The table which holds the work is of cast or wrought iron. The metal to be welded is laid on the table, and it is supposed that the contact between table and metal will be sufficient to carry the current. If the piece of metal is small, the positive lead had better be clamped directly onto the metal instead of the table.

The switchboard contains a single-throw switch and a rheostat connected with grids. Or the rheostat may be made of water-barrels with insulated sides and a terminal plate for a bottom. The other terminal is also a metal plate suspended over the barrel. It is lowered in and out of the water of the barrel.

Fig. 14. - Carbon negative pole and shield.

The trouble with barrel rheostats is that the water is liable to boil over under continuous usage, while the barrel hoops will rust rapidly. Circuit breakers should be used to prevent the armature from burning out, in case 'the operator accidentally short-circuits by touching his carbon pencil to his work.

The carbon pencils are made in sizes of 1/4-inch to 1 1/2-inch diameter by 6 to 12 inches long - of sound carbon. The carbon pencil is fixed into an insulated handle. Midway on the handle is a round shield to protect the operator from the flame of the arc and from sparks (see Fig. 14).