History Of The Incandescent Lamp - By John W. Howell And Henry Schroeder (1927)

You are reading: Chapter 6: Glass Construction



When Edison began experimenting on electric lamps he, like all other experimenters, made the glass chamber in two parts which were separably fitted together. This enabled him to renew a filament easily. Later when he realized that he must use a thin high resistance filament, he also realized that the very high vacuum, which was necessary to preserve this thin filament, could not be maintained in a two-piece glass envelope, as the joint was often subject to leaks. He then made a very bold decision. He abandoned the two separable piece construction and with it the ability to replace a broken filament. He fused the two parts of the bulb inseparably together, saying, "I will make the lamps so long lived and so cheap that they can be thrown away when the filament burns out."

This one-piece glass chamber was one of the elements of the combination which he patented and which the courts decided covered all successful incandescent lamps. This glass chamber consists of two principal parts: the bulb, and the inside part, or stem, which carries the leading-in wires and filament.

Bulb Making

At first the bulbs were made by hand from one-inch tubing. Shortly after the lamp factory was started, bulbs were made at the Corning Glass Works, being hand made and free blown from glass taken directly from the furnaces. These free blown bulbs were used by the Edison Lamp Works for about twelve years, although other lamp manufacturers adopted moulded bulbs much earlier. The hand made moulded bulbs were uniform in size and shape, while the free blown bulbs varied a great deal and had to be gauged and sized into groups of similar dimensions.

The hand made moulded bulbs were used for about twenty-five years before machines were developed to make them. Bulbs are now made by a ponderous automatic machine which takes the molten glass from the furnace in measured amounts, shapes it, blows it in a mould and delivers the moulded glass bulb to another machine which removes the superfluous glass from the neck of the bulb. The completed bulbs are then automatically delivered to a conveyor which carries them through an annealing furnace to the inspectors where they are handled for the first time. Each machine has twenty-four arms on which the bulbs are made, the machine making 70,000 bulbs per working day of 24 hours.


This ponderous machine turns out 50,000 bulbs per working day of 24 hours and greatly reduces their cost.

Automobile headlight bulbs and bulbs for most miniature lamps are made from tubing in automatic machines which blow them in moulds. A very few bulbs for special types of miniature lamps are still made by hand from tubing held in a horizontal lathe.

Stem Making

The inside part or stem is now and always has been made from tubing. Stems have passed through several changes of form and methods of manufacture. In the very early stems, an enlargement was blown at about the center of a piece of tubing, the enlargement serving as a foundation to which the neck of the bulb was fused. The tubing was left long enough to be used as a holder while the stem was being sealed to the neck of the bulb. After this sealing-in operation, the extra length of tubing was cut off and thrown away. Later the enlargement on the stem tubing was omitted, the stem consisting of a straight piece of tubing with the leading-in wires sealed in one end. The tubing was still made long enough to hold the stem while it was being sealed in the bulb, the extra length then being cut off and thrown away.


An enlargement was blown on a piece of glass tubing to which the neck of the bulb was sealed.


This was much simpler to make, less glass was used and the seal with the bulb was less liable to crack.

In 1893, the short stem with the flared end, which had been developed in the Thomson-Houston factory, was adopted. No glass was cut off and wasted in this stem and it made a seal which was less liable to crack than the older forms.

With this stem, as with all previous stems, the neck of the bulb was cut off the desired length and the stem sealed to the rim on the end of the neck of the bulb. Later this was changed, the long neck of the bulb was not cut off, the flared stem was inserted well inside the neck and the excess neck cut off by the sealing-in fires at the exact point where the flare joined the bulb. This was a great improvement. The seals had less glass in them and so were less liable to crack, and did not have to be annealed as was the case with the previous ones. The long bulb neck also kept the water vapor, formed by the combustion of the gas, from getting inside the bulb and so made the exhaustion of the lamps easier.

