174 



Mr. E. W. Smith on the Measurement 



at the Institute of Electrical Engineers. But, as this method 

 has its objections in addition to that of interrupting the 

 circuit, it is very desirable to determine the actual working 

 E.M.F. in some other way. 



The E.M.F. could be readily obtained from the terminal 

 P.D. and the current if the internal resistance were known. 

 It is for the determination of this latter quantity that I have 

 devised the following modification of Mance's method. Of 

 course this method is applicable to the measurement of the 

 resistance of other forms of battery besidej accumulators. 



Fig. 1. 



o 7 * 



<DS 



m x 



-H 



In fig. 1 let the cell of E.M.F. e, and internal resistance,^, 

 be the one experimented upon, r being the resistance of the 

 external circuit which may contain an E.M.F. E, for example 

 that of the dynamo used to charge the cell. This circuit is 

 connected at with an auxiliary circuit, in which the resist- 

 ances m l and n± are so adjusted that the points A and B are 

 at the same potential, the resistance of the cell of E.M.F., e l9 

 being included in n x . Suppose a current, C, from some ex- 

 ternal source to pass through both circuits in series. The 

 P.D. between A and B will now be V. Let P.D. between A 

 and be v lf and that between and B be v 2) (v l + v 2 )=Y, 

 We have 



therefore 



and 



Q_ v\+e | pi + E 

 b r 



brC — re — bJZ 

 b + r 



raj + n x 



^2— *i 

 n x 



+ 



V2_ 



m x 



»2 



V _& + C— re — 5E m^iiC + m^i 

 b + r mi + n x 



