296 Mr. W. Brown on the Effects of Percussion in 



for the first set of six magnets, when they were remagnetized 

 after a series of observations. 



In order to obtain the deflections for calculating the mag- 

 netic moments, the apparatus used consisted of a lamp and 

 scale, a magnetometer, and a cradle for holding the magnets. 

 The magnetometer was of the ordinary Bottomley type, con- 

 sisting of a small circular mirror, with two short magnetic 

 needles attached to the back of it, and suspended by a single 

 torsionless silk fibre, the whole being enclosed in a slot cut in 

 a pyramidal block of wood, and the slot covered by a plate of 

 thin glass. On the base of this pyramid were fixed three 

 conical feet, which fitted accurately into the conical hole, 

 groove, and plane arrangement of Sir William Thomson. The 

 hole and groove were cut out of a piece of thick plate glass, 

 which was firmly fixed to the table in a position where the 

 horizontal component of the Earth's magnetic force was 

 known. When the magnetometer was put in position, the 

 mirror and the attached needles of course placed themselves 

 in the magnetic meridian. 



Immediately to the west of the magnetometer, at a distance 

 of 40 centim., was placed a cradle for holding the magnets 

 during the deflection observations. The base of this cradle 

 was made on the same geometrical principle as that of the 

 magnetometer, and was so arranged that the magnet could be 

 reversed relatively to the magnetometer, without touching the 

 magnet by hand. This cradle, I may say, was made and used 

 by Mr. Gray in his recent determination of the Earth's hori- 

 zontal magnetic force ; the whole arrangement is fully 

 described and illustrated by a drawing in his paper*. 



To the east of the magnetometer, at a distance of 129 centim., 

 was a glass scale divided to millimetres, and having a lamp 

 placed immediately behind it. The deflection of the spot of 

 light from the lamp when reflected by the mirror of the mag- 

 netometer upon the scale could be read to j-q of a millimetre, 

 by means of the shadow cast by a fine wire stretched across 

 the orifice in the side of the copper funnel of the lamp. 



The magnet and magnetometer being placed in position, 

 the magnetic moment of the magnet, M, is given by the 

 following equation : — 



^_ Htan(9(V 2 --^) 2 



where r— the distance of the centre of the magnet from the 

 centre of the magnetometer-needle ; 



* " On the Measurement of the Intensity of the Horizontal Component 

 of the Earth's Magnetic Field," Phil. Mag. December 1885, pp. 484-497. 



