336 On the Question of the Unit of Electrical Resistance. 



laid down in Germany. In the construction of, and in the 

 necessary determinations of faults in, these lines, by the me- 

 thods I had given, "the practical engineer M had repeated op- 

 portunities of making exact resistance-measurements, and of 

 fully learning to appreciate the use of a knowledge of the 

 electrical laws which governed his work, even then. 



Complete sets of resistance-coils, from 1 to 10,000 units, 

 were arranged by the help of the weight system in 1859, and 

 used extensively in the cable-tests which my brother (C. W. 

 Siemens) and I made in England. Mr. Fleeming Jenkin can- 

 not have forgotten that he himself made tests of the Indian 

 cable, at Birkenhead, under my direction, with such resistance- 

 coils. 



In his sketch, Mr. Fleeming Jenkin should also not have 

 forgotten to bring forward the fact that, in our report upon the 

 Red- Sea line, in the year 1859, the relative resistances of the con- 

 ductor and insulator of the cable were given in mercury units, 

 and that the method we followed to measure the resistance which 

 the insulating covering offered to the electric current, and to com- 

 pare it with that calculated from the specific resistance of the in- 

 sulating material, forms the foundation of the rational system of 

 cable- testing introduced by us, and which, with very trifling 

 modifications, is in general use. 



Further, Mr. Fleeming Jenkin should not have passed over 

 in silence the paper read by my brother before the Eighteenth 

 Meeting of the British Association, in which our systems of cable- 

 testing, both before, during, and after laying the cable, are 

 fully developed. I have yet to learn that any practical me- 

 thods of determining faults in cables, besides those proposed by 

 me, exist. 



Speaking of the mercury unit, Mr. Fleeming Jenkin gives 

 Marie- Davy (without, however, quoting any published matter) 

 the honour of being the first who proposed mercury as a ma- 

 terial adapted to the construction of a normal resistance-mea- 

 sure, and affords me only the credit of having materially fur- 

 thered the exactness of measurements by the great care paid to 

 the adjustment of my coils and apparatus. He is, however, 

 silent as to the fact that they who had previously spoken of mer- 

 cury as a fitting material gave no clue whatever to a method 

 by which exact standards might be constructed with it. 



In conclusion, Mr. Fleeming Jenkin must himself allow that 

 his historical sketch is remarkably incomplete. 



