Calometric Absolute Measurements. 131 



The current-intensity i was measured in absolute electro- 

 magnetic measure by means of the already mentioned simple 

 tangent-compass (R = 165'7 millims.) according to the relation 



EH A 3Z 2 \ nj _m + 



for the measurement of u, mirror, telescope, and scale were 

 made use of. To eliminate the daily variations of H, which 

 on some days may reach J- per cent, of the mean value, H was 

 determined, for the place of the tangent-compass, before and 

 after each measurement. The variations of declination of the 

 earth's magnetic force (which towards noon are very consi- 

 derable for delicate measurements) were eliminated by regu- 

 larly recurring, rapidly executed reversals of the current. A 

 very powerful damper enveloped the magnetic needle of the 

 compass, and permitted the readings of the deviations of the 

 magnet to be taken again 20 seconds after the reversal of the 

 current. The intensity of the current was maintained constant 

 within ^ Jq or g J-^- of its value by aid of a Dubois-Reymond's 

 rheochord in the path of the current. The quantities I and 6 



3 I 2 

 were so small that the sum of the two corrections, — -r ^ + 0> 



amounted to only + 0*0008. 



The absolute value of the resistance w was determined by 

 the method described above in section II. As the tempera- 

 ture t a of the calorimeter-environment varied somewhat from 

 one day to another (up to 3°), the coefficient of the increase 

 of the resistance for 1° of rise of temperature had also to be 

 known. To obtain the latter the absolute value of the resist- 

 ance w was determined for the two temperatures (maintained 

 constant) 0° and 23°. At the same time the value of w for 

 the same temperatures was measured in relative Siemens 

 measure. The resistance of the platinum wire was found as 

 follows : — 



e , P r In absolute measure. In relative measure. Date, 



ture. 



22-5 14-498 xl0 1Q ( millim, > ) 15*141 S.U. Oct. 14, 1870. 

 \ sec. / ; 



22-9 14-419 xlO 10 „ 15-142 „ „ 15, 



23-7 14-486 x 10 10 „ 15-154 „ „ 16, 



14-141 xlO 10 „ 14-782 „ „ 17, 



14-121 xlO 10 „ 14-791 „ „ 18, 



14-130 xlO 10 „ 14-770 „ „ 19 ; 



K2 



