‘144 PHYSICS. 
(fig. 51). The apparatus is especially calculated for being used by 
travellers ; its application will be readily seen by an examination of the 
figures. | 
A rod of soft iron, about eighteen inches long, fixed in the direction of 
the dipping needle for a certain length of time, will become magnetic; rods 
even placed for a great length of time at other directions with the horizon, 
will exhibit feeble traces of magnetism. Thus the vertical gratings of 
buildings are almost always magnetic. Magnetism may be fixed in such 
bars of iron by a few blows of a hammer. 
The intensity of a magnet may be measured by suspending weights to 
the armature until this is torn from the magnet. The results thus obtained 
are, however, not very satisfactory, as a magnet can be greatly strengthened 
by gradually adding weights, not enough at any one time, however, to 
produce the above-mentioned separation. It is a little singular that such a 
separation of the armature should result in a considerable weakening of the 
magnet. We are indebted to Coulomb for the first indication of a more 
accurate method of determining the intensity of a magnet. For this 
purpose he first employed the oscillations of a magnet, viewing the needle 
oscillating under the influence of terrestrial magnetism, as a compound 
pendulum, and considering that the operating force depended upon the 
intensity of terrestrial magnetism and the magnetic condition of the needle. 
I'rom his experiments he found that the magnetic forces are inversely as 
the squares of the times of oscillation, and that the times of oscillation are 
inversely as the number of oscillations in a given time. 
The second method employed by Coulomb consisted in the use of his 
torsion balance, an apparatus in which a vertical metal thread, stretched 
by an appended weight, and experiencing a torsion, endeavors to return to 
its original position when left to itself, the force with which this takes place 
being proportional to the torsion. The instrument employed by Coulomb 
is represented in figs. 13 and 14, pl. 20. 
A metal thread, wound at its upper end around a horizontal axis, 
supported by two small posts, p and p’, hangs in a vertical cylinder, covered 
above by a circular disk, ss’, perforated in the centre. A second disk, mm’, 
turns centrally in a groove on the first disk with a slight degree of friction. 
The disk ss’ is graduated on its circumference to degrees, and an index 
on mm’ serves to read off the amount of rotation. The wire carries at its 
lower extremity a small brass stirrup, in which may be placed the needle 
or bar whose magnetic force is to be ascertained. First of all an 
unmagnetized needle is to be laid on the stirrup, and the disk mm' turned 
until the needle lies exactly in the magnetic meridian ; a magnetized needle 
is then to be substituted for it, and this will be retained in the same 
position, partly by the terrestrial magnetism, partly by the untwisted 
threads. If now the disk mm’ be turned by a certain angle, the 
needle will be affected on the one hand by magnetic force, and on the 
other by the torsion of the thread; it will consequently take up an 
intermediate position, depending on the ratio of the two forces, and from 
which this ratio may be determined. 
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