Carbon.
(a) One of the elements; atomic weight, 12. It exists in three
allotropic modifications, charcoal, graphite and diamond. In the
graphitic form it is used as an electric current conductor, as in
batteries and for arc lamp, electrodes and incandescent lamp filaments.
It is the only substance which conducts electricity and which cannot be
melted with comparative ease by increase of current. (See Resistance.)
(b) The carbon plate of a battery or rod of an arc lamp. To secure
greater conductivity in lamp carbons, they are sometimes plated with
nickel or with copper.
(c) v. To place carbons in arc lamps. This has generally to be done once
in twenty-four hours, unless the period of burning is very short.
Carbon, Artificial.
For lamps, carbons and battery plates carbons are made by igniting,
while protected from the action of the air, a mixture of carbon dust and
a cementing and carbonizable substance. Lamp black may be added also.
Powdered coke or gas carbon is mixed with molasses, coal tar, syrup, or
some similar carbonaceous liquid. It is moulded into shape. For lamp
carbons the mixture is forced from a vessel through a round aperture or
die, by heavy pressure, and is cut into suitable lengths. For battery
plates it may be simply pressed into moulds. The carbons are ignited in
covered vessels and also covered with charcoal dust, lamp black or its
equivalent. They are heated to full redness for some hours. After
removal and cooling they are sometimes dipped again into the liquid used
for cementing and reignited. Great care in securing pure carbon is
sometimes necessary, especially for lamps. Fine bituminous coal is
sometimes used, originally by Robert Bunsen, in 1838 or 1840;
purification by different processes has since been applied; carbon from
destructive distillation of coal tar has been used. The famous Carré
carbons are made, it is said, from 15 parts very pure coke dust, five
parts calcined lamp-black, and seven or eight parts sugar--syrup mixed
with a little gum. Five hours heating, with subsequent treatment with
boiling caramel and reignition are applied. The latter treatment is
termed "nourishing." Napoli used three parts of coke to one of tar.
Sometimes a core of different carbon than the surrounding tube is
employed.