274 Mr. W. Brown on Steel Magnets. 



The magnetometer M consists of a small circular mirror m 

 8 millim. in diameter, with two short magnetic needles 10 

 millim. long and 0"8 millim. diameter, attached to the back of 

 it and suspended by a single approximately torsionless silk 

 fibre 16 centim, long and gig of a millim. in diameter; the 

 whole being enclosed in a case of which the base and lower 

 part is wood and the upper part a glass tube 2 centim. in 

 diameter. The upper end of the silk fibre is attached to a pin 

 which can be lowered or raised by means of the nut a working 

 in a collar on a brass cap fixed to the top of the glass tube. 

 This pin is for the purpose of adjusting the height of the 

 mirror so as to allow it to hang centrally in a cavity cut in the 

 wooden block b at the lower end of the tube. The pin is also 

 capable of a lateral motion in any direction by means of three 

 screws (not shown in the drawing) through the brass cap 

 and impinging on the collar. The cavity in the wooden block 

 b for receiving the mirror is 12 millim. in diameter and 

 3 millim. deep, thus allowing 1 millim. of clearance all round 

 the mirror ; the front of the cavity is closed by a piece of thin 

 plate glass g. 



The base of the magnetometer is fitted with three conical 

 feet which fit accurately into the well-known hole-slot-and-plane 

 arrangement of Sir William Thomson, the hole and slot being 

 cut in a piece of thick plate glass which is fixed to a table in a 

 position where the horizontal component of the earth's magnetic 

 force is known. Eastwards from the magnetometer at a dis- 

 tance of 127 centim. is a boxwood scale S divided into half- 

 millimetres, and having an electric g^ow-lamp immediately 

 behind it. The deflexion of the light-spot from the magneto- 

 meter-mirror on the scale can be read to ^^q- of a millim. 

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

 the orifice of a copper funnel c through which the beam of 

 light passes from the lamp. A dark screen s serves to pre- 

 vent any undue reflexion from the lamp on to the scale. 

 Westwards again from the magnetometer, at a distance of 40 

 centim., is placed the grooved plane P for holding the magnets 

 during the deflexion observations. The base of this plane is 

 made on the same geometrical principle as the magnetometer, 

 and is arranged so that when it is in position a line passing 

 through the middle of the groove and the centre of the mag- 

 netometer-mirror shall be at right angles to the magnetic 

 meridian. 



The magnet and magnetometer being placed in their re- 

 spective positions, the magnetic moment per gramme of the 

 magnet was calculated from the w^ell- known formula: 

 Htan%2^2^ 



