Circa 1928, here is a very interesting indoor lighting set from Gacor, a small appliance and electrical novelty company. Utilizing eight intermediate base C-9 lamps wired in pairs and called a "Multiple Twinkle Set", the outfit has a control box (pictured below, third image from the left) that randomly flashes each of the four pairs of lights. This is the first outfit on the market that would allow a true "twinkling" effect, as other twinkling lights of the period flashed the entire string on and off at once. The set was offered first in 1922 using miniature base C-6 lamps, and later in 1928 with the intermediate base C-9 lamps as shown below. When the candelabra base lights came out in 1934, Gacor offered a set featuring those lamps as well. All three types of outfits were sold through about 1936, when the effects of the Depression made the $6.00 sets too expensive to sell. The price for the set is printed on the inner flap of the box as pictured below and to the far right.

The control box has a red switch that allows the lights to either randomly twinkle or burn steadily, and uses a simple bi-metallic system to flash the lights. This system has two strips of different metals wired into each pair of light strings. As current passes through these strips, one of them heats up a bit and curls away, breaking the circuit. It cools again almost instantly, again closing the circuit and lighting the lamps, and the cycle then repeats for as long as power is applied. Different lengths of wire for each socket allow the lights to be easily distributed about the tree for the most pleasing effect. It is unusual for C-9 intermediate base lamps to be specified for indoor use, as they were invented specifically to be durable and weatherproof outside, but this outfit is decidedly not weatherproof and could not safely be used outdoors. The system is quite ingenious and well made, and really produces a nice effect on the Christmas tree. Note the unusual metal sockets in the set as shown in the picture below.