Measurements of Precision in Platinum Thermometry. 559 



and can be taken at any time. Let the thermometer leads 

 be connected to the bridge and suppose the switch C to be 

 m the "out" position. Then for the two positions of the 



Fig. 8. 



01 - 02 -02. 05 "I 2 A- & 16 32 64- 



A / W| A A/ V^ jWV\ ^VW| fWV^ jVW\ jWV\ A AA/^ /WV\ MA/1 MM 



, 1 ^fe#y* gg«3Jpa -fc^-^ j GTpsfe c:.... I:"* <zkzz£p> c.jsT'h) czizz^z> dkur^D L ' , t . ^t , ** ess^s — 



N ^_! OUT 



The rotating arms of the three dials and of the switch C carry con- 

 ducting straps which connect diametrally opposite studs. The switch C 

 connects four studs together at one time. With C in the IN position 

 and the commutator as shown in the figure, the connexions are those of 

 fig. 6. With the commutator rotated 90° anti-clockwise the connexions 

 are those of fig. 7. With the commutator and C (in OUT position) as 

 shown in the figure the connexions are those of fig. 9 ; rotating the 

 commutator 90° anti-clockwise now gives the connexions of fig. 10. 



commutator the bridge connexions are as shown in figs. 9 

 and 10 (p. 560), the first balancing condition being 



and the second 



X-f&' = §(R + a'), 



R + Z/ = ^(X' + «'), 



(10) 



(11) 



where a' and b' have values slightly smaller than a and h 

 respectively due to the shunting of the portion (a + b) by the 



portion (L 2 + P + L 3 ). Since ^ is nearly unity we have 



