334 Mr Searle, On a compound magnetometer for [May 16, 



is placed near the magnet G and deflects it through an angle 

 proportional to the strength of the current in L, the deflections 

 being kept very small. The solenoid M is placed in a vertical 

 position east or west of the mirror, its upper end being about in 

 a horizontal line with the mirror. The small coil N can be 

 adjusted so that the effect on the magnet F of the solenoid itself 

 is completely neutralised. R is a resistance box for varying the 

 current, P a battery, and Q a commutator. A lens S of about 40 

 inches focal length forms the window of the case in which the 

 suspended part is hung. A lamp and screen are placed at about 

 40 inches from the lens. Cross wires are placed in front of the 

 lamp and a sharp image of these is thrown upon the screen by 

 the action of the lens and mirror. The spot of light may be 

 made to take up any desired position on the screen by properly 

 adjusting small permanent magnets in the neighbourhood of the 

 two magnets G and F. I had expected that a good deal of trouble 

 would have been caused by change of zero in the vertical direction 

 owing to changes in the silk fibre on which the mirror is strung, 

 but I was agreeably surprised to find that the spot of light would, 

 if the mirror were disturbed, return to the same horizontal posi- 

 tion to within -^ inch. The zero position seemed to be quite 

 permanent. 



In order that the two motions of the spot of light should take 

 place in horizontal and vertical lines, the axis DE must be ad- 

 justed so as to be accurately perpendicular to the axis of suspension, 

 AK. The necessary fine adjustment is easily made by slightly 

 bending the suspending wire near the point B. I found that a 

 small block of cork formed the best means of connecting the wire 

 AB with the fork DBE. To get rid of any secondary effect of 

 the coil L upon the lower magnet a second small " compensating " 

 coil may be included in the circuit. In order to bring the spot of 

 light quickly to rest a suitable mica vane was attached to the 

 vertical wire AB. This rapidly stops the motions in azimuth. 



When I exhibited the instrument to the Society, the magnet 

 G was slightly affected by the induced magnetization of the speci- 

 men of iron in the solenoid. This effect can not be compensated 

 by another coil, since a coil through which the magnetising current 

 flows will not imitate the magnetic behaviour of the iron. To 

 remedy this defect I have fitted a second magnet of moment 

 nearly equal to that of C to the vertical wire a short distance 

 below C, its axis pointing in the opposite direction to that of G. 

 The effect of the magnetised specimen on the astatic system is 

 very small and I hope that all trouble from this source has now 

 been got rid of. 



The indications of the instrument can easily be reduced to 

 absolute measure (at least approximately) in the following way. 



