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 XLVII. Intelligence and Miscellaneous Articles. 



NEW METHOD OF DETERMINING THE MAGNETIC INCLINATION 

 WITH THE INDUCTION-COMPASS. BY M. WILD. 



IN the number of the Comptes Bendus for November 26, 1883*, 

 M. Mascart published an article " On a Magnetic Induction- 

 Compass/' in which he mentions the method which I pointed out for 

 the use of this instrument; but his description might lead to the 

 belief that my method is only a somewhat useless complication of 

 the old one of M. Weber. I may be permitted here to describe my 

 method, in order to show that it has a real advantage over that of 

 Weber. 



The method adopted by M. Weber for determining the magnetic 

 inclination by means of currents induced in a coil by the horizontal 

 and vertical components of the earth's magnetism is an easy and 

 exact experiment, and does not require a complicated calculation. 

 It is simpler than the modification which M. Mascart and I have 

 pointed out, for it does not require a divided circle, while the orien- 

 tation and the levelling of the axis of the coil are almost the same 

 for the three methods. It requires, moreover, less time than the 

 determination of the inclination with an ordinary dip-circle, and 

 yet gives more constant results. That its use is not more widely 

 extended is due to the fact that the apparatus requires more time 

 for its installation, as well as a special locality. 



In several observatories where the induction-compass has been 

 introduced, considerable differences have been found (as much as 

 15') between the inclinations deduced from experiment and those 

 obtained with ordinary compasses; and preference has generally 

 been given to the former as implying fewer sources of errors. By 

 more thorough researches at the observatory of Pawlowsk, with 

 two induction-compasses of different construction, and a very good 

 Dover's circle, I showed that this supposition was wrong, and that 

 the greater part of the error arose from the induction-compasses. 

 One of the two compasses which I used gave a difference of 1 5' 

 and the other only one of 5', in comparison with same compass of 

 the ordinary kind. In the memoir (Memoires de TAcademie imjpe- 

 riale des Sciences de Saint Petersbourg, vol. xxvi. No. 8, Nov. 1878) 

 in which I described these experiments, I showed that the error 

 arose from the fact that the deflections of the galvanometers 

 usually employed in these experiments were not exactly proportional 

 to the strength of the current, even when these deviations only 

 amounted to from 2° to 3°-5. Hence, in order to avoid these errors, 

 it was necessary either to determine the functions according to 

 which the sensitiveness of the galvanometer varies with the deflec- 

 tion, and to develop the theory of the oscillation of a magnet under 

 such conditions, or to find a method of working with the induction- 

 compass which should be independent of this want of constancy of 

 the sensitiveness of the galvanometer. 



* Phi . Mag. Jan. 1884, p. 78. 



