the Unit of Electrical Resistance. 333 



Dr. Matthiessen must be well aware of the fact that points of 

 contact between solid metals have always a changing resistance ; 

 that therefore the contact-plugs of a set of resistance-coils through 

 which, either wholly or in part, the current has to pass must 

 give rise to a want of exactness. Pie will himself have frequently 

 found and be fully able to appreciate the great difficulties in the 

 way of summing up 5000 units for the first time in this way 

 without considerable error creeping in. 



The set of resistance-coils which Dr. Matthiessen calls 

 " Siemens, London/'' was one of the first made for private use in 

 1859, by a very incomplete method of combining the several 

 normal resistance-coils arranged by the weight system. The 

 set of coils in question formed one of the branches of a Wheat- 

 stone's balance with which determinations were made during 

 and after the laying of the Red-Sea and Indian cables, and was, 

 on account of the historical interest attached to it as the first com- 

 plete and practical balance of the kind, considered to be deser- 

 ving of a place in the Exhibition ; for by its help the hitherto 

 crude way of testing submarine cables had been first reduced to 

 exact method. This set of coils was subsequently readjusted by 

 an improved system. 



Dr. Matthiessen asserts, further, that the sets of coils adjusted 

 by this improved system differ 0*5 per cent, from the standards of 

 1864<. He concludes this from the measurements of a copper 

 wire which Mr. Fleeming Jenkin, during the Exhibition of 1862, 

 compared with the sets of coils. What temperature the copper 

 wire had in the two measurements, made with an interval^of 

 some years between them, is not given. A difference of \\ de- 

 gree Gels, would, however, completely account for the apparent 

 discrepancy ! 



Under any circumstances, Professor Matthiessen and Mr. 

 Eleeming Jenkin were not justified in using a single doubtful 

 determination, made only by one of them and not avow- 

 edly for any scientific purpose, to force into the Tables in the 

 Reports of the Committee, as well as in their own papers, the 

 columns " Siemens, Berlin" and " Siemens, London," next to a 

 column headed "Siemens, 1864" (containing the honest value 

 of the unit), with the too evident purpose of throwing a false 

 colouring upon the value of the work I had attempted, by 

 making it appear that the coils issued by me from time to time 

 have represented different resistances. 



Analogous to this is the oft repeated assertion that between 

 my determinations of the unit differences of 1*6 per cent, occur, 

 and that this, therefore, must be the limit of its exactness. 



It is true that in my first published experiments on the sub- 

 ject one such difference occurs. I gave at that time the cause 



