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XXVIII. On the power of masses of Iron to controul the Attractive Force of a 
Magnet. By William Snow Harris, Esq. F.R.S. 
Read June 16, 1831. 
i. It may not be unfavourable to the further elucidation of the interesting 
subject of screens, treated of in my last paper, and which I had the honour of 
laying before the Royal Society, to give a short account of some subsequent 
experiments concerning the effects of masses of iron on a magnet, as they have 
more particularly arisen out of the investigation above alluded to. 
2. The principal part of the apparatus which I employed in these experi- 
ments is represented in Plate XIV., fig. 1 & 2 ; it is extremely simple, and will 
be readily understood by the following description of it. 
A light beam of dry clean-grained deal, of about fourteen inches in length, 
and somewhat less than a quarter of an inch square, is allowed to rest freely, by 
means of a delicate axis, on two horizontal bars of glass ; the glass bars are 
secured on a convenient frame and stand, and the axis of the beam, as in many 
similar cases, is formed of a fine sewing-needle ; it was passed in this case 
through the opposite angles, directly at the centre. From the extremity of 
one of the arms is suspended a small cylindrical piece of iron, about an 
inch and a half long and one fifth of an inch diameter, which is counterpoised 
by an equivalent weight, placed in a small pan suspended from the other ; 
the method of suspension is by light hooks and rings, so as to obtain every 
possible degree of motion. The under part of the centre of the beam carries 
an index of about a foot in length, constructed of short pieces of straw, which, 
being tubular, are easily secured at their extremities one within the other ; an 
index thus formed is very straight and true. By means of this index and a 
graduated arc the slightest motion of the beam is apparent. 
3. The beam and index may be so managed, that for a short distance the de- 
viations from the horizontal position, with equal and very small weights placed 
in the pan, will correspond to equal divisions on the arc, or very nearly so. 
