Edison Medalist Traces History of the Lighting Unit, Recounting the Repeated Improvement in Its Efficiency and Quality – The Three Steps in Filament Manufacture – How Present Day Lamps Are Made

By JOHN WHITE HOWELL*

As a man gets along in years his mind often turns back and dwells on the past, and some of my pleasantest thoughts are remembrances of pieces of work which have been well done. Work well done makes a man's life of value to his fellow men. And now to have this committee tell me that I have done good work by awarding this medal is a very great comfort to me, and it will sweeten the rest of my life.

As I came from my home in Newark to this meeting tonight I noticed the lights, and I believe that every lamp I saw, from the largest to the smallest, from large street lamps to small automobile lamps, was an incandescent filament. And yet when I was twenty-one years old Mr. Edison had not yet invented the incandescent lamp, and we knew only gas, oil and candles – and an occasional electric arc lamp. When I worked at Menlo Park I often went home at night on the Pennsylvania Railroad, and the conductors carried lanterns on their arms to enable them to read the tickets.

The Edison Lamp Works were started in Menlo Park in the fall of 1880, so that 1881 was the first complete year of their operation. In 1881 they made 35,000 lamps. Now the General Electric Company makes one thousand lamps every minute of every working day, and the General Electric Company, together with other lamp manufacturers all over the world that work under the patents of the General Electric Company, makes more than three thousand lamps per minute every day.

THREE PERIODS OF DEVELOPMENT

During these years two great developments have taken place, one affecting the quality of the lamp and one affecting the cost and manufacturing facility. Improvements in quality have been made by changes in the filament, vacuum and other details inside the lamp. I will consider this quality development in three periods:

1. The period of the carbon filament.

2. The short period in which the metallized filament and the tantalum filament held the field.

3. The period of the tungsten filament.

The carbon filament was alone in the field from 1880 to 1905. During this time a great advance was made.  The treated filament was developed to its full value, the squirted cellulose filament was introduced, and so was the phosphorus exhaust. These and other minor developments improved the quality of the lamp greatly.

Any improvement in lamp quality may be utilized in two ways – the life of the lamp may be kept the same and its efficiency increased, or the efficiency may be kept the same and the life increased. It has been the policy from the beginning to keep the life constant and utilize all the improvements increasing the efficiency. So during the carbon-filament period the efficiency increased from 1.68 lumens per watt to 3.4 lumens per watt. In order to compare the relative quality of two lamps one may compare the seventh power of the two efficiency figures, or one may compare the lives of the two lamps at the same efficiency. If this is done it will be found that the 1905 carbon-filament lamp, if tested at the efficiency of the 1881 lamp, will have a life 139 times as long as the life of the 1881 lamp; so we say the value of the 1905 lamp was 139 times the value of the 1881 lamp. The metallized carbon filament invented by Dr. Whitney of the research laboratory of the General Electric Company was introduced in 1905, and the result was a lamp which had a value 4.77 times the value of the 1905 carbon-filament lamp. A year or so later the tantalum-filament lamp invented by Von Bolton was introduced, with the result that there became available a lamp having a value 2.71 times that of the metallized carbon-filament lamp.

ERA OF THE TUNGSTEN FILAMENT

Both the metallized carbon-filament and the tantalum-filament lamp promptly gave way and were displaced as a result of the remarkable improvement brought about by the introduction of the tungsten-filament lamp in 1907. This is the lamp which reigns supreme today. Like all other forms of the incandescent lamp, it has been improved and its efficiency increased by the engineering and development work which has been spent upon it, but it is still the tungsten-filament lamp and owes its efficiency and value to the use of tungsten as the filament. Since 1907 the filament has been changed from a squirted filament to the draw-tungsten wire invented by Dr. Coolidge of the research laboratory. The exhaust process has been developed and improved, and tungsten filaments of different crystalline structure have been developed for use in different types of lamps. This work and development include a number of improvements which are inventions in themselves. The efficiency of the 40-watt tantalum-filament vacuum lamp of 1906 was 4.9 lumens per watt. The efficiency of the tungsten-filament vacuum lamp today is 10.3 lumens per watt.

Compared on the basis of lives at the same efficiency, the tungsten-filament lamp of 1925 has 180 times the value of the tantalum-filament lamp of 1906. During the forty-four years of the history of the incandescent lamp its efficiency has improved from the 1.68 lumens per watt of the original carbon-filament lamp of Edison to the 10.3 lumens per watt of the modern vacuum tungsten-filament lamp of today. In other words, the value of the tungsten-filament vacuum lamp of 1925 when compared with the carbon-filament lamp of 1881 is 325,000 times as great.

So far I have considered only the vacuum lamp. The gas-filled lamp of Dr. Langmuir made an enormous improvement, especially in large lamps. During the year in which the gas-filled lamp was introduced we made 500-watt tungsten-filament lamps of both kinds – vacuum and gas-filled. The vacuum lamps gave 8.17 lumens per watt and the gas-filled lamps gave 14.44 lumens per watt. The gas-filled lamp would last fifty-four times as long as the vacuum lamp at the same efficiency.

The gas-filled lamp completed the triumphs of the incandescent lamp. It has enabled the incandescent lamp to enter the field of very large lamps and special lamps; it has virtually driven the arc lamp out of the market and made street lighting better and cheaper than ever before. Its use is being extended constantly and will continue to be still more extended.

