366 Br. Matthiessen on the so-called Mercury Unit, 



Weber) was found by him in 1861 equal to 4*025 Siemens 

 (London) units. 



That Thomson's coil had not altered during this time was 

 proved by comparing it again with a coil of wire of the gold- 

 silver alloy which I made, and compared with the coils before 

 they were sent to Weber in 1860. The new comparison agreed 

 identically with the first one. 



Thomson's coil =4-025 S. (London) units =15*543 of an arbi- 

 trarily chosen unit, made for my own experiments, 

 Mile coil . =53780 ditto; 



and therefore the mile coil = 13*93 Siemens (London) units, 

 proving that the mile coil has not altered, for Mr. Jenkin gives 

 the value equal 13*95*, showing a difference of rather more 

 than 0*15 per cent., a difference, considering the roundabout 

 way the value is arrived at, which is very small ; in fact it is 

 much smaller than the differences of which I have now to speak. 

 Now the mile coil equals in resistance 13*59 B. A. units. 



Mile coil equal 13*95 Siemens (London) units, 

 „ „ 14*12 „ (Berlin) units, 

 therefore 



Siemens (London) unit =0*9750 B. A. units. 



(Berlin) „ * =0*9632 



A new unit issued by the firm in 1864 . =0*9564 „ 



It would be of interest to know which of these units Messrs. 

 Siemens consider the correct odc Owing, however, to their 

 using the wrong specific gravity for mercury, the values for com- 

 parison with others must be corrected by multiplying them 



13*559 

 by y when we arrive at the following values for a metre 



of mercury of a square millimetre section at 0° according to 



rue value in B. A. units. 

 . 0*9723 

 . 0*9605 

 . 0*9534 



Siemens (London) unit , 



(Berlin) „ . . 

 „ (1864) „ . . 



Mean .... 0*9620 

 Value deduced from experiments"] 



made by Mr. Hockin and myself > 0*961 9 

 (1864) . J 



From the above we see how closely the Siemens (Berlin) coil 



* Mr. Jenkin informs me that the values given in his Table may possibly 

 be 0*1 per cent, wrong ; he does not think that any of them are 025 per cent, 

 wrong. 



