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LXIX. On a Modification of Mance's Method of measuring 

 Battery Resistance. By Oliver J. Lodge, B.Sc* 

 [Plate V.] 



THE modification here suggested consists simply in using 

 a galvanometer and condenser instead of a galvanome- 

 ter alone, so as to detect variations in difference of potential 

 instead of variations in current. 



By this change it is converted into a strictly null method. 

 Moreover it is now possible entirely to get rid of the effects of 

 variations in the electromotive force of the battery, which are 

 very annoying in any of the ordinary methods and prevent ac- 

 curate measure. This is accomplished by breaking the galva- 

 nometer-circuit the instant after the battery is short-circuited. 

 Fig. 1 (PI. V.) is a diagram of the connexions for measuring 

 the resistance of the battery d, with the keys shown on a large 

 scale : m partially short-circuits the battery when depressed ; 

 n closes the galvanometer and condenser circuit unless de- 

 pressed. The two keys are electrically independent ; but the 

 stand of the upper one is balanced so as to rest partly on the 

 spring of the lower one (which must be strong). On depress- 

 ing the upper key, the first effect is to close the circuit marked 

 r at the point m ; the second, and immediately succeeding, 

 effect is to break the circuit marked g at the point n. The 

 same object would be accomplished* more conveniently by a 

 single double-contact key made on purpose, as shown in fig. 4. 

 The object of the double key is fully explained below. A B C D 

 represents a box of resistance-coils; a and b are large and 

 equal resistances ; and c will be equal to d 7 the resistance of 

 the battery, whenever the galvanometer-needle is unaffected 

 by pressing down the keys. 



Resistance-measurements in general. 



Consider the arrangement of six conductors joining four 

 points (commonly known as the Wheatstone's bridge) as form- 

 ing the edges of a tetrahedron or triangular pyramid (fig. 2). 

 It is obvious, (1) that, as far as position is concerned, every con- 

 ductor has precisely the same properties as any other, and (2) 

 that any one conductor is adjacent to four of the others and 

 opposite to the remaining one. Call the resistances of pairs of 

 opposite ones a and c, b and d, r and g, and let electromotive 

 forces be caused to act in any manner through any of them ; 

 then it can be shown that when ac = bd, r and g are "conju- 

 gate conductors," or that variations in the conductor r have 

 no effect whatever on the current in </, and vice versa, no 

 * Communicated by the Physical Society. 

 2*L 2 



