[ 484 ] 



LIX. On Measurements of the Intensity of the Horizontal 

 Component of the Earth's Magnetic Field made' in the Phy- 



• sical Laboratory of the University of Glasgow. By Thomas 

 Gkay, B.Sc, F.R.S.E* 



[Plate XI.] 



IN the course of some measurements of the horizontal com- 

 ponent of the intensity of the earth's magnetic field, which 

 have recently been made in the physical laboratory of the 

 University of Glasgow, several improvements in the apparatus 

 and mode of conducting these experiments suggested them- 

 selves. The following paper is a description of these mea- 

 surements. 



The method adopted was, in general principle, that of Gauss ; 

 that is, the determination by a deflection-experiment of the 

 ratio M/H, and by an oscillation-experiment of the product 

 MH, where M is the magnetic moment of a magnet, called in 

 this description the deflector, and H is the horizontal intensity 

 of the earth's magnetic field at the place of experiment. 

 The earlier experiments, of which the results are given below, 

 were, with the exception of the mode of determining the 

 effective length of the deflector, conducted in the way that 

 has for several years been practised in Sir William Thomson's 

 laboratory, and descriptions of which have already been 

 publishedf. 



The Deflection Expebiment consists in finding the deflec- 

 tion produced by the deflector on a second needle, called the 

 magnetometer-needle , suspended at a known distance from it, 

 and thus furnishing an equation of the form 



M 

 **=-^=Htan0j (1) 



/(2rt a , r) K ' 



where /(2<2j,r) is a function of 2% the effective length of the 

 deflector, and r the distance between the centre of the deflector 

 and the centre of the magnetometer-needle, and is the 

 angle of deflection. The function f(2a ly r) depends on the 

 relative direction of the line joining the centres of the deflector 

 and magnetometer-needles, the direction of the axis of the 

 deflector and the direction of the lines of force in the magnetic 

 field, and on the distribution of magnetism in the deflector and 

 magnetometer-needles. Strictly f(2a { , r) should be /(2%, b, r) y 



* Paper read before the British Association at the Aberdeen Meeting, 

 September 1885. Communicated by Sir William Thomson, F.R.S. 



T Vide Phil. Mag. for November 1878 ; < Electrician > for July 8, 1882 

 1 Nature ' for November 9, 1882. 



