332 M. W. Siemens on the Question of 



value. Mr. Sabine, in a note to his paper on the unit, gave the 

 following comparisons between the calculated values of tubes 

 which had been used in previous determinations : — 



No. of tube. 



Original 



determination, 



1859. 



First 



reproduction, 



1860. 



Second 



reproduction, 



1863. 



3 



5 



7 

 8 



555-87 

 193-56 



555-99 



193-73 



191732 



260057 



55605 



19373 



1917-54 



2601 46 



The greatest difference therefore between the assigned values 

 of these tubes by the first and last determination does not exceed 

 01 per cent. 



From the first determination only a few standards and sets of 

 resistance-coils were made, originally for our own use. Those 

 sets of coils which have been made subsequently, for our own use 

 or for sale, have been, without exception, adjusted according to 

 the second determination of the unit. Lastly, from the mean 

 values of the third determination I have had about a hundred 

 standards adjusted, which have been presented to the various phy- 

 sicists, technologists, and scientific institutions, in the hope of 

 furthering the adoption of a rational measure. When sent from 

 my atelier these standards were exactly alike, and, if they have not 

 since altered, agree to within 0*05 per cent, with the mercury 

 unit when, allowing for the corrected specific gravity, at a tem- 

 perature 10° *5 Cels. above that at which they were adjusted. 



The proof advanced by Dr. Matthiessen in support of his 

 assertion that the units which I have determined do not repre- 

 sent the same resistance, is based upon the measurement of two 

 of my resistance-coils, of each from 1 to 10,000 units, by Mr. 

 Fleeming Jenkin, who was employed by the jurors of the 1862 

 Exhibition to report upon the electrical instruments. Mr. Jen- 

 kin found a difference of 1*2 per cent, between the values of these 

 two sets of resistances. Whether Mr. Jenkin measured cor- 

 rectly or not* is of very little importance; but it is perfectly 

 incomprehensible how a man of Dr. Mattbiessen's experience 

 and knowledge of the subject could make use of a comparison 

 between coils adjusted for practical use years ago, when the art 

 of copying resistances was scarcely known, with resistance-stan- 

 dards carefully adjusted in 1864; and still more incomprehensible 

 is it that he could have based such an assertion upon the uncor- 

 roborated evidence of the Jury -report of an Exhibition. 



* The difference given by Mr. Jenkin as 1*2 per cent, really amounted 

 to 1*8 per cent. 



