2O2 



APPLIED SCIENCE 



used resistance coils for practical experiments connected with 

 telegraphy; but it was not till 1857, during the manufacture 

 of the last seven or eight hundred miles of the Atlantic Cable, 

 that the copper was systematically selected. This example was 

 followed in the lied Sea Cable, when the resistance of the con- 

 ductor was regularly tested by Mr. Fleeming Jenkin at Birken- 

 head, and by Messrs. Siemens during the laying. The copper 

 of the first portion of the Atlantic Cable was not selected in this 

 manner, and was of very indifferent quality. Since then the 

 improvement has been continual. Dr. Matthiessen reported to 

 the Joint Committee appointed by the Board of Trade, and the 

 Atlantic Company, in ] 858, that chemically pure copper was 

 superior to all alloys, and that the best copper for electrical pur- 

 poses was to be obtained from Lake Superior and Burra-Burra, the 

 worst from DemidofF and Rio Tinto. The gradual improvement 

 since that date may be gathered from the following table : 



The smaller the figure in the last column the better the 

 material ; the last figure represents perfection . The specific 

 resistance is the resistance of a foot of wire weighing one grain. 

 The unit in which it is measured is that selected by a Committee 

 appointed by the British Association in 1861, from whose 

 yearly reports may be learnt the reasons for preferring this to 

 other rival standards for it is by no means a matter of indif- 

 ference what unit is employed. 



The improvements in the methods and instruments used to 

 measure resistance have far more than kept pace with the prac- 



1 The writer believes that the 1866 At]antic Cable has better copper than 

 any of the cables in the above table, but he does not know the exact figure of 

 merit. 



