ON THE DIRECTIVE POWER OF MAGNETS. 
487 
introduced, and to observe the deflection produced. The measure of the force of the 
large magnet was then the cotangent of the angle of deviation. The observation of the 
deflected needle by dots &c. was the same as before ; but the angle of deviation was 
never measured by degrees. Instead of that measure, a circle upon semitransparent 
paper was graduated by cotangents, and thus the measure of the force of the large 
magnet was read off at once. 
The arrangements in this state were confided to Mr. Carpenter, Assistant of the Royal 
Observatory, by whom all the subsequent arrangements were planned and all the obser- 
vations were made. I need not say that they were made with the utmost skill and 
delicacy. 
A small frame was constructed, carrying a floor at a definite position about T8 inch 
above the compass-needle. As it was my object to make the observations at small 
distances from the great magnet, where its power is great, it was necessary to use a 
large power in the deflecting magnet. Mr. Carpenter selected a horse-shoe magnet 
about 4 inches long, consisting of sixteen plates, each (M36 inch thick; these were 
retouched a few days before they were used. From the consistency of the results 
obtained at the beginning and end of each circuit of the great magnet, I am entitled to 
conclude that no sensible change took place in the magnetism of the horse-shoe magnet. 
The magnet was placed in a vertical position, its two poles resting on the raised floor 
above mentioned. In all cases the deflecting magnet was used in the two positions to 
produce deflection right and deflection left. 
These arrangements sufficed for observation of the powers of the great magnet in both 
positions, and also for observation of the galvanic coil carrying the soft iron core, the 
intensity of the battery having been in some measure adjusted to make the power of the 
coil with core not very different from that of the magnet. But when the coil was used 
without core, the force was so enormously reduced that the arrangement which applied 
well in the other cases failed totally in this. A small magnet was then used, T25 inch 
long, not very strongly magnetized ; its deflecting power was compared with that of the 
horse-shoe magnet in the following way : — The small compass being under the influence 
of the earth’s magnetism, the horse-shoe magnet and the small magnet were successively 
placed on the raised floor above mentioned, then 05 inch higher, then TO inch higher, 
and the cotangents of deflection were compared. Thus the following proportions were 
obtained : — 
Magnets upon the raised floor . . . . 
Magnets 0 - 5 inch above the raised floor . 
Magnets TO inch above the raised floor . 
O 
power of small magnet 
power of horse-shoe magnet 1 0 7 ° 
power of small magnet T 
power of horse-shoe magnet 13 6* 
power of small magnet 
power of horse-shoe magnet 1 2 5 ‘ 
With so great inequality the results are necessarily irregular. I use x 4q as the pro- 
