" Direct-reading " Platinum Thermometer. 63 



purposes, temperature is never required to be measured with 

 a precision greater than one tenth of a degree. It is my 

 object to consider how the reserve of precision may be utilized 

 in facilitating the processes of testing. The results seem to 

 indicate that it is possible, with a platinum thermometer, to 

 determine ordinary temperatures by a single operation, of 

 such a kind that the readings are " direct " — that is to say, 

 no calculation requires to be applied to them. 



In the course of some experiments upon the variation of 

 the electrical resistance of dielectrics with temperature, an 

 apparatus was needed which, by a quick and simple operation, 

 would measure the temperature within the substance of the 

 dielectric. As dielectrics, generally, are bad conductors of 

 heat, it is desirable to take simultaneous readings of tempe- 

 rature at many points within their mass. A platinum 

 thermometer may be regarded as having an extensible bulb, 

 for the wire may be distributed to various points within the 

 dielectric ; the temperature deduced from the platinum 

 thermometer, at any instant, is therefore equivalent to the 

 mean of the simultaneous readings of a large number of dis- 

 tributed mercurial thermometers. Further, the dielectrics, 

 in their protecting covers, are submerged in a tank of water ; 

 the stems of ordinary mercurial thermometers placed, verti- 

 cally, beneath a water-surface would be difficult to observe ; 

 the platinum instrument is far more convenient for this work, 

 for it has the advantage of a portable stem and scale. 



The resistance of a dielectric* is by no means such a 

 definite quantity as the resistance of a metallic conductor. 

 The difference in quality between samples of the same name, 

 the age of the substance, the nature of the contact between the 

 dielectric and its electrodes, and the so-called " electrification," 

 are such variable factors that the term " dielectric resistance " 

 has only an empirical significance. To arrange for the mea- 

 surement of temperature with extreme precision is therefore 

 unnecessary. 



Range. 



For certain practical reasons, the temperatures mentioned 

 in this paper are referred to the Fahrenheit scale. The 

 required range was 32°-120° F.; and, within the present 

 limits of accuracy, it is safe to assume that the resistance, 

 B^ of the platinum coils, at any temperature t, within this 

 range, is represented by the expression 



R,=B M [l + «(*-32)] (1) 



* See " Dielectrics," Proc. Physical Soc. xiii. p. 155, 1895 ; Phil. Mag. 

 1894, xxxviii. p. 396. 



