Siemens Mercury Unit of Electrical Resistance. 343 



Intensity of Terrestrial Magnetism. 



Professor Klinkerfues had the goodness to arrange that, du- 

 ring the period of the observations, local magnetic influences 

 should be avoided in the Astronomical Observatory where the 

 variation-instruments were suspended. For observing the latter 

 I am indebted to Dr. Riecke. 



With regard to the arrangement of the bifilar magnetometer, 

 together with accessory needle, I refer to Weber's article, " De- 

 termination of the rectangular component of the Earth's Mag- 

 netic Force in Gottingen, 1834-1853"*. The value of a division 

 of the scale of the bifilar magnetometer at that time 1 found to 

 be equal to O000105 in parts of the whole. The intensity cor- 

 responding to a position 8 of the bifilar will be obtained in the 

 sequel from the expression 



T= 1-83846 (1-f 0-000105 .S). 



Absolute measurement. — The moment of inertia of the principal 

 bar was, according to a new determination which agreed to within 

 TO o o o w ^ n one f° rmer ly made by Weber, 



K =42997 . 10 6 millimetre 2 milligramme, 



the temperature in two successive observations being 15°. 



The ratio of torsion was 0-01085. 



The time of vibration, e, observed with the arc of p divisions, 

 gives that reduced to zero, since the scale was at a distance of 

 4125 divisions from the mirror, 



K 1 - 3565i2^)= T ( 1 - 000000000023 -^- 



If, further, during the observations of vibration 8 was the 

 mean position of the bifilar magnetometer calculated from a de- 

 finite point of the scale (to which all the observations are reduced), 

 the time of vibration actually observed must be multiplied with 

 1 + i • 0-000105 . 8 ; we therefore put 



t =t(1-0-00000000023 .^« + 0-000052 . 8). 



Let, finally, M denote the magnetism which the vibrating bar 

 possesses of itself — that is, in the east-west direction in which 

 it was afterwards used as deflecting bar. In the vibrations M 

 has a magnetism of position, which Weber had determined in the 

 case of this bar to be equal to 780000 absolute units. Then 



(M + 780000)T = t1 ^ 08 , . . . (A.) 



For the purpose of making observations of deflection, the bar 

 * Abh. d. Gott. Ges. 1855, vol. vi. 



