A bubble light is a decorative device consisting of a liquid-filled vial that is heated and lighted by an incandescent light bulb. Because of the liquid's low boiling point, the modest heat generated by the lamp causes the liquid to boil and bubble up from the vial's base. The liquid is almost always methylene chloride, but some early bubble lights used a lightweight oil. The light from the lamp illuminates the bubbles from underneath, causing them to shine.
Bubble lights for Christmas decoration were introduced in 1946 by NOMA, one of the largest American manufacturers of Christmas lights. Bubble lights were also used in operating accessories for Lionel and American Flyer model trains (in the case of American Flyer as bubblng oil wells), and they were also used in a larger form as decoration for Wurlitzer jukeboxes.
As Christmas lights, bubble lights were very popular during the 1950s and 1960s, and into the 1970s, before miniature "fairy" lights became popular. The original design used a miniature screw base (E-10), such as those used on C-6 cone Christmas lamps. These early designs were nominally designed to operate with 8 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 an already-owned light string. Modern incarnations use candelabra base (E-11), 120-volt bulbs.
The clear light bulb is enclosed in a plastic base made up of a "bowl" and a "cap," usually of different colors. Bubble lights are manufactured in just about any color, including the liquid in the tube. The plastics used are most commonly semi-opaque red, yellow, blue, and green. Liquids are generally amber, red, blue and green. Uncolored liquid is also available, being lit with colored bulbs instead.
In recent years bubble lights have gotten fancier. Glitter is sometimes added to tubes for extra sparkle. This is most common on specialty types such as those used in decorative nightlights. The bases are now often made to look like objects such as Santas or snowmen, rather than the plain ribbed plastic. Like many other Christmas decorations, they have been converted for Halloween use, usually with orange liquid and a base that looks like a jack-o-lantern, or the head of a black cat or witch, among other Halloween symbols.
Bubble lights of all kinds operate best when the top of the tube is significantly cooler than the bottom, thus increasing the pressure gradient. The tubes must be kept upright, and occasionally need to be tapped or even shaken to begin bubbling after warming up. Bulbs should not be stored in a hot (or very cold) attic, as this will cause them to not bubble as well later on.
Modern imitations of bubble lights are made from acrylic or other clear plastic rods, with permanent bubbles deliberately manufactured into them, and lit with fixed-color or color-changing LEDs. Other bubbling lights are much larger and sit on a table or floor, occasionally with fake fish which "swim" up and down in the changing buoyancy. These tubes are filled with distilled water and have one or more airstones at the bottom, and normally a light, along with the air pump.