the plate between them as it revolves, and a brass conductor C,
insulated on glass posts, is fitted with points like the teeth of
a comb, which, as the electrified surface of the plate passes by,
collect the electricity and charge the conductor with positive
electricity. Machines of this sort have been made with plates 7
feet in diameter, and yielding sparks nearly 2 feet long.

The properties of the "electric fire," as it was now called, were
chiefly investigated by Dufay. To refine on the primitive
experiment let us replace the shreds by a pithball hung from a
support by a silk thread, as in figure 2. If we rub the glass rod
vigorously with a silk handkerchief and hold it near, the ball
will fly toward the rod. Similarly we may rub a stick of sealing
wax, a bar of sulphur, indeed, a great variety of substances, and
by this easy test we shall find them electrified. Glass rubbed
with glass will not show any sign of electrification, nor will wax
rubbed on wax; but when the rubber is of a different material to
the thing rubbed, we shall find, on using proper precautions, that
electricity is developed. In fact, the property which was once
thought peculiar to amber is found to belong to all bodies. ANY
SUBSTANCE, WHEN RUBBED WITH A DIFFERENT SUBSTANCE, BECOMES
ELECTRIFIED.

The electricity thus produced is termed frictional electricity. Of
course there are 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