306 M. F. Kohlrausch on the Absolute Value of the 



cause the same strain on the suspending- thread in all the obser- 

 vations, and likewise admit of very accurate measurement of the 

 distance between them. Finally, the bifilar magnetometer is 

 supplied with a Weber's auxiliary needle*. 



I had myself the gratification of using these arrangements for 

 the first time in their entirety, while determining the terrestrial- 

 magnetic elements at Gottingen in 1867, and of convincing 

 myself of their excellence. During these determinations, and in 

 a subsequent series, I acquired an accurate knowledge of the 

 constants of the instruments and sufficient practice in their use. 



While referring to the next section for the observations them- 

 selves, I will adduce here two pairs of determinations of intensity, 

 from each of which variations in the terrestrial magnetism are 

 eliminated, and which can therefore be directly compared. The 

 following are these values : — 



1867, with an interval of sixteen days, 1-83960 and 1*83849; 

 difference =0*00011. 1869, with an interval of three days, 

 1-83860 and 183832; difference =0*00028. 



Expressed in terms of the whole, these differences amount 

 to 0*00006 and 0*00015. But the error arising from the moment 

 of inertia and the measurements of distance is not included in 

 these differences ; hence for the possible error AT of the hori- 

 zontal intensity we may take about ten times the above amount 

 — that is ^-p i 



2. The moment of inertia K of the astatic pair of needles was 

 determined by two different methods. One of them gave 

 K= 1,135,700,000 ; and the other, which was not quite free 

 from objection, 1,132,800,000. We will assign to the former de- 

 termination the weight two, and assume as the error AK the 

 deviation of the two determinations from the mean — that is, 



AK_ 1 

 K ~~ ± 750' 



3. The inductor -surface S is the only magnitude which I 

 have not measured myself. It was determined by Weber when 

 he constructed the inductor ; he measured the length of the wire 

 by coiling it upon a large wheel about 3 metres in diameter f. 

 It was moreover controlled by measuring the individual wind- 

 ings. I put the possible error 



AS = + 1 



S ~ 2000 



4. The time of vibration t amounted to about 34'4 seconds. 



* Abhandl. d. Gotting. Gesellsch. d. Wissensch. vol. vi. (1855). 

 t Abh. der Gott. Ges. 1853, page 53 of the separate impression. 



