MAGNETISM AND ELECTRICITY. 145 
Coulomb has also the credit of determining the law, according to which 
the strength of the magnetism in a magnetic bar decreases from the 
extremity to the neutral line. The results of his observations are shown 
graphically in fig. 15, pl. 20. Here om represents the half of a magnetic 
bar, m being the neutral line and o the extremity; the perpendiculars 
erected at different points along om exhibit, according to a given unit, the 
observed intensity of magnetism at these places. As is evident from the 
figure, the intensity is greatest at the extremity 0, diminishes towards the 
middle, very rapidly at first, and finally disappears entirely at or near 
the neutral line. 
3. On the Method of Preparing Artificial Magnets. 
Steel may be magnetized or rendered magnetic in various ways; of these 
the follewing are best known and most convenient in practice: 1. The 
method of Duhamel, or the separate touch. This consists (fig. 17) in so 
placing two powerful bundles of magnets (fig. 16), with their opposite poles, 
f and f’, towards each other, that their axes fall in the same straight line. 
fhe bar or needle to be magnetized is laid on the poles of the two bundles 
of magnets, and supported in the middle by a piece of wood. The two 
touching magnets, g and g’, are taken, one in each hand, and held at an 
angle of 25°—30°, with their opposite poles nearly in contact, and resting on 
the middle of the bar. They are then to be separated, being drawn along to 
the opposite extremities of the bar, which are to be reached simultaneously. 
They are then brought back again, and the operation repeated until the 
magnetization is completed. It will be understood that the touching and 
stationary magnets must have their poles directed the same way. 
The method of Duhamel is best calculated for magnetizing fine needles, 
such as those required for compasses. For thicker bars, as from one to two 
lines in diameter, the method of Cipinus, cr the double touch, is much 
preferable. Here (fig. 18) the bar is placed as before, and the other 
conditions are nearly the same, except that the touching magnets are held 
at a more acute angle, and instead of being separated they are kept nearly 
in contact, the stroking extending alternately from one extremity of the bar 
to the other. The contiguous extremities of these magnets must be kept 
from actual contact by a small piece of wood, / (fig. 18). After the 
operation has been continued for a sufficiently long time, the magnets are 
to be brought back to the middle of the bar and raised up. The strength 
of the magnetism communicated to the bar depends upon that of the 
touching magnets; there is a point of saturation, however, beyond which 
there is no increase in intensity. The intensity of a strong magnet 
is often considerably impaired when brought in contact with a weaker 
one. 
The most powerful artificial magnets are unquestionably those composed 
of perfectly hardened steel ; they are, however, besides a liability to fracture 
on account of their brittleness, very apt to have more than two poles 
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