98 Messrs. Duncan, Wilkes, and Hutchinson on the Value 



of German silver. Accordingly a series of experiments were 

 carried out on this question, with the result that the linear 

 expansion of the specimen used was found to be '0000154 per 

 degree Centigrade. 



The relative extra expansion of platinoid wire is therefore 

 2-7 

 151' or 5V- 



XIV. A Determination of the Value of the B.A. Unit of 



Resistance in Absolute Measure, by the Method of Lorenz. 

 By Dr. Louis Duncan, Gilbert Wilkes, and Cary T. 

 Hutchinson*. 



THIS work was done at the Physical Laboratory of Johns 

 Hopkins University during the spring of 1 888. Lord 

 Rayleigh's modification of Lorenz's original method was used. 

 In this, as is well known, a measured part of the current 

 flowing through the inducing coils is balanced by the current 

 induced by the rotation of the disk. 



The apparatus employed is that designed by Prof. Rowland 

 for his determination of the ohm undertaken for the United 

 States Government. A detailed description of it is contained 

 in his forthcoming report, so only a few words will be given 

 to it here. The induction-coils, four in number, were wound 

 in square channels cut in heavy flanges, which were cast on 

 the exterior of a hollow brass cylinder open at both ends. 

 The coils were respectively 30*171, 9*786, 10*545, and 30*775 

 centim. from the mean plane of the disk, itself placed as 

 nearly as possible midway between the ends. The cylinder is 

 about 66 centim. long, 100 centim. in diameter, and 1 centim. 

 thick. It is thus the longest ever used in work of this kind. 

 The flanges and cylinder were cast in one piece, and the 

 tooling was all done without removing the casting from the 

 lathe. The walls of the channels were left very thick to 

 prevent spreading during the winding of the coils. The 

 radius of the disk was so chosen that an error in its value 

 should enter as slightly as possible in the value of the co- 

 efficient of induction. 



The disk was brass, 21*5 centim. radius and *5 centim. 

 thick. It was fixed to a brass axle, 3 centim. diameter, turn- 

 ing in bearing-boxes carried by suitable framework fixed 

 inside the cylinder. There was a cone of grooved pulleys 

 toward one end of the axle, used for getting different speeds 

 of the disk. The motor for running the disk was in the 



* Communicated by the Authors. 



