (1), (3) 

 and (4), which are simplest and most generally applicable. The ad- 

 vantages of the direct method of comparison are very great. (1) The 

 comparison may be extended continuously throughout the scale, and 

 is not confined to a few arbitrary selected points. (2) It is easy 

 to apply the electric method of heating, which is of all methods the 

 most easily regulated. (3) It is easy to arrange the experiments in 

 such a way that there can be no question of difference of temperature 

 between the thermometers under comparison, which is the most 

 insidious source of error in high temperature measurement. 



In the comparison of the scale of the expansion of platinum (1), 

 with that of the platinum resistance thermometer (4), it is simply 

 necessary to observe simultaneously the expansion and the electric 

 resistance of a platinum strip, tube or wire maintained at a steady 

 temperature by means of an electric current. The expansion may be 

 measured, as in the meldometer of Joly, by means of a micrometer 

 screw ; but for lecture purposes it is preferable to adopt the method 

 of the optical lever employed by Laplace in his experiments on 

 expansion a century ago. By employing a direct-reading ohmmeter 

 to indicate the changes of electrical resistance, it is thus possible to 

 exhibit the diiference between the two methods by the simultaneous 

 advance of two spots of light on a single scale. If the two instru- 

 ments are adjusted to read correctly at 0° and 1000° C, the resistance 

 thermometer will be in advance at temperatures below 1000°, but 

 will lag behind at higher temperatures, because the rate of expansion 

 increases as the temperature rises, whereas the rate of change of 

 resistance diminishes. As the result of these experiments, it appears 

 that the two scales (1) and (4) differ from that of the gas-thermometer 

 to a nearly equal extent, but in opposite directions. 



The resistance of platinum at its melting point is more than six 

 times as great as at 0° C, whereas the whole expansion amounts to 

 only one-fiftieth part of the length. The electrical method is for 

 this reason by far the most accurate and sensitive. It also possesses 

 in a very striking degree the merit of pliability and adaptability to 

 the needs of each particular problem. For this reason the scale of 

 the platinum resistance thermometer has come to be regarded as the 

 platinum scale par excellence, and has been adopted as the standard of 

 reference in many recent researches. 



As an illustration of the facility of applying this method, the 

 determination of the fusing point of platinum on the platinum scale 

 may be taken. This is a difficult experiment to perform by any 

 other method. In performing the experiment by the measurement of 

 the electrical resistance, it suffices to take a fine wire of which the 

 electrical constants are accurately known, and to raise it gradually to 

 its melting point by steadily increasing the current. The observation 

 of the resistance of the central portions of the wire at the moment of 

 fusion gives directly the temperature required on the platinum scale. 

 In attempting to perform the same experiment by the expansion 