Stem Making Machines

All stems were made by hand until 1901, when J. W. Howell, aided by W. R. Burrows, made a successful stem making machine which is essentially the same as the present day machine. It was a four-head vertical machine which enabled unskilled labor to make two or three times as many stems per day as a skilled operator could by hand. The flared stem tube was inserted in the heads, the two leading-in wires placed inside the tube, and the anchor wire (of the carbon lamp) put in a holder which held it in position so that its end was inside the tube. These were heated in three positions while the work rotated, the hot end of the tube being squeezed into a flat mass, in the third position.

Flaring the Stem Tube

A number of different machines have been made for flaring the end of the stem tube. At first the pieces of glass tubing were placed in chucks by hand, the chucks rotating the tubing in the gas fires and the flare being formed by a hand tool. Later entirely automatic machines were made which placed the tubes in the chucks, formed the flares and delivered the flared tubes to the stem making machines.


Stems were made by hand until this machine was developed by J. W. Howell, aided by W. R. Burrows. It enabled unskilled labor to make more than twice as many stems as skilled labor could previously produce by hand.

Other automatic machines have been developed which make the flares on the ends of long tubes, the gas fires cutting off the desired length of flared stem tube. This method is considered the best because the tubing is cut by the fires while it is soft, whereby cracked and irregular edges are eliminated.


The first glass working tool was the "bulb punch", developed by William Holzer, of the Edison Lamp Works, early in 1883. This tool punched a tit in the round end of the bulb, the glass at this point being softened by a gas flame. The protruding glass of the tit was afterward cut off, leaving a hole where the exhaust tube was later sealed on to the bulb.

About 1903, William R. Burrows developed a tubulating machine. A fine pointed gas flame was allowed to play on the rounded end of the bulb, a slight air pressure being put in the bulb.


This machine was developed by W. R. Burrows. On the left, a hole was blown in the bulb by air pressure while the glass was softened by a gas flame. On the right, the exhaust tube was welded to the hole in the bulb.

As the glass became softened, the air pressure blew a hole through the softened glass, blowing the flame away from the glass and so making the hole of uniform size. The exhaust tube was then welded to this hole. This machine remained in use about twenty years, or as long as bulbs were tubulated on the round end, and until the invention of the Mitchell and White method of tubulating the stem seal, which is described later.

Sealing-in Machines

The first glass working machine was a sealing-in machine, called the "Dufunny" and made by Edison about 1889, to seal the stem in the bulb. This was a single-head machine which simply held the bulb and stem in their proper relative positions while the two parts were sealed together. The machine held the work in a horizontal position, which is the natural position in hand working. While the machine enabled unskilled operators to perform the sealing-in operation, it did not increase the number an operator could produce in a day. The work rotated while it was being sealed in.

The first of the modern glass working machines was the four-head vertical sealing-in machine, made by John W. Howell in 1896, and which was essentially the same as the sealing-in machine of the present day. The work rotated and was heated in three positions, increasing the speed of operation very much. With this machine an unskilled operator could complete 600 lamps a day, which was more than twice as much as could be done before. These machines were made just in time to enable the factory to take care of a large increase in production without increasing floor space.


This was developed by J. W. Howell and was the first of the modern machines. It is essentially the same as those now used and enabled the production per operator to be doubled.

"Tipless" Construction

The tip of glass left on the round end of the bulb has always been recognized as an objectionable feature and many efforts have been made to get rid of it by tubulating the glass chamber in a position which would enable the tip to be covered by the base of the lamp, making a so-called "tipless" lamp. Many lamps have been made in previous years which were tubulated in the stem or at the seal of the bulb and stem, but the methods by which they were made were expensive and slow.

One tipless method of construction was to weld a tube on the rounded end of the lamp bulb as was done in making the standard tipped lamp, and weld the glass stem holding the filament to the bulb. After this weld had been made a fine pointed flame was allowed to heat the glass at the seal where the stem is welded to the bulb. When the glass became soft, air was blown into the tube on the bulb and blew a hole through this soft glass part of the flare.

