Mr. F. Jenkin on the New Unit" of Electrical Resistance. 479 



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 resistance- 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. Varley* states that, even at that date, he used a 

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

 published two methods for determining, by experiments made at 

 distant stations, the position of " a fault" — that is to say, a con- 

 nexion 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 J, 

 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 of 4 millimetres diameter. Several other units 

 were from time to time proposed by Langsdorf H, Jacobi^ff, Marie- 

 Davy**, Weberff, W. Thomson:^, and others, with a gradually 

 increasing perception of the points of chief importance in a standard ; 

 but none of these were generally accepted as the one recognized 

 measure in any country. To remedy the continually increasing 

 evils arising from the discrepancies invariably found between dif- 

 ferent sets of coils, Dr. Werner Siemens (in 1860§§) constructed 

 standards, taking as unit the resistance of a column of chemically 

 pure mercury 1 metre long, having a section equal to 1 millimetre 

 square, and maintained at the temperature of 0° Centigrade || ||. 



* Letter to writer, 1865. 



f Pogg. Ann. vol. lxxix. p. 481. + Patent No. 14,331, dated Oct. 21, 1852. 



§ 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. 



|| Liebig's Ann. vol. lxxxv. p. 155. ^f Pogg. Ann. vol. lxxviii. p. 173. 



** Ann. Chim. et Phys. 3rd series, vol. ix. p. 410. 



tt Pogg. Ann. vol. lxxxii. p. 337. 



$ | Phil. Mag. Dec. 1851, 4th ser. vol. ii.p.551. §§ Pogg. Ann. vol. ex. p. 1. 



|| I Dr. Siemens, while retaining his definition, has altered the value of his 

 standard about 2 per cent, since the first issue ; and it is doubtful whether even 

 the present standard represents, the definition truly : his experiments were made 

 by weight ; and in reducing the results to simple measurements of length he 

 has used a specific gravity for mercury of 13*557 instead of 13-596 as given by 

 Eegnault, 13-595 by H. Kopp, and L3-594 by Balfour Stewart. 



