Measurements of Precision in Platinum Thermometry. 555 



If this is done we have as the first balancing condition 

 (fig. 2) 



P + L 3 =(1+«)(R + L 2 ) 

 and for the second (fig. 3) 



P + L 2 = (R' + L 3 )/(l + a) 

 Combining (I) and (5) we have 



R + R' a f /T1 r , 



+ (L 2 -L 3 



(*) 



(5) 



r[« B 



+ L 



(6) 



If Q is equal to S within 2 parts in 10,000, then « = 0'0002 

 and P is equal to (R + R'j/2 within 2 parts in 100 millions. 

 This is on the assumption that L 2 =L 3 . If L 2 and L 3 each 

 have a resistance of about 01 ohm but differ in resistance by 

 10 per cent., then the error introduced by neglecting a and 

 taking the equation 



P=(R+R')/2 



as an exact one is equivalent to about 0°0001 C. The want 

 of equality of the leads and the want of equality of the 

 ratio-coils may therefore be easily eliminated as sources of 

 error. 



Practical Application. — The reversals which have been 

 indicated are conveniently made by means of a six-pole 

 switch with connexions as shown in fig. 4. It is of course 

 easy to arrange for the change in position of the battery lead 

 to be made simultaneously. 



Yvr 4. 



If the link a always forms a part of L 3 , then b will always be in series 

 with L 2 and c with R ; equality of the resistances of the links a, b, c is 

 then of no importiuice. Mercury contacts are employed. 



As the method is practically useless unless R is a resistance 

 capable of being changed in steps varying from 0*00001 ohm 



