Recent tests show the most efficient light bulb is the circline lamp system. General Electric and Sylvania have just started making an entire system, a screwbase adaptable to incandescent sockets and a fluorescent bulb. Since the 1940s, Westinghouse has made a long-life circline bulb that fits into a fluorescent base.

The circline lamp is a fluorescent circular tube with a Y-shaped center that looks like a steering wheel. Like all fluorescent lights, the tube is coated with a phosphor that glows when electricity passses through. A cone-shaped device in the center of the Y contains the ballast that controls the flow of electricity. The cone also contains an adapter that plugs into any ordinary light socket.

Last year the Department of Energy published a report on the cost effectiveness of long-life incandescent lamps by Rudy Verderber and Oliver Morse from the Lawrence Berkeley Laboratory at the University of California.

The study concluded that "the most cost-effective long-life replacement lamp for the standard 100-watt (120 volt) incandescent lamp is the adaptive fluorescent circline lamp. The cost of light with this source is more than twice as cost-effective as any lone-life incandescent lamp."

The circline is efficient because it uses less than half the electricity for the amount of light it produces. For instance, according to GE spokesman Larry Muehling, GE's Circlite 100 lamp system uses 40 watts but is as bright as a 100-watt standard bulb. GE also makes two other Circlite bulbs: a two-step bulb that gives the light equivalent of 100/50 watts and one that gives the light of a 60-watt bulb. Each uses, respectively, 44/16 watts and 22 watts. GE claims the fluorescent tube lasts about four years; the adapter base lasts from 20 to 25 years.GE's Circlite has been on the market about six months.

Its efficiency, as well as the color and softness of the circline tube (it's not harsh light, as are many fluorescent systems), make the circline very appealing. GE's Circlite costs about $20; without the adapter, the bulb is under $10.

Sylvania's Circlite lamp system is newer than GE's -- about two months old, says sales representative Dave Phillips. Like the GE version, it is a direct replacement for 40-, 60-, 75- and 100-watt incandescent bulbs. it sells for $12 to $16. its rated life is 12,000 lumen hours.

Westinghouse's circline bulb is a circular fluorescent tube that, unlike the others, fits only into a fluorescent socket. Westinghouse's John Hoffman believes its circline bulb was introduced as early as the late 1940s. The circline bulb runs about $7 to $9.

A long-lasting light bulb has been the ambition of light bulb inventors and producers since Edison, according to Bernard Finn. Finn is director of electricity and modern technology at the National Museum of American History and its curator for the permanent exhibit "Lighting a Revolutaion," which opened in 1979.

Edison's first light bulbs (1879) lasted as few as two to three hours -- then a great achievement. Edison's early experiments produced either a flickering light or light that burned out instantly. The key was finding a material that was highly resistant to be used as the filament. A carbon filament turned out to be the answer.

Today the search for the long-lasting, energy-efficient bulb continues.

In 1971, General Electric came out with the long-life light bulb. During the past 10 years, Westinghouse has come out with a long-lasting bulb. It lasts from 750 to 1,000 hours.

A long-life 75-watt bulb does last longer than a standard bulb. The catch is the longer-lasting bulb gives less light.

As Westinghouse's John Hoffman points out, the consumer has to go one size higher -- to 100 watts -- to get the same wattage as with a 75-watt bulb. Hoffman is the product planning manager for fluorescent light at Westinghouse.

Dave Phillips, sales representative with Sylvania, says its 75-watt standard bulb lasts about 750 hours.Sylvania's long-lasting light bulb, the Supersaver, lasts 2,500 hours but its light output is diminished by 5 percent.

Nancy Christensen, GE's lighting specialist, advises using a standard-life bulb "rather than a long-life bulb to get more light for your energy dollar. Long-life bulbs do last longer but they cost more to buy and are less efficient. Use them only in hard-to-reach places where the convenience of extra life makes higher bulb costs worthwhile."

Hoffman adds that in the home, replacing light bulbs in table lamps is simple. "For street lights it's quite expensive and time-consuming to change a light -- it takes at least two people and a cherry picker-type truck. Here, a long-lasting bulb makes more sense. Westinghouse's long-lasting street lights are rated at 12,000 hours." At home, hard-to-reach places like an attic, basement or garage might be places a long-life bulb would come in handy.

Sample long-life bulb prices:

General Electric's Longlife White light bulb comes in 60, 75 and 100 watts and costs about $2.20 per pair (as compared with a standard 60-, 75-, or 100-watt bulb, which costs about $1.86 a pair). Westinghouse's Super bulb, which contains krypton gas and lasts 750 hours, costs about $2.15 per bulb. Westinghouse also sells a limited number of Turtle Life bulbs (which it claims last 33 percent longer than standard bulbs) in 40, 60, 75 and 100 watts for about $1.90 per pair. Sylvania's Supersaver bulbs come in 40, 60, 75, 100 and 150 watts and range in price from $1.19 to $2.01 per bulb.