A lamp rheostat is sometimes required for experimental purposes when receptacles for the lamps are not available. Where they are watched sufficiently to avoid any danger from fire, Edison base lamps may be held in place by nails, thereby overcoming the need of receptacles. The illustration shows such an arrangement, in which a lamp is held in place on a board by three nails, A, B, C. No dimensions are given for the location of the nails, as their proper position is very readily obtained by using a lamp as a gage. The edge of the board should be about at the junction of the base to the globe, as shown. See that the nails for holding the various lamps are so located that the globes do not crowd one another. The wires are connected to the nail B and to either A or C. They are twisted around the nails and may be soldered thereto if desired. When in use the board should be laid on a table somewhat larger than itself, and with the lamps in a horizontal position.

Fig. 230 - A simple lamp receptacle.

In case a lamp accidentally becomes loose, the tabic will catch it and prevent its being broken. The lamps are inserted by pushing them downward between the nails A and C, and screwing them up until contact is made with a nail B. Notice that, with certain connections, a short circuit will result if the shell of the base touch the nail B at the same time that it touches C.