Siemens Mercury Unit of Electrical Resistance. 351 



w _ 3 o^_ {<>_(_* X \ AB /A\ ■ 



VI. Calculation of the Absolute Resistance. 

 If we insert these numbers for A, B, X, T, and further those 

 for X , t , K, and S in the formula (page 305) 



S^T 2 



K 'rf 



we obtain for the absolute resistance of the inductor and galva- 

 nometer for the four determinations, taking as unit the resist- 



, „, n millim. , . n earth-quadrant 



ance 10 10 r , or briefly - — -. , 



second J second 



^=3-9687 3-9937 3-9903 3-9849 earth -q uadran t . 



second 



The greatest difference of these numbers corresponds to a 



change of temperature in the wire of less than 2°, which cannot 



be controlled. 



VII. Comparison of the Resistance w with Siemens' s Standards. 



The two standards, Nos. 1135 and 1143, each of which con- 

 tained 4 mercury units, were guaranteed to be correct at 19°*4 

 and 18°*3 respectively. The increase of the resistance in the 

 German-silver wire amounted for 1° to 0'0004 of the whole. 

 Hence the ratio, if both were at the same temperature, must 

 have been 



No- U43 _ 



No. 1135 — l WU44 - 



I found, by comparison with third resistances, 



1-00050, 1-00046, 1-00055; mean = l-00050. 



The difference from the above ratio corresponds to an error of tem- 

 perature of only o, 15, and may therefore serve as a test for the 

 accuracy of the copies and of the method of comparison. 



At the beginning of the observations I did not possess the 

 standards; and instead of them I constructed four provisional 

 German-silver resistances (I will denote them as A, B, C, D), 

 each nearly equal to 4 Siemens's units, and which in the deter- 

 mination I. served for comparison with the inductor. They were 

 subsequently compared with the standards, which had meanwhile 

 reached me. 



A = 4-1021 B = 4-0977 C = 4-1095 D=4'0965 Siemens. 



For interpolation two tenths of a Siemens unit were necessary. 

 They were constructed of two pieces of German-silver wire, sol- 

 dered in lengths of 325 millims. to copper rods. As it was 

 found, moreover, that 3250 millims. of the same wire had the 



