64 wenner: four-terminal conductor 



If the current leads are connected to the potential terminals 

 there is, when a current is flowing, a difference in' potential be- 

 tween the current terminals. The ratio of this potential differ- 

 ence to the current is the same as with the regular connections. 

 If the current leads are connected to one of the current terminals 

 and to a corresponding potential terminal, there may be a differ- 

 ence in potential between the other two terminals when a current 

 flows. The ratio of the difference in potential to the current is 

 what we may call the cross resistance. 



Y 



Fig. 1. Thomson Bridge Method 



Where the cross resistance is appreciable, errors may be intro- 

 duced unless the method of measurement is such as to eliminate 

 its effect as well as the effect of the connecting resistances. 



Of the different methods available for the comparison of low 

 resistances, we shall consider here only the Thomson bridge 

 method. This method has been in use for about a half century 1 

 and many of its advantages were pointed out in the original paper. 

 The arrangement of the conductors in the bridge is shown in Fig. 

 1. Here X and Y designate the low resistance conductors to be 

 compared and A and B the conductors of the main ratio set. 



1 Sir Wm. Thomson, Phil. Mag., 24: 149. 1862. 



