ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 121 



resistances. In this latter case if P and Q are exchanged, P is equal to 

 Q when there is no change in the deflection, and no error is introduced 

 by want of symmetry in the galvanometer coils or inequality of the 

 resistances of the galvanometer circuits, always supposing that these 

 latter remain constant throughout the ol)servations. Unfortunately, the 

 resistances of these circuits do change, for the reason previously given, 

 and the error introduced may be considerable. Let P=Q=0'1 ohm, 

 and let the resistance of the leads of P=0-0001 ohm, and of Q=00002 

 ohm. (In some low resistances the potential leads are of the order 0*0 1 

 ohm.) Then if G = l, and no correction is applied for the inequality of 

 the leads, the error of measurement is 0-01 per cent. If G = 100 ohms, the 

 error is O'OOOl percent., but the sensitiveness is reduced to one-fifth of its 

 former value. Such errors are abolished if the Kohlrausch method of 



Fig. 12. 



Fig. 13. 



overlapping shunts ' is used, of which a diagi-am of connections is given 

 in fig. 13. In fig. 12 let P=Q. Then, unless there is symmetry of the 

 galvanometer coils and equality of resistance of their arms, there will be 

 a deflection. Supposing that G and g can be exchanged in position by 

 substituting for X a re.sistance practically identical with it, then the 

 deflection will be of the same magnitude and of the same sign as before. 

 In practice P is made equal to Q by shunting one of them, the equality 

 being determined by the equality in magnitude and sign of the deflection 

 before and after interchanging G and g. The exchange is eflfected by a 

 six-pole switch as shown in fig. 13. The resistance of the galvanometer 

 circuits is thus constant, and it is apparent that the Kohlrausch method 

 of using the differential galvanometer is the only one so far suggested 

 that can be used for precision measurements. 



' Wied. Ann., 188.3,20,76. See also article hv Jaeger, ZoitseJ/r. I7istri(menten- 

 hinde, 1901, 288. 



