the Unit of Electrical Resistance. 335 



terminations can be put forward as a proof of the incorrectness 

 of mine. 



Mr. Fleeming Jen kin, in his paper * " On the New Unit of 

 Electrical Resistance proposed and issued by the Committee," 

 &c, brings forward no new features whatever, but turns the 

 conclusions and experiments of Dr. Matthiessen to account 

 in a still more extended way. The communication which he 

 makes, that four of my 1864 standards were compared with four 

 different copies of the B. A. unit by four different observers, 

 with four different measuring-apparatus, giving the values 

 1-0456, 1-0455, 1*0456, and 1-0457 respectively, is interesting. 



The mean value of these observations (1*0456), multiplied into 

 the coefficient of correction for the right specific gravity of 

 mercury, or 



§§gx 1-0456=10486, 



is therefore the value of the B. A. unit in mercury units, or 

 1 mercury unit=0*9536 B. A. units. 



In the present uncertainty of the true relation between the 

 values of the B. A. unit and 10 7 s " c ^ units, we can translate 

 the value of a resistance given in mercury units into terms 

 of the 10 10 Weber's unit, or 10 7 ^~ units measure by sub- 

 tracting 5 per cent. 



The historical sketch of the order in which different pro- 

 positions for resistance-measures have been made, and the pro- 

 gresses in the field of resistance-measurements, with which Mr. 

 rieeming Jenkin commences his paper, induce me to make a 

 few observations in order to correct some errors and omissions 

 of things which interest me personally. 



Complete sets of resistance-coils, from 1 to 100 units, each 

 unit equal to the resistance of one geographical mile of copper 

 wire of 1 line diameter, at a temperature of 20° C, have been, 

 since 1848, made in the Berlin branch of my establishment, 

 repeatedly described, and distributed far and wide. Mr. Jenkin 

 says that "until about the year 1850 measurements of resist- 

 ance were confined, with few exceptions, to the laboratory ; but 

 about that time underground telegraphic wires were introduced, 

 and were shortly followed by submarine cables, in the exami- 

 nation and manufacture of which the practical engineer soon 

 found the benefit of a knowledge of electrical laws." 



Mr. Jenkin ought surely to be aware that, as early as 

 1847 and 1848, subterranean lines of considerable lengths were 



* Phil. Mag. June 1865, p. 477. 



