346 EBPOBT — 1851 



Report to Francis Ronalds, Esq., on the Performance of his three 

 Magnetographs during the Experimental Trial at the Kew Observa- 

 tory, April 1 till October 1, 1851. By John Welsh, Esq. 



In making a preliminary report on the performance of the three Magneto- 

 graphs during the experimental trial of them which lias just expired, I shall 

 confine myself to a statement of — 1st, the methods adopted for adjustment 

 of the several instruments; 2nd, the means of preserving a record, nume- 

 rical and grapliical, of the photographic registers ; and 3rd, the general 

 capabilities of the instruments of affording data for magnetical investigation. 

 The instruments having been already minutely described in your various 

 reports on the Observatory, it is unnecessary to make any reference to their 

 mechanical construction. 



I. Adjustments, &c. 



Declination Magnetograph. — This instrument was put into adjustment on 

 March 27-29. The suspending thread, of untwisted silk, was examined 

 throughout its whole extent, and found to have retained its original condition. 

 The magnet having been removed, a brass bar of the same weight was in- 

 serted in its place, and the amount of torsion existing in the thread examined. 

 This amounted to only 10° and was eliminated. 



The value in arc corresponding to a given ordinate on the registering 

 plate depends upon, — first, the distance from the centre of motion of the 

 magnet and its appendages to the slit in the moveable shield; and second, 

 the number of times by which the image of a certain motion of the slit is 

 magnified when represented upon the registering plate. The first of these 

 is obtained by direct measurement with a beam compass; for the determi- 

 nation of the second, the following contrivance was resorted to : — A scale 

 divided on plane glass to joth of an inch was placed in such a position that the 

 lines of the graduation were at the same distance from the lens as the slit in 

 the moveable shield when the magnet is in full adjustment. A scale of the same 

 value divided on ground glass was placed in the sliding plate-frame, and upon 

 it the magnified image of the first scale was received at the proper focus. 

 Both the scales were in this way visible on the same surface: the one 

 magnified, and the other of the natural size. It was then easy to observe 

 the value of the one in terms of the other; the number of divisions of the 

 real scale corresponding to one division of the apparent scale representing 

 the magnifying power of the lens. If a be the required arc value correspond- 

 ing to an ordinate d on the plate, r the distance of the slit from the centre 

 of motion of the magnet, and m the magnifying power of the lens, we have 



d „ <^ 



o=tan~i , or when the angular motions are small, a:=arc~' very 



m.r ° m.r •' 



nearly. The value of r was found by measurement to be 18'0 inches ; and 

 that of m, by the process above-described, 6*706 ; whence we have the arc- 

 value of an ordinate of one inch=28''48. 



By turning the arms of the torsion-circle through different angles, it was 

 found that a twist of 90° in the thread deflected the magnet through 44' ; 



whence the value of the torsion coefficient I 1 +^ | = 1*008. 



(-?)=■ 



This torsion effect being taken into account, the arc-value of one inch 

 ^28'*71. The scale employed in the process of tabulation being divided 

 to j'^th of an inch, the factor for converting the recorded numbers into 

 minutes of arc is 0''574. 



