432 EEPOET— 1881. 



An investigation into tLe causes of these variations was therefore 

 desirable ; and at the request of your Committee it has been undertaken 

 by myself. As yet, no very definite result has been reached, but the 

 observations already made, involving much care and a very large ex- 

 penditure of time, and the method of experimenting employed, may 

 perhaps be worth recordiug. 



I should here say that thi-oughout the investigation I have had the 

 benefit of the co-operation of Mr. Charles Hoekin, by whom many of the 

 observations were made. 



To better observe the variation in the temperature-coefficient with 

 change of diameter, rods of considerable sectional area, to be afterwards 

 drawn into wire, Avere obtained from Messrs. Johnson & ]\latthey. 



The first rod (called hereafter Bar A) was of the commercial alloy 

 manufactured specially for electrical purposes ; the metals used are 

 commercially pure, and are melted together in large quantities. The 

 alloy is then cast as a flat ingot, not more than an inch or an inch and a 

 half in depth, and this ingot is next rolled into a large sheet about 0'3 

 inch thick, which is cut by shears into narrow strips. 



These strips are finally passed between grooved rollers, to give them 

 an approximately circular section, and the rods thus formed are ready for 

 the draw plates to reduce them to wii'e of the required diameter. 



The bar experimented on was, in the ' rod ' stage, abotit 8 inches long, 

 and 2 7 inch in diameter. 



The second rod (called hereafter Bar B) was specially made for these 

 experiments of pure materials, by Messrs. Johnson and Matthey. 



It was cast in the form of a bar, about 8 inches long. On leaving 

 the mould it was about 0"3 inch square, slightly tapering, and more or 

 less irregular in section, but was reduced in the lathe and by filing to a 

 section almost absolutely square and uniform. 



The third rod (called Bar C) was of an alloy made by the same firm 

 for the use of dentists — the method of casting and rolling being the 

 same as that described for the electrical alloy, but rather less attention is 

 paid to the purity of the components. 



It was procured in the form of a narrow strip, about a quarter of an 

 inch thick, and reduced to a uniform square section by the lathe and file. 



As it was necessary to observe accurately the small percentage- 

 variation, due to change of temperature, in the resistance of these bars, 

 which was itself exceedingly minute, I'ecourse was had to the method of 

 observation originally proposed by Mr. C. Hoekin, and described and 

 figured in Clerk Maxwell's ' Electricity and Magnetism,' pp. 406 and 

 407, vol. 1, by means of which the unavoidable resistance of the con- 

 nections can be altogether eliminated. 



Instead, however, of using, as shown in the figure referred to, a 

 comparative'iy short wire, with resistance-bobbins at its ends, a wire 

 40 feet in length, wound on a cylinder, was employed, so that the bobbins 

 could be dispensed with, without loss of accuracy, and with a great gain 

 in simplicity of calculation and in range. 



To avoid the very great expense of a necessarily thick wire of iridio- 

 platinum, which was, however, recognised to be the best matei'ial, a 

 german-silver wire was in the first instance fitted to the cylinder and 

 calibrated, but after a short time it was found to get loose in its groove, 

 so that the readings obtained on it could not be depended on. 



A platinum-silver wire was next tried, and though most carefully 



