TO CONTROUL THE ATTRACTIVE FORCE OF A MAGNET. 
503 
sequently takes on an attractive power ; the experiment should therefore be 
so managed as to have the distance between C and b such, that whilst by the 
intervention of C the action of A is neutralized, its induced magnetic state 
does not become sensible upon b at that distance ; this will be always very 
evident when C is of some considerable thickness, and the previous distance of 
A and b taken just within the limit of the attraction. 
7. In this case C is said to screen or stop out the attraction of A upon b, and 
this probably explains the way in which screens operate in impeding the mag- 
netic influence. It seems therefore not unreasonable to infer, that substances, 
possessing the greatest inductive energy, are at the same time the most power- 
ful neutralizers. Hence in employing various bodies as screens, those are the 
most efficient which are susceptible of the greatest transient magnetic state : 
thus zinc is more efficient than lead ; copper more efficient than zinc ; silver 
than copper, and iron the most efficient of any. 
8. As the distance within the magnet, to which the neutralizing force can 
extend, must necessarily depend on the magnetic energy of the substance 
employed, it would be difficult with non-ferrnginous bodies to controul any 
very sensible portion of the action of a magnet by placing them at its ex- 
tremity, Fig. 1, or beneath it, Fig. 3, except in the latter case we suppose the 
magnet to be extremely thin ; but by intervening a considerable mass, Fig. 4, 
immediately between the magnet and the substance acted on, we operate 
directly on the contiguous attracting surface of the bar, and thus the neutral- 
izing effect at length becomes sensible. 
9. The attractive force exerted between a magnet and a mass of iron is in 
the direct ratio of this neutralizing power of the iron ; the distance between 
the magnet and the iron being the same. 
(e). Let a magnet A, Fig. 5, of about ten inches in length, and three eighths 
of an inch square, be placed at some convenient distance, immediately under 
the suspended iron b, and the observed force carefully counterpoised by small 
weights placed in the opposite pan at p, so as to bring the index of the beam, 
Fig. 1, to zero of the graduated arc ; then the neutralizing power of a few small 
pieces of very soft iron, w, x, y, z, about the same diameter as the magnet, 
and varying from a quarter of an inch to two inches in length, may be easily 
estimated on the graduated arc. Fig. 1, by bringing each piece successively in 
3 T 
MDCCCXXXI. 
