three matching early 20th century vintage industrial double light socket cluster with edison screw base plugs. the keyless socket clusters contain yellow brass shells or enclosures held in place with two porcelain socket rings or insulators. socket clusters are connected groups of lamp sockets that may be used in a single socket or as part of a fixture to permit several lamps to be burned simultaneously. they are frequently used for indirect and semi-indirect lighting fixtures or with glass balls or domes. this arrangement breaks up the light source into several smaller units and thus reduces the intrinsic brilliancy; it often gives a more artistic effect than a single lamp. the clusters are frequently arranged for mounting on an iron pipe or fixture stem, and in some forms are made with a plug at one end to screw into a medium-base socket. they are generally arranged so that only two wires need be connected to the cluster. from two to seven lamps are commonly used and the connections may be made so that the lamps are in series or parallel: the series connection is used chiefly for clusters used on 500 to 600-volt electric railway systems. special forms of clusters are made for domes or other large fixtures having as many as 20 lamps in the cluster, and arranged in various groupings. manufactured by the signal electric mfg. co., menominee, mi. priced for the lot of three.