A piece of curved glass tubing was then welded to this hole for the subsequent purpose of exhausting the air from the lamp. The tube on the bulb was then melted off and the hole closed up by allowing the soft glass to flow together, so that the bulb looked the same as before. The air was then pumped out through the curved exhaust tube, which, when sealed off, was covered by the base.

Another method was to make a nick in the edge of the flare of the glass stem so that when the stem was sealed in the bulb this nick left a hole in the edge of the seal. The curved exhaust tube was then welded to this hole in the flare. This did away with the necessity of welding the glass tube on the bulb of the lamp and later removing it.


By welding a curved exhaust tube to the seal, the tip on the sealed exhaust tube would be covered by the base, making a "tipless" lamp. The construction was too expensive for the general product.

Meridian Lamps

H. D. Burnett and S. E. Doane, of the General Electric Company, obtained a patent in 1894 for a tipless construction which, however, was not commercially used until about twelve years later, and then used only on a special type of lamp called the Meridian lamp, designed for decorative purposes to compete with the Nernst lamp. It was possible to obtain a higher price on the Meridian lamp, as compared with the standard line of incandescent lamps, which warranted the expense of making it tipless.

A machine was developed by Mark H. Branin, of the General Electric Company, for which he obtained a patent in 1906, to reduce the amount of handwork otherwise necessary in making the Meridian lamp. Inside the stem tube, in which the leading-in wires were later imbedded, a smaller diameter tube was placed through which the lamp was later exhausted. The end of the exhaust tube toward the inside of the lamp was flared and rested on a mandrel which projected into the exhaust tube. The stem tube with the leading-in wires and exhaust tube were then heated at the end near the mandrel and when the glass was soft the parts were pinched together by a pair of pincers, which had a hole in the middle. This pinched the two glass tubes together so that a pair of glass "ears" were formed in which the leading-in wires were imbedded. The mandrel and hole in the pincers prevented the exhaust tube from collapsing.

This process had many difficulties and caused a large amount of spoilage. The operator had to watch the conditions existing in the machine very closely. If the mandrel supporting and holding the exhaust tube open during the pinching process in making the seal became heated too much, the glass would stick to it and be drawn out of shape when the stem was removed. If the mandrel was too cool it was apt to cause cracks in the glass, so that a considerable percentage of the product was spoiled. With the advent of the more efficient tungsten lamp the popularity of the Meridian lamp soon waned, its manufacture being stopped in 1910.


The exhaust tube was placed inside the stem tube, the two sealed together at the end. The leading-in wires were imbedded in glass protuberances made while the two were sealed together.

Jaeger Tipless Lamp

In 1903, Herman J. Jaeger obtained a patent on a tipless construction which consisted of an "L" shaped exhaust tube sealed to the inside of the stem tube away from the pinched seal. After the stem had been made in the usual way, this "L" shaped exhaust tube was inserted in the stem tube and a fine pointed flame heated a spot on the side of the latter. The bent portion of the exhaust tube was then welded to this heated spot in the stem tube and by blowing through the exhaust tube a hole was made through the stem tube. Thus the exhaust tube, when sealed off, was covered by the base, making a tipless lamp. This lamp was marketed for a number of years by the Tipless Lamp Company.


An "L" shaped exhaust tube was sealed to the inside of the stem tube away from the pinched seal.

The Stemless Butt Seal

Low volt miniature lamps used as indicators in telephone switchboards have largely been made tipless since about 1898. In 1913, this construction was applied to flashlight lamps and, in 1915, to side and rear automobile lamps. These lamps have no glass stem to support the short filament, it being supported entirely by the two leading-in wires held rigidly together by a globule of glass. The leading-in wires, with the filament, are put inside the bulb, the wires bent over the edge of the neck of the bulb (which is of small diameter) and the flared end of a glass exhaust tube welded to the neck of the bulb, the leading-in wires being imbedded in the weld. This method of construction was practicable only with the stemless filament supported by the leading-in wires. The standard lighting lamps for 110-volt service require an additional filament support that is too heavy for the leading-in wires to carry.


