OP THE NATIONAL PHYSICAL LABORATORY. 369 



XXIII. Method of calculating Comparison Experiment. 



As an example of the method employed in making and calculating out an 

 experiment, the results of the observations made in Experiment No. 13 of Series II. 

 are given in full. During this experiment the steadiness of the temperature of the 

 furnace was perhaps a little above the average, but the room temperature and that 

 of the resistance-boxes and of the various mercury columns of the gas thermometer 

 and barometer were rising more rapidly than usual. According to observations taken 

 on the thermojunctions and platinum thermometer, the steady state had been 

 maintained for about 10 minutes before the first recorded reading on the gas 

 thermometer was taken. As only two observers were available, one of these took 

 alternate observations on the gas thermometer and thermojunctions, while the second 

 took the platinum thermometer readings. 



* O 



By graphic interpolation the mean thermojunction readings, corresponding to the 

 times at which the other instruments were read, were obtained and are given in the 

 example. "" 



The readings of the gas thermometer are made as independent of one another as 

 possible by lowering the mercury each time before a setting, raising it again slowly so 

 as to make a new meniscus. 



The calculation to the accuracy here necessary of the air temperature corresponding 

 to definite platinum temperatures is somewhat laborious, if the formula 



T = MO + 50) - AA 5 T + 50Y - 10 ' 6 > 

 \ o / \ 8 / S 



has to be applied for each observation. The graphic methods used by HEYCOCK 

 and NEVILLE cannot easily be made sufficiently accurate. Since the value of 8 for pure 

 platinum wire has been found to be 1'5, varying from one specimen to another within 

 very narrow limits, the most suitable method of effecting this conversion was found 

 to be to construct a table giving, for a sufficient number of points, the value of T for 

 given values of pt when S = 1'5. A second table gives the correction to apply to 

 the value of T thus obtained, if the S differs from the standard value by a small 

 amount. 



* The letters AB, BA refer to the position of the reversing switches leading to the potentiometer. 

 Owing to small Thomson and Peltier effects, there was generally a small difference between the two 

 positions. In this case it is rather above the average. There was no difficulty in setting to '1 microvolt, 

 so the readings are given to this figure, though it is not considered as having any significance in temperature 

 measurement. 



VOL. CCIII. A. 3 B 



