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, unle the period of burning is very short.