Siemens Mercury Unit of Electrical Resistance. 345 



The observations of deflections of the accessory needle, upon 

 which the bar M acted in the two east-westerly positions at an 

 angle of 180° with each other, gave on a scale, the centre of 

 which was at 770 millims. : — 





Adjustments. 



f- 



S. 



<Po- 



Aug. 19. North... 

 South... 



millims. 

 1250-48 

 1210-55 



millims. 

 207-25 

 167-34 



O 1 II 



4 54 32 

 4 54 22 



+3-5 



+6-2 



O 1 II 



4 54 38 

 4 54 35 











Mean = 4 54 36 



Aug. 22. North... 

 South... 



124706 

 1229-33 



202-84 

 185-62 



4 54 47 

 4 54 36 



-0-9 14 54 46 

 +0 5 1 4 54 36 



Mean = 4 54 41 



We find from this for the zero-point of the bifilar magneto- 

 meter the values mentioned above (page 306), 



August 19. 

 „ 23. 



Mean 



T = 1-83860 

 T = 1-83832 



T„= 1-83846 



(The magnetism M of the principal bar is found in close 

 agreement, =538630000 or 538710000.) 



From the degree of agreement of all these numbers, it seems 

 to result that, w T ith such means, a determination of the hori- 

 zontal intensity to within an error of at most 0*1 per cent, may 

 be safely undertaken. 



Determination of Absolute Resistance. 



I shall always collate the measurements of one kind. In so 

 far as refers to operations which belong to any single operation 

 out of the four, they will be discriminated by la., lb., IT., 

 and III. 



Terrestrial Inductor. — This has been described in Weber's 

 paper, " Application of Magnetic Induction to the Measurement 

 of Inclination" {Abh. der Gbtt. Ges. 1853, vol. v. p. 53). The 

 surface of the coil is there given at 39216930 square millims. 

 From a comparison of the standard then used with the normal 

 metre, one part of the former is equal to 1*00086 millim. ; so 

 that the above number is changed into 



S = 39284000 square millims. 



The galvanoscope (PL IV. fig. 1 in J of the true size) consists 

 of a multiplier of about 250 coils of copper wire 3 millims. in dia- 

 meter, in 10 coils, on a wooden frame 100 millims. in breadth. 

 The cylindrical magnets of the astatic couple are each 170 mil- 



