330 M.W. Siemens on the Question of 



British Association, has undoubtedly been made with the greatest 

 possible exactness. I am, on the contrary, certain that science 

 has materially profited by a most serviceable work. 



But I think that the Committee, on convincing itself that 

 Weber's absolute unit even was not adapted for a normal mea- 

 sure, would have done better not to have set up another arbitrary 

 unit, but to have had the mercury unit, as defined by me, re- 

 produced with all possible care, with the great talent and the 

 perfect arrangements at their command, and to have distributed 

 copies of it. This unit is already very generally employed, 

 and is found to answer perfectly all practical wants. The cor- 

 rection-coefficient to reduce it to absolute measure could then 

 have been determined as exactly as possible and published. By 

 this procedure the Committee would have conformed with Kirch- 

 hoff's proposition, published in the appendix to the first Report, 

 that both measures should be retained, and not exclusively Weber's 

 system — an opinion in which Wilhelm Weber himself perfectly 

 coincided, and with which the Committee, in its first Report, de- 

 clared itself satisfied. 



There is no doubt that with the rich means at their command, 

 the Subcommittee could have determined the mercury unit with 

 an accuracy quite equal to present requirements — that is to say, 

 equal to the comparison of two different resistances — as is proved 

 as well by my original experiments as by the more recent and 

 careful measurements by Robert Sabine. 



Future and more complete determinations of the unit would 

 become necessary as the exactness of physical measurements 

 progresses. This, however, could scarcely give rise to inconve- 

 nience, as the true value of the unit may be definitely settled, — 

 the differences which have shown themselves between the three 

 determinations made in my laboratory being so small as to 

 enable them to be neglected entirely in practice in ordinary 

 measurements of electrical resistances; while for exact measure- 

 ments it would be necessary to repeat from time to time the ad- 

 justment of the coils, on account of their possible want of per- 

 manency. 



Unfortunately the Committee has not thought proper to follow 

 the way which Kirchhoff and I proposed to them. On the con- 

 trary, two of its members (Professor Matthiessen and Mr. Fleem- 

 ing Jenkin) have attacked my proposition not only in the reports 

 of the Committee, but also in special papers*, in a way not hitherto 

 customary, I think, in scientific critiques. The plan followed 

 by these gentlemen in common does not consist in opposing the 

 principle of the system by any reasonable grounds, but in attack- 

 ing the trustworthiness of my labours. 



* Phil. Mag. S. 4. vol. xxix. p. 361, and p. 477- 