This construction has been in use for several years on miniature lamps, producing a tipless lamp.

Mitchell and White Tipless Construction

L. E. Mitchell and A. J. White, of the General Electric Company, invented a method of tubulating the stem seal which made a great improvement in lamp construction. Their method eliminated tubulating as a separate operation, thus reducing the cost of lamp making and eliminating the exposed tip on the lamp. All lamps for standard lighting service are now so made.

In this method the exhaust tube is placed inside the stem tube in the stem making machine. The inner ends of the two tubes are sealed together, closing both tubes, making a mass of glass in which the leading-in wires are imbedded. While this mass of glass is still soft, air is blown in the outside end of the exhaust tube, the air pressure blowing a hole through the soft glass at this soft mass. Through this hole the exhaust tube communicates with the inside of the bulb.

Thus the tubulation of the lamp is done on the stem making machine. When the lamp has been exhausted and, in the case of gas-filled lamps, the gas has been allowed to flow in, the exhaust tube is sealed off close to the lamp so that the tip is completely concealed by the base.


The exhaust tube is put inside the stem tube with the leading-in wires, the end fused and pinched together. While the seal is still soft, air is blown through the exhaust tube making an opening at the seal. All standard lamps are now made this way.


The light from a clear bulb incandescent lamp is exceedingly dazzling on account of the high brilliancy of the filament. In a carbon filament lamp this brilliancy is about a hundred times that of the ordinary candle and in a tungsten filament lamp from 200 to 2500 times. Thus while clear bulb lamps should always be shaded, in many cases the bare lamp must be used for various reasons. Under these circumstances "frosted" lamps have been occasionally used in place of clear ones, since the brilliancy is reduced about a hundred fold by frosting. Originally, the frosting consisted either of acid etching or of a coat of mineral paint sprayed onto the surface of the bulb.

The higher cost and slight loss of light due to absorption by the frosting prevented the use of frosted lamps in many places where they should have been used. These objections, and the limitations they imposed on the frosted type of lamp, have been eliminated by a recent invention of Marvin Pipkin of the General Electric Company, which not only cuts the loss by absorption to a third of its former value, but, since it is practical for quantity production, has reduced the price of the lamps.


If a lamp bulb is acid frosted on the inside, the bulb becomes fragile. Marvin Pipkin restored the strength by a chemical treatment which rounded out the minute cracks made by the acid frosting. Inside frosting absorbs less than two per cent of the light, which is about one-third that absorbed by outside frosting.

The advantages of frosting an incandescent lamp on the inside of the bulb have been realized for many years, but until recently no satisfactory method has been devised. It is obvious that a lamp having a smooth outer surface, will be more apt to stay clean than one having a roughened outer surface.

The absorption due to frosting is considerably less with inside than with outside frosting. If the frosting is on the outside, the light from the filament goes through the glass wall of the bulb to the irregular frosted surface where some of it is diffused. The remainder of the light is reflected back through the glass to the opposite wall of the bulb. This process is repeated again and again until most of the light gets out. Some light, however, is absorbed each time it passes through the glass. If the frosting is on the inside surface of the bulb, the cross reflections from the frosting do not have to pass through the glass walls of the bulb each time, which may be an explanation for its lesser absorption.

The ordinary acid frosting on the inside of the bulb weakens the bulb and renders such lamps subject to breakage. It etches the bulb and causes minute cracks or splits to appear just below the surface of the glass. When the bulb is evacuated, the inside surface of the glass is under tension from the air pressure on the outside surface, and the cracks on the inside surface of the bulb cause it to break easily just as a steel truss will break with a crack at the bottom and pressure on top. If the frosting is on the outside, the glass is not materially weakened, as then the etched glass surface is on the outside under compression, the compressive strength of a piece of glass being greater than its tensile strength.

