OF THE NATIONAL PHYSICAL LABORATORY. 



353 



from 5 up to 1280 units, with an additional coil of 100 units for the determination ot 

 the fundamental interval. The coils are of fine platinum-silver wire wound into 

 loose hanks not paraffined, so as to follow with as little lag as possible the tempera- 

 ture of the massive copper tank in which they are placed. The working of the 

 resistance box has under the conditions which obtained for this work been sufficiently 

 good to render unlikely the introduction of any systematic errors. The sensitivity of 

 the platinum thermometer bridge was such that even at the highest ranges a change 

 of about '01 C. could be detected, though an accuracy of '1 at 1000 C. is probably 

 about the limit in the absolute measurement of temperature even under favourable 

 conditions with resistance coils of platinum-silver, such as those employed here, with 

 a temperature coefficient of - 00026 per degree C. 



The flexible leads which were never detached from the thermometers during any 

 series were carefully checked before and after the experiments, and the possible 

 checks, got by using different combinations of coils in measurement of the same 

 temperature, always agreed reasonably well. 



XI. The Potentiometer for Thermocouple Measurements. 



The potentiometer used in these experiments was specially designed and made in 

 the laboratory for accurate thermocouple work, and was described in detail in a paper 

 in the ' Phil. Mag.' for July, 1903. It is direct reading, and i.s arranged so that for 



ST. | CELL 



ai. I ^t 



r'h 



-o o-o 



ACC. 



\ 



REG. RES. 



6 



/W\A 



4O I I I -5 I 1-5 



b 6=6 6=6 6=6 6=0 o 



Fig. 4. Connections to potentiometer. 



ordinary thermoelectric work where the maximum electromotive force to be measured 

 is of the order of '02 volt the individual settings could be made to about '1 microvolt 

 if required. A diagrammatic representation of the connexions is shown in fig. 4 and 

 VOL. ecu r. A. 2 z 



