34 Mr. A. Campbell : Measurements 



approximately O'l, O'Ol, and O'OOl international ohm re- 

 respectively/ For convenience o£ reference I shall allude to 

 them as X, Y, and Z. Three other standard resistances ot 

 1, 1, and 10 ohms, whose values have been accurately deter- 

 mined, formed the starting-point in each o£ the methods. 

 Let them be named P, Q, and R respectively The proce- 

 dure was to find first the ratio o£ X to P, then that of Y to 

 X, and, finally, that of Z to Y ; this gave X, Y, and Z in 

 terms of P. the following methods were used :— 



(1) Shunt Potentiometer. 



(2) Kelvin Bridge. 



(3) Two-step Bridge. 



(4) Differential Galvanometer. 



(5) Matthiessen and Hockin's Method. 



As most of these methods are well known I shall discuss each 



^^(1) ^Shmt Potentiometer— As shown in fig. 1 the potentio- 

 meter consisted of the known standards R and Q, one or 



Fi-. 1. 



other of them (say Q) being shunted by a high resistance S, 

 while A and B were the resistances whose ratio was to be 

 found. A special double key was arranged so that the cross- 

 connexions (the wire F and the galvanometer G) could be 

 quickly switched over from A and R to B and Q. The main 

 current through A and B was kept constant, and the current 

 throuo-h R and Q was adjusted till a balance was obtained in 

 the first position (A against R). The switch was then turned 

 to the second position, and Q was shunted so as to balance 

 ao-ain, the process being repeated with successive adjustments 



