SCIENCE AND PRACTICE. 323 



exactness is desired, to make two observations with the same 

 batteries, E and E', with other resistances, r x and E^, in order 

 to eliminate x altogether. 



Thus, by the first measurement with R x and r, we have 

 JE_ r + x + E,, 



i; = ~RT 



and by another measurement, with E' x and r lf 



_E_ r,+ x + B'i 

 E, ~~ ~ R', 



which, being combined* and x eliminated, give 



_E_ (r-QCR.-R',) 

 E, - (R, - R',) 



a formula in which differences of resistances only appear. 

 The measurement may be made without knowing the values 

 of either r or K^ ; as it is only necessary to note the difference, 

 A> the addition to or subtraction from the side c E b, which 

 compensates a difference, A', in the same sense, in the circuit 

 c E, x 5, and we have 





Mr. Yarley has pointed out to us that which he considers 

 a source of objection to this method. It is that the tension 

 of the constant cell is different in the circuit in which its 

 current is 0, to that which it would be if the circuit were closed 

 without this condition, while we measure the force of the 

 battery E, whose elements have only a tension due to an 

 ordinary closed circuit. This objection does not, however, 

 interfere with the correctness of the system ; for as the unit cell 



* These two measurements enable the operator to ascertain the resistance 



-p 



of the battery E. If, instead^'of eliminating x, we eliminate , and seek the 



value of x, we get 



_ Ri O'4-R'.Q R'l 



And by dividing this expression by n, the number of elements in the battery 

 E, we get the average resistance of each of the elements composing it. 



Y 2 



