EXPERIMENT STATION BULLETINS. 561 



lettered exactly as in Fig. 1. The letter H represents the battery which 

 actuates the induction coil K. The known resistance or set of resistance 

 coils is Y, the unknown resistance or the solution Avhose resistance is 

 desired is Z. T is the telephone. At this place it is not necessary to go 

 further into the theory of conductance measurements but merely to call 

 attention to the fact that this network illustrates the "Null" method in 

 which the quantity being measured is balanced against some other quan- 

 tity and the point of equilibrum determined by means of a telephone. 

 It is less rapid than the deflection method but on the other hand is much 

 more accurate and there is less necessity for applying corrections. "Null" 

 methods are generally to be preferred for all precision work. 



The Wheatstone Bridge network described below is the outcome of a 

 systematic study of the various sources of error in the ordinary method. 

 We were led to such a study because of all the known methods for de- 

 termining the resistance of electrolytes none proved suitable for the par- 

 ticular problem then under investigation. Not only were there certain 

 errors that could and should be eliminated, but the method we thought 

 could also be simplified and made rapid and easy. It has been assumed, 

 in the case of dilute nutrient cultures that the external errors due to the 

 nature of the experiment are greater than the errors due to the apparatus. 

 If this is true it is hardly advisable to improve the apparatus. However, 

 the methods of control can be greatly improved so that a corresponding 

 improvement in the apparatus is welcomed. It is obviously better to con- 

 trol all the errors possible, whether of the apparatus or experimental 

 technique, than to be satisfied to "let well enough alone." In addition to 

 its value to the biologist this method can be used by the most exacting 

 physical chemist. It has a precision ten times that of the Kohlrausch 

 method, and the measurements can be made with ease and rapidity. 



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