438 REPORT— 1881. 



with tlae two ends of the wire to be calibrated, properly mounted on its 

 drum. To these copper bars the poles of the battery could be connected 

 by means of a contact-key. 



The series of coils was arranged, in the order above enumerated, in a 

 trough of water, to maintain uniform temperature. One galvanometer 

 wire was permanently attached to the sliding contact of the drum, whilst 

 the loose end of the other galvanometer wire could be dipped in either 

 of the mercury cups joining the coils, the arrangement thus forming a 

 Wheatstone's bridge. 



The loose galvanometer wire, being first inserted in the mercury cup 

 joining the outer or left-hand terminal of the coil la with the connecting 

 bar, a balance was obtained, the reading j-q on the drum being of course 

 quite close to the end of the wire. 



The galvanometer wire was then shifted to the cup between la and 

 lb, and a balance and reading r, obtained by moving the druai-contact. 



The coils la and lb were then transposed in position, and a new 

 balance and reading r,, very near to rj, were obtained. The loose wire 

 was then moved to the cup between la and 2, and the balance and read- 

 ing rg were observed. 



This process of transposition and reading was repeated until the coil 

 la had been moved unit by unit from the left-hand to the right-hand 

 extremity of the series. 



As it is evident that the resistance of the length of wire between pairs 

 of readings such as r,„ r,, and r^, r^, &c., bears the same ratio to that of 

 the whole wire to be calibrated as the resistance of the coil la does to 

 that of the series of which it forms part, and that the latter ratio is a 

 constant one, being independent of the position in the series which the 

 coil occupies, it follows that every such length has the same resistance ; 

 and the lengths being expressed in terras of divisions of the drum circle, 

 it is easy to make a table showing the proper corrections. 



In practice, the coil la was further subdivided. Four intermediate 

 points having been determined on the wire of which it was composed, 

 dividing its resistance into fi.ve equal parts, wires were soldered to these 

 points and were connected with small mercury cups sunk in the ebonite 

 bridge-piece of the coil. The case was then filled in with paraffin wax as 

 usual. In calibrating the drum-wire, therefore, besides the 20 pairs of 

 readings corresponding with the 20 transpositions of the coil, there were 

 interpolated between each such pair four additional readings, thus cali- 

 brating the wire into 100 parts. 



The following table gives the I'eadings corresponding to the twenty 

 transpositions of the coil, their diSerences, which are proportional to the 

 conducting power of the wire between the points at whicli the readings 

 are taken, and the percentage- variation from mean conducting power. 



In Col. IV. are given the total percentage-corrections for the readings 

 in Col. V. They are obtained by taking from Col. III. the algebraic 

 sum of the percentage- variations for all observations included in the 

 reading, and dividmg by the number of terms summed ; thus the correc- 

 tion for drum-reading 3,000, and thereabouts is l'Ql''> + Q-835+ 0-513 



o' 

 = 0'788% of the readings. All these corrections happen to be 

 subtractive. 



