The Story of Electricity | Page 3

some materials, such as amber, glass, and wax, which display
the effect much better than others, and hence its original discovery.
In dry frosty weather the friction of a tortoise-shell comb will electrify
the hair and make it cling to the teeth. Sometimes persons emit sparks
in pulling off their flannels or silk stockings. The fur of a cat, or even
of a garment, stroked in the dark with a warm dry hand will be seen to
glow, and perhaps heard to crackle. During winter a person can
electrify himself by shuffling in his slippers over the carpet, and light
the gas with a spark from his finger. Glass and sealing-wax are,
however, the most convenient means for investigating the electricity of
friction.
A glass rod when rubbed with a silk handkerchief becomes, as we have
seen, highly electric, and will attract a pithball (fig. 2). Moreover, if we
substitute the handkerchief for the rod it will also attract the ball (fig. 3).
Clearly, then, the handkerchief which rubbed the rod as well as the rod
itself is electrified. At first we might suppose that the handkerchief had
merely rubbed off some of the electricity from the rod, but a little
investigation will soon show that is not the case. If we allow the
pithball to touch the glass rod it will steal some of the electricity on the
rod, and we shall now find the ball REPELLED by the rod, as
illustrated in figure 4. Then, if we withdraw the rod and bring forward
the handkerchief, we shall find the ball ATTRACTED by it. Evidently,
therefore, the electricity of the handkerchief is of a different kind from
that of the rod.
Again, if we allow the ball to touch the handkerchief and rub off some
of its electricity, the ball will be REPELLED by the handkerchief and
ATTRACTED by the rod. Thus we arrive at the conclusion that
whereas the glass rod is charged with one kind of electricity, the
handkerchief which rubbed it is charged with another kind, and,
judging by their contrary effects on the charged ball or indicator, they
are of opposite kinds. To distinguish the two sorts, one is called
POSITIVE and the other NEGATIVE electricity.
Further experiments with other substances will show that sometimes
the rod is negative while the rubber is positive. Thus, if we rub the
glass rod with cat's fur instead of silk, we shall find the glass negative

and the fur positive. Again, if we rub a stick of sealing-wax with the
silk handkerchief, we shall find the wax negative and the silk positive.
But in every case one is the opposite of the other, and moreover, an
equal quantity of both sorts of electricity is developed, one kind on the
rod and the other on the rubber. Hence we conclude that EQUAL AND
OPPOSITE QUANTITIES OF ELECTRICITY ARE
SIMULTANEOUSLY DEVELOPED BY FRICTION.
If any two of the following materials be rubbed together, that higher in
the list becomes positively and the other negatively electrified:--
POSITIVE (+).
Cats' fur. Polished glass. Wool. Cork, at ordinary temperature. Coarse
brown paper. Cork, heated. White silk. Black silk. Shellac. Rough
glass.
NEGATIVE (-).
The list shows that quality, as well as kind, of material affects the
production of electricity. Thus polished glass when rubbed with silk is
positive, whereas rough glass is negative. Cork at ordinary temperature
is positive when rubbed with hot cork. Black silk is negative to white
silk, and it has been observed that the best radiator and absorber of light
and heat is the most negative. Black cloth, for instance, is a better
radiator than white, hence in the Arctic regions, where the body is
much warmer than the surrounding air, many wild animals get a white
coat in winter, and in the tropics, where the sunshine is hotter than the
body, the European dons a white suit.
The experiments of figures 1, 2, and 3 have also shown us that when
the pithball is charged with the positive electricity of the glass rod it is
REPELLED by the like charge upon the rod, and ATTRACTED by the
negative or unlike charge on the handkerchief. Again, when it is
charged with the negative electricity of the handkerchief it is
REPELLED by the like charge on the handkerchief and ATTRACTED
by the positive or unlike charge on the rod. Therefore it is usual to say
that LIKE ELECTRICITIES REPEL AND UNLIKE ELECTRICITIES
ATTRACT EACH OTHER.
We have said that all bodies yield electricity under the friction of
dissimilar bodies; but this cannot be proved for every body by simply
holding it in one hand and rubbing it with the excitor, as may be done
in the case of glass. For instance, if we take a brass rod in the hand and

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