1884.] On a New Reflecting Galvanometer, tyc. 



287 



February 14, 1884. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table and thanks ordered for 

 them. 



The following Papers were read : — 



I. " On a New Reflecting Galvanometer of Great Sensibility, 

 and on New Forms of Astatic Galvanometers." By 

 Thomas Gray, B.Sc, F.R.S.E., and Andrew Gray, M.A., 

 F.R.S.E. Communicated by Prof. Sir William THOMSON, 

 F.R.S. Received February 5, 1884. 



The object of this paper is, first, to describe a galvanometer in 

 which a principle which is, we believe, novel in its application to this 

 particular purpose, has been employed to give an instrument of very 

 great sensibility ; and, second, to describe some forms of astatic gal- 

 vanometers which are, we think, improvements on astatic combina- 

 tions as ordinarily made and arranged for use. 



Fig. 1 shows in elevation and horizontal section the coils and 

 arrangement of needles adopted in an instrument of very great sensi- 

 bility which we designed in the summer of 1882, and which with 

 assistance from the Government Research Fund, we have had con- 

 structed for use in our experiments on the Electric Resistance of 

 Glass and Allied Substances. It consists of two pairs of coils, c c c c, 

 with hollow cores, arranged so that the axes of each pair are parallel 

 and in a vertical plane. Each pair is carried by a vertical brass 

 plate, and the two plates are inclined to one another at an angle of 

 about 106° ; and thus the vertical planes containing the axes of the 

 coils are inclined to one another at an angle of about 74°. Two 

 horseshoe magnets, m, m, made of steel wire of about 1 millim. in 

 diameter, are connected together by a very light frame of aluminium, 

 .and are at such a distance apart as to hang, when the needle system 

 is in equilibrium, with no current in the coils, freely within and 

 nearly along the axes of the cores of the coils. The horseshoes are 

 not plane, but are bent round so that they form approximately 

 portions of one vertical cylinder of which the suspension-thread is the 

 axis, and to which the axes of the coils form horizontal tangents near 

 their middle points at the approximate positions of the poles of the 

 needles. This form was given to the needles to lessen the chance of 



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