It is a well known fact that if a round hole is drilled at the end of a crack or split in a steel truss, it will withstand a greater weight. Mr. Pipkin discovered that if an inside frosted lamp is subjected to the proper treatment, the entire area of the inside surface will become etched in such a manner as to round out the bottoms of these cracks. The effect of this action is to restore the strength of the bulb to its former value. This he accomplished by chemically treating the inside of the bulb after it had been acid etched.

The new inside frosted lamps were first put on the market in 1925 with a new shape of bulb which is considered more pleasing in appearance and which is expected to replace many of the different shaped bulbs used in the past. This new standard line of six lamps is intended to replace approximately forty-five different lamps heretofore supplied.


These are the six standard lamps of the new line which replaced the forty-five different types and sizes for standard lighting service previously used. The lamps have a new shaped bulb which is frosted on the inside.

The Unit Machine

The process of making a lamp consists of a succession of steps, each of which is an independent operation. A glass tube is made into a stem, a glass rod welded to it and little wire supports set into it to hold the filament. The filament is draped on anchors and pinched fast to the ends of the leading-in wires. The stem with its filament is inserted in the bulb and the stem and bulb are fused together. The air is exhausted and gas inserted if it is to be a gas-filled lamp. The base is cemented on. The leading-in wires are soldered to the base. The lamp is tested, wrapped and packed in a carton and then in a case. It is one long succession of delicate little operations.

One of the difficulties has been the problem of maintaining a balance in the quantity of the different parts manufactured. This has led to the necessity for storage of parts between operations. It has required a great deal of floor space and an expenditure for labor in handling and rehandling materials. Various individual machines used in the different operations ran at their own particular speed of efficiency and the effort was to keep a balance amongst the number of machines or operators in each department that would maintain a uniform production.


In this machine, having four operators making standard lighting lamps, the heretofore individual processes in lamp making are coordinated. The result has been that the floor capacity of lamp factories has been tripled and the output per operator doubled.

Soon after the close of the war, when industrial men began to turn their thoughts once more to plant improvements, W. R. Burrows began to see the possibilities for correlating the machine steps in the manufacture of a lamp. His first move was to take one of the various machines out of each department and set them up side by side to work in sequence with each other and with the different hand operations required. There gradually developed the conception of balancing these machines and the hand work processes so that materials would flow evenly into the unit of machines and all storage of parts between operations might be eliminated.

By gradual evolution, the result of endless experiment and a tremendous amount of machine development, this very end was accomplished. Out of it finally came a unit lamp-making machine, one single combined mechanism into which glass bulbs, tubes and rods, filament wire, anchor wire, bases and packing materials are fed. Out of the other end come finished lamps, marked, tested, wrapped, packed and laid in a case upon a conveyor belt that carries them away to be shipped.

In the present unit machine there are three to seven operators, depending on the type of lamp, turning out twice as many lamps per operator as were made by the old departmental method. It is expected that machines will soon be available requiring a lesser number of operators, perhaps as low as two, in which much of the hand work now done will be made automatic. Another great advantage of the unit machine is that it has tripled the capacity for a given floor space, because it has eliminated the storage of parts between operations.

These advantages have materially reduced the cost of manufacture, making possible a reduction in prices. At the present time the price of lamps is more than one-third below the pre-war level, an accomplishment which few industries can claim and which is even more remarkable when it is considered that the present average price of commodities is over 50 per cent above their pre-war figure.

Another interesting thing about the unit machine is that the quality of lamps has improved through its use. This is due to the ability to locate definitely imperfect manufacture in any given part of the lamp which was almost impossible to fix by the old departmental method where the part may have been made in any one of a great many individual machines.