EXHAUSTING THE LAMP

The problem of lamp exhaustion has been with us from the beginning. The chief difficulty in exhausting a lamp is taking care of the moisture which adheres to the surface of the glass. The amount of this moisture is very considerable, and it can be liberated by heating the glass. As the glass is heated hotter and hotter, even up to the softening temperature of the glass, more and more moisture is liberated, so during or immediately before exhaustion the lamp must be heated to a temperature considerably higher than it will attain under any condition of use.

In practice we heat the lamp as hot as practicable and to at least 300 deg. C. After the moisture is driven from the glass, the problem is to get rid of it. During the first fifteen years of the history of the lamp the exhausting was done on mercury pumps which would not pump out this water vapor, so it was absorbed by phosphoric anhydride in a glass cup placed to bring the drier as close as possible to the lamp; but even then the absorbing action took place through the exhaust tube which was of small diameter and about 2 � in. long.

This absorption took considerable time, and exhausting an ordinary lamp in this way took a half an hour. The modern way of getting rid of this water vapor and other residual gases is by chemicals. An Italian engineer – Malignani – discovered that phosphorus vaporized in the lamp under proper conditions precipitated all water vapor and all other gases remaining in small quantities in a lamp after it had been exhausted to a vacuum less than one millimeter pressure. He painted the inside of the exhaust tube with red phosphorus and exhausted the lamp on a fast mechanical pump.

When the vacuum was under one millimeter the lamp was lighted to high incandescence and a blue glow appeared all through the bulb. The connection with the pump was then closed and the phosphorus heated, driving a lot of phosphorus vapor into the bulb while the blue glow filled the bulb. The blue glow instantly disappeared and a good vacuum resulted. The exhaust tube was then sealed off from the bulb. By this method lamps were exhausted in about one minute, the lamps being thoroughly heated before they were put on the pump. In the present day practice the phosphorus is applied as a coating on the filament. The lamp is exhausted on a highly developed rotary vacuum pump to a pressure less than one-tenth of a millimeter of mercury. The lamps are well heated beforehand, but are not lighted up during exhaustion. After the lamps have been sealed off and based they are lighted up brightly. A blue glow appears in the lamp which immediately disappears, leaving a good vacuum of less than one-thousandth of a millimeter pressure of mercury.

Although we have used this phosphorus exhaust for thirty years, we do not yet fully understand its action.  We believe its action is not entirely chemical because other materials will condense or clean up water vapor, oxygen, hydrogen, nitrogen, CO or CO2. We believe that a chemical action takes place with oxygen and water vapor and that the products of these combinations are carried to the surface of the glass and held there. We also know that under the condition of ionization indicated by the blue glow gases which do not combine chemically with Phosphorus are carried to the glass an held there permanently. These gases may be liberated in the lamp in their original condition if the glass be heated again hot enough to vaporize the phosphorus. We believe that phosphorus has a continuing action during the life of the lamp in case any oxygen or water vapor is liberated in the lamp. We believe also that the phosphorus acts solely to get and keep the vacuum. It acts on gases only. Phosphorus used in this way is called a "getter" – it gets the vacuum – it is used in all vacuum lamps.

ACTION OF THE "GETTER"

There is another kind of "getter" also used in all vacuum lamps, the action of which is to reduce the blackening of the lamp. This getter is usually a halogen compound, such as a fluoride, and it is applied to the filament as a coating. In practice it is mixed with the phosphorus and the mixture is put on the filament.

When the lamp is flashed after exhaustion the getter vaporizes and condenses on the bulb, where it remains.  During the life of the lamp atoms of tungsten fly from the hot filament to the bulb and slowly blacken the bulb. The getter reduces this blackening very much.

Developments affecting the cost and manufacturing facilities have been going on during the entire history of the lamp. In 1881 no glass-working machinery existed. All glass work was done by skilled glass workers. Today no skilled glass workers are employed in the factory. In the beginning bulbs were blown from glass tubing by skilled labor. Later they were blown directly from the glass furnace by skilled labor. Now they are made by an entirely automatic machine. One of these machines will make more than fifty thousand bulbs in twenty-four hours.

The inside part or stem is also made on an automatic machine. The three glass pieces which compose it are automatically fed to the machine, held in their proper relative positions, and sealed together.  The stem is then put on another automatic machine which inserts the filament supports, after which the filament, which is a very strong tungsten wire, is draped on the supports by hand. Then the two parts, the bulb and the mount, are put in their proper relative positions on the sealing-in machine and the flared end of the stem is sealed in the open neck of the bulb, thus completing the structure of the lamp.

While on the sealing machine the glass bulb and stem are heated very hot, and they are placed on the exhaust machine in this hot condition. The exhaust machine connects the lamp to four pumps, one after another, and these pumps produce a vacuum in the lamp of about one-tenth of a millimeter of mercury. The filament is not lighted during exhaustion. The lamp is automatically sealed off and delivered to the basing machine.

After basing the lamp is lighted to high incandescence, the blue glow comes and goes and the lamp is finished.

As a result of the improvements briefly referred to, the number of lamps produced per operator per hour is now 150 times the number produced per operator per hour in 1881.