wenner: four-terminal conductor 65 



Later we shall use X and Y to designate the values of the low 

 resistances and A and B to designate the values of the resistances 

 in the ratio set between p and y, and y and p.' We shall also use 

 X m and Y m to designate the inductances of the low resistance con- 

 ductors. The equations generally given for use with this method 

 have been derived without taking into consideration the cross 

 resistances of the four terminal conductors. The relations given 

 by the equations are therefore not exact as has generally been 

 supposed. Recently Prof. Searle has derived new equations 2 

 giving the relations between the various resistances including the 

 cross resistances. These equations are necessarily somewhat 

 complicated and in order to be able to use them in calculations, 

 it is necessary to know the values of a large number of resistances. 

 If, however, adjustments are made so that the bridge is in balance 

 using alternately p and p,' and q and q' as branch points and if the 

 balance is not disturbed on removing the connector Z it can be 

 shown that 



X/Y = A/B (1) 



Low resistance standards are sometimes used in alternating 

 current measurements. In such cases it is generally necessary 

 to know both the resistance and inductance or to know that the 

 inductance is so small that at the frequency used, the phase angle 

 may be considered zero. The Thomson bridge method may be 

 used for measuring both the resistance and inductance. 



A general equation, giving the relations between the resistances 

 inductances and frequency of the alternating current necessary 

 for a balance, would be very complicated. If however, adjust- 

 ments are made so that, under the conditions given above, the 

 bridge is balanced both when using direct current and when using 

 alternating current the relations become much simpler and as a 

 particular case it can be shown that 



X m /Y m =X/Y = A/B (2) 



Here it is assumed that the inductances of A and B are so small 

 in comparison with their resistances that their time constants 

 may be considered zero. 



5 Electrician, 67: 57. 1911. 



