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LXV. Note on. a Determination of the Specific .Electrical Be- 

 sistance of certain Copper-Tin Alloys. By Oliver J. Lodge, 

 D.Sc, Assistant Professor of Physics in University College, 

 London*. 



ON the 5th of November I received from Mr. Chandler 

 Roberts three rods of certain very brittle copper-tin 

 alloys which he had with some difficulty cast, in order that I 

 might determine their specific resistance per unit of volume 

 by ordinary processes, because the readings which the induc- 

 tion-balance gave for disks of these three metals indicated that 

 their conductivities did not by any means agree with Mat- 

 thiessen's curve of electric conductivity for copper-tin alloys, 

 though, on the other hand, they did agree with some severely 

 criticised and discarded experiments of Messrs. Calvert and 

 Johnson on the conductivities of these same alloys for heat. 



The rods, which are hereafter spoken of as A, B, and C, were 

 about 9 millimetres thick and from 30 to 40 centimetres long. 

 The method which I employed to determine their conductivity 

 was scarcely altered from that which was described by Prof*. 

 Foster and myself in this Journal in 1875 f ; and the results 

 which I have obtained I believe to be somewhat accurate. 

 It was not worth while to aim at any very excessive accuracy, 

 though the method is quite capable of it (by a calibration of 

 the bridge- wire), because the temperature of the alloys was a 

 little indefinite, being simply that of the room. 



The three rods were laid end to end in a groove in a long 

 piece of wood so that they just projected above its surface ; and 

 their ends were then screwed up into secure contact with pads 

 of tinfoil between. The resistance of the contacts is not of the 

 slightest consequence, provided it remains constant and is mo- 

 derately small, all measurements being made within the length 

 of the rod itself. 



A bridge-slider was arranged so as to make contact with one 

 of the rods at a point whose position could be read by a milli- 

 metre-scale fixed at the proper level. The length of the portion 

 of the rod examined could be ascertained within the tenth of a 

 millimetre ; and the resistance of this portion could also be read 

 with great accuracy. The least satisfactory part of the mea- 

 surement is the gauging of the diameter of the rod ; for they 

 are not perfectly cylindrical, and the average cross section has 

 to be taken. I would have had the rods turned ; only two of 

 them, viz. B and C, are far too brittle to be touched with any 

 tool. 



* Communicated by the Physical Society. 



t Foster and Lodge, "On the Flow of Electricity in a Plane. — Part II.," 

 Phil. Mag. December 1875. 



