112 Dr. J. A. Fleming on a new 



" bridge"-wire consists of the arm H.H' and the wire under 

 the base-board together with the galvanometer inserted be- 

 tween G and G 7 . By moving round the arm H H 7 and press- 

 ing the button w, we can find a position where there is no 

 current through the galvanometer. The copper strips k k are 

 made of copper so thick that their resistance is practically 

 nothing. Having established a balance between the conduc- 

 tors and read the vernier, the next operation is to lift up the 

 legs of the coil which were inserted in the cups a and c and 

 drop them into the cups b and d. Likewise a similar change 

 is effected on the other side ; the terminals of the coil inserted 

 in V and d' are changed to a' and c r . An examination of the 

 connexions as shown in fig. 1 will show that the result of the 

 operation is as if the coils had changed places whilst preserving 

 their former connexion. Now let the arm be moved round 

 and a fresh position of equilibrium found by pressing the 

 trigger and reading the vernier. A little consideration will 

 show that the difference of these readings gives the difference 

 between the resistances of the coils in terms of a length of the 

 bridge-wire ; for the amount by which one coil exceeds the 

 other in resistance is equal to the resistance of that part of the 

 bridge-wire included between the two readings*. In order 

 to render this method of determining the difference of the two 

 coils practicable, the platinum-iridium wire must be exceed- 

 ingly uniform in resistance, or else a Table of calibration will 

 have to be made. Great pains were taken to procure a length 

 of wire as uniform in size and resistance as possible ; and con- 

 siderable care was taken, in laying the wire in its groove, not 

 to strain it in any way. It lies evenly in its groove, just suf- 

 ficient tension being put upon it to keep it in its place. The 

 whole resistance of the wire from end to end is not far from 

 ■£$ of an ohm at about 15° C. 



The wire was carefully calibrated by measuring the differ- 

 ence in the resistance of two pieces of thick brass wire of such 

 lengths that the difference of their resistances was about equal 

 to that of thirty divisions of the bridge-wire ; and this differ- 

 ence was measured at about a hundred different equidistant 



* This method of obtaining the difference of two resistances in terms 

 of a length of the calibrated bridge wire was suggested by Prof. G. C. 

 Foster, F.R.S., in a paper read before the Society of Telegraphic Engi- 

 neers, May 8, 1872. In this paper is given an account of the method of 

 calibrating a wire. It is obvious, without any further proof, that if the 

 coil placed in a and c exceeds in resistance that placed in b and d, then 

 on exchanging them, since the united resistance of coils and bridge-wire 

 remains the same, that the contact knife-edge must be moved back along 

 the bridge-wire by a length exactly equal in resistance to the excess of 

 one coil over the other. 



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