If you use vintage Christmas lights, you need this 1950s Bakelite Wonder Plug. Old light bulbs burn hot, which gives them a shorter life and takes more electricity. This plug will reduce the current and keep your light bulbs from burning out. It works with 7-1/2 watt or C-7 type light strings, including figural bulbs (I don't think it will work with C6 bulbs however). The instructions on the back say you can use up to 100 light bulbs on the plug without overloading. The plug has been taken out of the package, the seal is no longer intact on the card.
1950s Christmas Lights Wonder Plug Current Saver