ON STANDAEDS FOK USE IN ELECTIUCAL MEASUREMENTS. 433 



drawn, the calibration showed such a want of uniformity in the con- 

 ducting power at different parts of its length that it was at once 

 discarded. 



Finally, an iridio-platinum wire was obtained from Messrs. Johnson 

 and Matthey, fitted to the cylinder and calibrated, but the result not 

 being considered quite satisfactory, the wire was removed, carefully 

 annealed, drawn through one hole in a draw plate and remounted, but 

 with very little better results. This operation was repeated without 

 advantage, and it became evident that the want of uniformity in the 

 conducting power was not due to irregularities in the section of the wire, 

 but was to be attribvited, in all probability, to want of uniformity in the 

 composition of the alloy. 



To avoid further loss of time it was therefore decided to make use of 

 the wire as it then was, and to correct all readings by the result of an 

 accurate and close calibration. 



The wire was therefore calibrated in 100 equal parts by a method 

 devised after trying one or two others, and found to be very accurate 

 and convenient. 



It is fully described at the end of this paper. 



The wire is wound in twenty convolutions in a spiral groove, 

 accurately formed in the cylindrical surface of an insulating drum. The 

 ends of the wire ai"e soldered to massive bars, each brazed to one axle 

 of the drum, which terminates in an amalgamated copper disc, half 

 immersed in a cup of mercury. 



The mercury cups are themselves connected with the rest of the 

 apparatus by means of very stout copper rods. 



The contact-piece, by means of which the galvanometer is put in circuit, 

 is mounted on a brass block, moving between two brass rods, and 

 traversed by a screw after the fashion of the slide rest of a lathe. One 

 end of the screw carries a toothed wheel, gearing with another wheel 

 attached to the drum and concentric with it. The pitch of the screw 

 and the gearing are so calculated that, when the drum is made to 

 revolve, the contact-piece, whilst moving in a horizontal line parallel 

 with the axis of the drum, is always in contact with some point of the 

 wire, upon which it presses lightly by means of a spring. 



The brass toothed wheel has a slightly greater diameter than the 

 drum to which it is attached. On the flat exterior side near its circum- 

 ference, it is divided into 1,000 equal parts, and by means of a microscope 

 with cross-wire eyepiece, the divisions can be read by estimation to 

 tenths and easily to fifths. 



As there are twenty turns, the whole wire can therefore be accurately 

 divided into 100,000 parts. 



Whole turns of the wire are shown by the divisions of a horizontal 

 scale close to the contact-piece. 



To maintain the wire throughout its length at a uniform temperature, 

 the drum is enclosed in a wooden case in which openings are left for the 

 contact-piece and microscope. A sketch of the apparatus is given in 

 Plate VI. 



To determine the temperature-coefficients of the various bars and 

 wires, their resistances at two diff'erent temperatures were compared with 

 that of a standard, maintained as nearly as possible at a constant 

 temperature. 



The higher temperature of comparison was generally nearly that of 



1881. F F 



