of Small Resistances, 



35 



until a balance was obtained in each position. To eliminate 

 thermoelectric errors the two batteries were simultaneously 

 I'eversed and the same process repeated ; from the mean value 

 of S, B was found in terms of A. The main advantage of 

 the method lies in the simple and straightforward way of 

 using the potential terminals, no current being taken from 

 them when a balance is attained. The main defect is that 

 large steady currents must be maintained, and this almost 

 necessitates running the currents for a considerable time and 

 altering the temperature of the coils. 



In connexion with this I may mention the following simpli- 

 fication of the method, proposed by Mr. S. W. Melsom for 

 the purpose of comparing two nearly equal resistances. In 

 fig. 2 R and R' are the resistances to be compared, while D 



Fi?. 2. 



-A/VV\AA 



AA/VVVVVV 



I'WWVWwwvNAAN 



s 



^NAA'VVX 



.A^ 



may be any unknown resistance of convenient magnitude. 

 The double switch is arranged to switch the ends of F and Gr 

 from R to W. Thus a standard E, can be copied at R^ with- 

 out the use of any other known resistances. 



Thomson Bridge. — The setting up of this was similar to 

 that used at the German Reichsanstalt, the connexions being 



Fi?. 3. 



IVWWVVWWVWVWj 



as in fig. 3. As the current entered and left by the potential 

 leads at K and L, the leads HK and LM were included infthe 



D2 



