Mounting of carbon electrodes on a holder of an arc lamp

In combination, a plate-shaped holder for a plurality of carbon electrodes for use in an arc lamp for a weatherometer or a light fastness tester, said holder being of an electrically conductive material having a plurality of threaded portions projecting from the surface of said plate-shaped holder corresponding to the number of electrodes to be held thereon, and a plurality of carbon electrodes each having a threaded portion on one end thereof, said electrodes being mounted on said holder and electrically connected thereto solely by the threaded engagement with a corresponding threaded projection on said holder.