Low Temperatures by Platinum-Thermometers. 517 



if, however, the current is kept constant, the absolute zero 

 works out too high. In the determination of the con- 

 stants of thermometers N, a rough attempt was made to 

 keep the current constant j but in the remaining thermo- 

 meters no alteration was made in the electromotive force 

 as the resistance of the coil, and therefore of the bridge, 

 increased. It appears probable that some of the discrepancies 

 in the above table are due to this cause. 



A simple method of graduating platinum- thermometers is 

 thus suggested. Assuming, as we are fully entitled to do, 

 that the curve t— j J t i s a parabola, three points only are 

 necessary for its complete determination : the points hitherto 

 adopted have been 0°, 100°, and 444°'5. The necessity of 

 guarding against loss by radiation, gain by superheating, &c, 

 when determining the resistance in sulphur, renders it a 

 somewhat troublesome operation to those observers who are 

 not provided with the necessary apparatus. In cases where 

 a high order of accuracy is not a sine qua non, and where the 

 platinum is known to be fairly pure, we may assume that 

 when t=— 273°*7 then R = : therefore, if E x and R are 

 determined by direct observations in steam (760 millim.) and 

 melting ice, the instrument may be considered as graduated, 

 since 



- 273*7 -j^ 



rhere 



8 = 



pt = 



273-7 



100 





273-7 

 100 



xlOO. 



As an example of the order of accuracy which could be 

 thus obtained, we append the following table, the first column 

 giving the true temperature, the others the error introduced 

 by graduating the various thermometers by the suggested 

 method. 



True 

 temp. 







Callendar's. 



Na+b. 



Na. 



N B . 



H. 



M r 



M 2 . 































50 



- 01 



+•08 



+•08 



+ •08 







-•14 



-•14 



100 ! 































150 



+•03 



-•25 



-•25 



-•23 







+•44 



+•43 



