Dr. Matthiessen on the so-called Mercury Unit. 365 



is based on the fact that for the calculations the wrong specific 

 gravity of mercury has been used, viz. 13*557 at 0° instead of 

 13*595 as found by Regnault, Kopp, Balfour Stewart, and 

 Neumann. These observers all give the same value for the spe- 

 cific gravity of mercury at 0° as compared with water at 4°. (Their 

 values only vary up or down 007 per cent, from the above value, 

 the maximum being 13*596, and the minimum being 13*594.) 



These data prove either that no true mercury units have been 

 issued, or if 13*557 be the true specific gravity of mercury at 

 0°, then the experiments of Regnault, Kopp, B. Stewart, and 

 others must be all incorrect — a highly improbable result ; in fact, 

 one is irresistibly led to the conviction that it is utterly impos- 

 sible that those distinguished observers can all have made such 

 mistakes, and therefore that the value used, viz. 1B*557, is wrong, 

 and the units issued, supposing them to be otherwise quite 

 correct, will each have a resistance about 0*3 per cent, greater 

 than it ought to be. 



2ndly. That the units issued from time to time have not the 

 same resistance. In the Exhibition of 1861 were two sets of 

 coils said to be reproductions of the mercury units ; now the 

 one set was exhibited by Siemens (London), the other by Sie- 

 mens (Berlin), and when the two sets were compared with each 

 other they showed a difference of about 1 *2 per cent. 



The value of the resistance of these two units in terms of the 

 B. A. unit is as follows : — 



Siemens (London) .... 0*9750 

 Siemens (Berlin) .... 0*9632 



These values have been obtained in the following manner : — 

 Mr. F. Jenkin, in his Report on Electrical Instruments 

 (Jurors' Report, 1862, p. 82), gives a Table containing the values 

 of different coils, and on reference it will be seen that the coil 

 whose resistance equals a mile of pure copper, &c, is equal to 

 13*95 Siemens (London) and 14*12 Siemens (Berlin) units. 

 Now, when I adjusted this mile coil for Messrs. Elliots Brothers, 

 I made another coil of the same resistance of different German- 

 silver wire, and arranged it in a perfectly different manner from 

 the coil tested by Mr. F. Jenkin. On comparing these two coils 

 with each other after the lapse of three years, they were found 

 exactly equal, showing that they had not altered ; for it is hardly 

 probable that two different specimens of wire put up in different 

 methods should alter in resistance to a like extent. 



Again, another proof of the mile coil not having altered is the 

 following. Mr. F. Jenkin sends me the following value : — Thom- 

 son's coil (one in my possession, a duplicate of that sent to 



