ing.

Sec. 112. Paraffin.

This substance has long enjoyed great popularity in the physical
laboratory. Its specific resistance is given by Ayrton and Perry as
more than 1025, but it is probably much higher in selected samples.
The most serviceable kind of paraffin is the hardest obtainable,
melting at a temperature of not less than 52 deg. C. It is a good plan to
remelt the commercial paraffin and keep it at a temperature of, say,
120 deg. C. for an hour, stirring it carefully with a glass rod so that
it does not get overheated; this helps to get rid of traces of water
vapour.

Hard paraffin, when melted, has an enormous rate of expansion with
temperature, so great, indeed, that care must be taken not to overfill
the vessels in which it is to be heated. Castings can only be
prepared by cooling the mould slowly from the bottom, keeping the rest
of the mould warm, and adding-paraffin from time to time to make up
for the contraction. The cooling is gradually allowed to spread up to
the free surface.

The chief use of paraffin in the laboratory is in the construction of
complicated connection boards, which are easily made by drilling holes
in a slab of paraffin, half filling them with mercury, and using them
as mercury cups.

Since paraffin is a great collector of dust, it should be screened by
paper, otherwise the blocks require to be scraped at frequent
intervals, which, of course, electrifies them considerably. This
electrification is often difficult to remove without injuring the
insulating power of the paraffin. A light touch with a clean Bunsen
flame is the readiest method, and does not appear to reduce the
insulation so much as might be expected. The safest way, however, is
to leave the key covered by a clean cloth, which, however, must not
touch the surface, for a sufficient time to allow of the charges
getting away.

The paraffin often becomes electrified itself by the friction of the
key contacts, so that in electrometer work it is often convenient to
form the cups by lining them with a little roll of copper foil twisted
up at the bottom. In this case the connecting wires should, of course,
be copper. Small steel staples are convenient for fastening the
collecting wires upon the paraffin; or, in the case where these wires
have to be often removed and changed about, drawing-pins are very
handy.

With mercury cups simply bored in paraffin great trouble will often be
experienced in electromete