THE MEASUREMENT OF RESISTANCE 191 



It has been assumed that the resistance a remains constant. 

 Consequently all clamps and connections must be firmly set up 

 and a itself should be of so large a cross-section that it will not 

 heat with the largest current which is used. The following 

 points should be attended to: the resistance of a must be made 

 as small as possible; the total bridge current I B should be as 

 large as is consistent with absence of heating in the various 

 bridge arms; R l , and consequently R c and Rb, should be 

 small; their magnitude is limited' by the fact that they are 

 usually adjustable to single ohms, and a certain definite per- 

 centage precision is usually required in the results. They should 

 be large enough so that there is no danger of heating, and so 

 large that the resistances of 

 the connection wires }G and dE 

 are negligible. If it should 

 prove that R c and R b are 

 much smaller than R l , it 

 would be better to make the 

 adjustment by changing R l 

 rather than as suggested 



above. The standard resist- Rheo * 



o v i j i i FIG. 108. Circuit diagram for 



ance 8 should have ample Thomson bridge. 



current carrying capacity. It 



may be necessary to keep the temperature of X down by immer- 

 sion in an oil bath. 



Thomson Bridge or Kelvin Double Bridge. 6 The elimina- 

 tion of the intermediate resistance a may also be accomplished 

 by means of the Thomson bridge. The scheme embodied 

 in this instrument is that most frequently employed for the pre- 

 cision comparison of low resistances; it is also commonly used for 

 special bridges designed for the rapid measurement of the con- 

 ductivity of samples of wire. 



Inspection of the theoretical diagram will show that this 

 arrangement differs from the Wheatstone bridge in the addition 

 of two auxiliary resistances, m and n, which are placed in series 

 and shunted around the resistance a, which is to be eliminated; 

 one galvanometer terminal is connected to the junction of m 

 and n. See Fig. 108. 



The conditions necessary for a balance may be shown thus: 



