﻿312 Dr. S. R. Milner on the Use of the Secohmmeter 



tapping the key which makes k zero, but method (1) is 

 somewhat simpler to carry out. 



Practical Considerations — When any one of the three 

 partial equalities of (1) has been obtained by a balance at 

 the corresponding lead, the attainment of the complete equality 

 becomes of course only a matter of the adjustment of the re- 

 maining variable to balance at any other lead. In practice, 

 much the best method in general is to adjust to r=k at & = %, 

 as described on p. 308, and then, turning the commutator 

 until 8=0 (where the galvanometer is most sensitive to 

 variation in p) balance the resulting deflexion by the adjust- 

 ment of P or Q. The advantage of the r = k balance is that 

 the corresponding commutator position is both a fixed one 

 independent of the speed, and easy to ascertain without the 

 necessity for introducing any extra tapping key into the 

 bridge system. It has the disadvantage, however, that it 

 contains both the unknowns S and K, and consequently no 

 information can be drawn from it until the complete balance 

 has been ascertained. The p — k and p = r balances are not 

 subject to this disadvantage, and for this reason they may 

 occasionally be usefully em} loyed to make a direct measure- 

 ment of one member of a combination when a knowledge of 

 the other is not required. I have found them useful in 

 saving time in making a rapid preliminary survey of electro- 

 chemical combinations which were varying in value in the 

 course of an experiment. 



The only objectionable feature of these methods is the re- 

 striction theoretically necessary on the magnitudes of the 

 resistances F and /of the battery and galvanometer branches 

 (i.e., F must be large and/ small). Both conditions tend to 

 reduce the sensitiveness of the arrangement, but the dis- 

 advantage in this respect is more apparent than real, and the 

 last sensitiveness may be easily regained. Thus in the ar- 

 rangement of fig. 1, where the condition (F large) requires 

 in practice the insertion of a high resistance in series with. 

 the battery, the E.M.F. of the battery may be increased in 

 proportion to F without any possibility of overstepping the 

 limit of permissible current through any part of the system. 

 This is not the case in the arrangement of fig. 3 ; on the 

 other hand, in this case the use of an electrometer in place 

 of a galvanometer will completely satisfy the theoretical 

 requirements. 



As regards the practical obtaining of a complete balance 

 t is not, however, necessary to attempt to satisfy accurately 

 he theoretical conditions. The neglect of the second con- 

 dition (/ negligible) makes so very little difference to the 



