THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



♦ 



[FOURTH SERIES.] 



MARCH 1863. 



XXI. On a New Determination of the Mercury Unit of Electrical 

 , Resistance in Dr. Siemens's Laboratory. By Robert Sabine, 

 Esq.* 



THE result arrived at by the Committee appointed by the 

 British Association to report on standards of electrical 

 resistance has induced Dr. Siemens to take up the subject 

 again, and he has in consequence had the mercury unit deter- 

 mined for a third time. 



Following the original determination described by Dr. Siemens 

 in his paper "Vorschlag eines reproducirbaren Widerstands- 

 maasses"t> the comparison of six tubes, of very different resist- 

 ances, with one Jacobi's unit gave rise in some instances to an 

 employment of the measuring apparatus beyond the limits of its 

 exactness. This occasioned the observed resistances of the tubes 

 numbered 1, 4, 5, and 6 to appear greater or less than their 

 calculated values, according as a was less or greater than Z>J; 

 where this was not the case, the differences were inconsiderable. 



At the time of the first reproduction §, only three of the six 



* Communicated by the Author. 



f PoggendorfFs Annalen, vol. ex. p. 1. 



X o, and b were the readings of the bridge wire. This will completely 

 answer the criticism of Dr. A. Matthiessen (Pogg. Ann. vol. cxiv. p. 312), 

 who repeats Dr. Siemens's figures of the differences in question as a proof 

 that the method of a reproducible standard by means of mercury is not 

 more to be depended upon than that of his gold-silver alloy, but which 

 indeed only prove that the observed resistances are not so exact as the cal- 

 culated, — in other words, that the measure produced was more exact than 

 the instruments by which it was employed. 



§ The reproduction alluded to took place in the summer of 1861 (Pogg. 

 Ann. vol. cxiii. p. 94). 



Phil Mag. S. 4. Vol. 25. No. 167. March 1863. M 



