156 Mr. F. Jenkin Report on the New Unit [April 6, 



in the same year reduce the resistance of their experiments to lengths of a 

 given German-silver wire, and as a further definition they give its value as 

 compared with pure silver. To avoid the growing inconvenience of this 

 multiplicity of standards, Jacobi* (in 1848) sent to Poggendorif and others 

 a certain copper wire, since well known as Jacobi' s standard, desiring that 

 they would take copies of it, so that all their results might be expressed in 

 one measure. He pointed out, with great justice, that mere definition of 

 the standard used, as a given length and weight of wire, was insufficient, 

 and that good copies of a standard, even if chosen at random, would be 

 preferable to the reproduction in one laboratory of a standard prepared and 

 kept in another. The present Committee fully indorse this view, although 

 the definition of standards based on weights and dimensions of given 

 materials has since then gained greatly in precision. 



Until about the year 1850 measurements of resistance were confined, 

 with few exceptions, to the laboratory ; but about that time underground 

 telegraphic wires were introduced, and were shortly followed by submarine 

 cables, in the examination and manufacture of which the practical engineer 

 soon found the benefit of a knowledge of electrical laws. Thus in 1847 the 

 officers of the Electric and International Telegraph Company used resist- 

 ance-coils made by Mr. W. F. Cooke, apparently multiples of Wheatstone's 

 original standard, which was nearly equal to the No. 16 wire of commerce ; 

 and Mr. C. F. Varleyf states that, even at that date, he used a rough 

 mode of "distance testing." In 1850, Lieut. Werner Siemens published 

 two methods for determining, by experiments made at distant stations, the 

 position of " a fault" that is to say, a connexion between the earth and 

 the conducting-wire of the line at some point between the stations. In one 

 of these plans a resistance equal to that of the battery is used, and the 

 addition of resistances is also suggested ; and Sir Charles Bright, in a Patent 

 dated 1852 , gives an account of a plan for determining the position of a 

 fault by the direct use of resistance-coils. Since that time new methods of 

 testing for faults and of examining the quality of materials employed, and 

 the condition of the line, have been continually invented, almost all turning, 

 more or less, on the measurement of resistance ; greater accuracy has been 

 continually demanded in the adjustment of coils and other testing-apparatus, 

 until we have now reached a point where we look back with surprise at the 

 rough and ready means by which the great discoveries were made on which 

 all our work is founded. 



The first effect of the commercial use of resistance was to turn the " feet " 

 of the laboratory into " miles " of telegraph wire. Thus we find employed 

 as units, in England the mile of No. 16 copper wire||, in Germany the 

 German mile of No. 8 iron wire, and in France the kilometre of iron wire 



* Comptes Rendus, 1851, vol. xxxiii. p. 277. t letter to writer, 1865. 



I Pogg. Ann. vol. kxix. p. 481. Patent No. 14,331, dated Oct. 21, 1852. 



1| A size much used in underground conductors, and equal in resistance to about 

 double the length of the common No. 8 iron wire employed in aerial lines. 



