62 PHYSICAL SCIENCE 



change of temperature. The metal most usually 

 employed is platinum, since it is not attacked by 

 acids, and has a very high melting point. Platinum 

 thermometers are now used extensively for physical 

 research ; they have a very large range, and are 

 probably susceptible of greater sensitiveness than 

 any other form of thermometer. At ordinary 

 temperatures a difference of temperature of one 

 ten- thousandth of a degree can be detected with 

 moderate ease, while, with great precautions, the 

 hundred-thousandth of a degree can be estimated. 

 At high or low temperatures such accuracy is 

 impossible, but measurements, correct to the 

 nearest degree, can be made up to about i ioo° C. 

 and as low as — 200° C. Below the latter tem- 

 perature the rate of change of the resistance 

 alters in a manner to be described below, and the 

 instrument ceases to be trustworthy. 



The standard to which the readings of all 

 other thermometers are referred, as we have 

 indicated when considering the absolute scale of 

 temperature, is the gas thermometer containing 

 hydrogen or helium. Not only is the hydrogen 

 thermometer thus used for purposes of reference, 

 but it can also be employed as a practical instru- 

 ment at temperatures too low to be measured by 

 the platinum resistance thermometer. It might 

 be thought that, as the point of liquefaction was 

 approached, a gas would cease to be trustworthy 

 as a thermometric substance, but experiment has 

 shown that, as long as the pressure of the gas is 

 kept well below the saturation value at which 

 condensation would occur, the gas still expands 

 or contracts proportionally to the absolute tem- 

 perature. Dewar has found that thermometers, 



