Bubble lights have been made in two different lamp base sizes — E10 (C6) 15 volt, and E12 (C7). The original design used a miniature screw base (E10), such as those used on C6 cone-shaped Christmas lamps. For 120 V power, these early designs were nominally designed to operate with eight on one string, in series, at 15 volts each. However, they were often packaged with a nine-socket string to extend the life of the bulbs. Bubble lights could also be purchased individually for use in existing light strings. Modern incarnations either use candelabra base (E12) 120-volt bulbs, or push-in wedge base "midget" size miniature bulbs (most commonly ten 12-volt bulbs wired in series).