THE CALIBRATION OF INSTRUMENTS 607 



each, which should be capable of adjustment in single ohm 

 steps. In addition, a suitable galvanometer and k<>\ an 

 required. The wire ab causes the points a and b to assume tin- 

 same potential. If the current through the ammeter be denoted 

 by 7, the potential at d differs from that at a by IR. If the poten- 

 tial difference at the terminals of the battery BI be P.D., that 

 between b and c will be, 



P.D. ^~^- 

 When the potentials at c and d are the same, 



IR = P.D. r , 1 , 

 ri + r% 



and if they are the same, the galvanometer will not deflect when 

 the key is depressed. Consequently, if r\ or r 2 has been adjusted 

 so that the galvanometer gives no deflection, 



7 - P ' D V " 



-t r* XN 



12 f i + r, 



One cell of storage battery is the most satisfactory for use 

 at BI. In order that its P.D. may remain constant, the cell 

 should be partially discharged. A standard cell cannot be used 

 at BI because it is incapable of supplying even a small current 

 without alteration of its e.m.f. through polarization. 



The first step in carrying out this test is to determine the P.D. 

 of the auxiliary battery BI. This is to be done by Poggendorff's 

 method. 



The connections shown in Fig. 370 are then made; it is in 

 sary that BI and R be connected + to +. 



In general, on closing the key there will be a deflection which 

 is to be brought to zero by adjusting r\ or rz. When a balance 

 is obtained, the ammeter is to be read immediately. 



If ri -f r 2 greatly exceeds the battery resistance of BI, P.D. 

 will be approximately the e.m.f. of the cell. It is well to re- 

 member that imperfect connections to the cell and excessive 

 lead resistance have the same effects on the results as high bat t cry 

 resistance at BI, and that this battery resistance should be 

 negligible both during the test by Poggendorff's method and the 

 subsequent use of the arrangement in determining the current. 



