WEBER ON A TRANSPORTABLE MAGNETOMETER. 5G9 



shown*, that if both bars are six times smaller, the deflecting 

 bar may be brought six times nearer to the magnetometer, with- 

 out its being necessary to take more exactly into account the 

 distribution of free magnetism in the bars. If, then, the length 

 and breadth be diminished, and the thickness be left unaltered, 

 (the large bars are 600™"" long, 36'"°» broad, and 9"»"' thick ; and 

 the small bars 100"in^ long, 9"^ broad, and 9""» thick,) it follows 

 that as much may be gained in the small magnetometer, by in- 

 creasing the angular deflection, as is lost by diminishing the di- 

 stance of observation. In fact, the experiments of deflection 

 admit of a precision which leaves nothing to be desired, and 

 which harmonizes perfectly with the degree of accui-acy which 

 is known to be of easy attainment in the eocperiments of vi- 

 bration. 



Of course the small magnetometer must be constructed in 

 such a manner that all its parts may form a solid whole, so that 

 their relative position may not be liable to be disarranged by 

 packing, unpacking, or putting up. It must be possible both 

 to set the magnet bar at Uberty, and to secure it again while in 

 its case, as is done in the common compass, and the torsion of 

 the thread must not be altered in so doing ; the access of air 

 must be quite cut off even from the mirror, which may be 

 observed through a thin plate of mica, if a pidce of plane glass 

 ground parallel is not to be obtained. It is very advantageous 

 to make the case entirely of copper, and even of strong 

 copper-plates, not only for the sake of the increased solidity 

 given to the whole apparatus, but also because the case will 

 thus act on the inclosed magnet as a damper, and all the 

 measurements may be made with much greater rapidity. The 

 instrument must be so strong and solid, even when used in the 

 open air, that it may carry two arms, which ser\^e for placing 

 the deflecting bar at equal measured distances east and west. 

 These arms being correctly placed, all the preparations for the 

 experiments of deflection which would otherwise be necessary, 

 — namely, placing the measuring bars horizontally, and in a di- 

 rection perpendicular to the magnetic meridian, and finding 

 the corresponding points on either side of the magnetometer, — 

 are spared, and the experiments are rendered much easier, and 

 require less time. 



• Sci. Mem. Part V. page 86. 



