CHAPTER  SEVEN

THE  BASE

In the latter part of 1879, when Edison had invented a practical incandescent lamp, it was apparent that a device must be made whereby the lamp could be readily connected to the circuit. The first attempt at such a device consisted of wooden stand having two ordinary binding posts. As this required fastening the circuit wires to the binding posts each time the lamps were replaced, the danger of making a short circuit with the loose wires soon indicated the need for a socket and a base to fit into it.

ORIGINAL LAMP BASE, 1879

This was before a socket had been invented, the circuit wires being attached to the binding posts on the wooden stand holding the bulb.

The first socket consisted of a hollow piece of wood containing two strips of copper, fastened at one end inside the wood on opposite sides of the socket. A thumb screw forced the two strips to make a rigid contact between two similar copper strips fastened to the neck of the bulb. One end of each of the copper strips on the lamp was soldered to the corresponding end of the leading-in wire and the other end was held against the neck of the bulb by wrapping string around it.

WIRE TERMINAL BASE, 1880

Copper strips were fastened to the end of the leading-in wires, the ends of the strips being secured to the neck of the bulb by string.

ORIGINAL SOCKET, 1880

This consisted of a hollow piece of wood containing two strips of copper. A thumb screw forced the two strips to make a rigid contact with the copper strips on the neck of the lamp, when the lamp was inserted in the socket.

This first base was superseded in the latter part of 1880 by a screw base. It consisted of a screw shell for one terminal and a ring for the other terminal. Wood was used to insulate and hold together the parts of the base. The base was cemented to the neck of the bulb by plaster of paris. This base was large and bulky and was soon changed,

ORIGINAL SCREW BASE, 1880

This was the first screw base. It consisted of a screw shell and a ring for terminals with wood for insulation. It was fastened to the bulb by plaster of paris and was a bulky affair.

ORIGINAL SCREW SOCKET, 1880

This was made of wood, the copper terminals inside being designed to accomodate the original screw base pictured above.

early in 1881, to a smaller sized base having a cone-shaped ring and screw shell for terminals. Soon afterward the use of wood for insulation was abandoned, plaster of paris being used instead, both for insulation and to hold the two parts of the base together and to the bulb.

IMPROVED SCREW BASE, 1881

The terminals of this smaller base were a cone shaped ring and a screw shell with wood insulation.

PLASTER SCREW BASE, 1881

To simplify matters, the wood insulation was omitted, plaster of paris being used for this purpose as well as for fastening to the bulb.

It was found, however, that when the lamp was firmly screwed into the socket, the pressure of the cone-shaped ring terminal of the socket against the similar terminal of the base produced a tension on the plaster of paris between the two terminals of the base so that it was liable to be pulled apart. A few months later, about the middle of 1881, this difficulty was overcome by changing the base terminals to a screw shell and an end contact so that by screwing the base in the new socket, changed of course to fit the base, pressure instead of tension was put on the plaster of paris insulation.

FINAL SCREW BASE, 1881

In the previous base, it was found that when the lamp was firmly screwed in the socket a tension was produced on the plaster insulation between the two terminals so that the base was apt to be pulled apart. This was overcome by changing the terminals to a screw shell and an end contact, as illustrated, producing a pressure instead of tension on the plaster insulation. This arrangement has been used ever since, and this base will fit present day sockets.

BASING RACK

This was used in basing lamps when plaster of paris was used.

This arrangement of the terminals for the base is the same as is standard today. While slight dimensional modifications have since been made, this base will fit present day sockets. This screw base, generally known throughout the electrical industry as the Edison base in honor of its inventor, has become the world wide standard and lamps fitted with it are annually made throughout the world in quantities many times greater than the combined quantity of lamps fitted with all other bases.

The base had a ring of plaster about the neck of the bulb for use as a handle to screw the lamp into the socket. In 1884, this ring of plaster was omitted. In 1888, the length of the screw shell was increased, more threads being put on. Owing to the fact that the necks of the free blown bulbs used were not of uniform size, various lengths of screw shells had to be used to fit the various lengths of bulb necks. With the adoption of the moulded bulb, this requirement was no longer necessary.

EDISON LAMP, 1900

Moulded porcelain was used for insulation in the base which was fastened to the bulb with a waterproof cement in place of plaster of paris.

The method of attaching the base to the lamp was to put the two terminals in a mould mounted on a rack, pour plaster of paris in the mould, thread one leading-in wire through the hole in the end contact terminal, bend the other leading-in wire back on the neck of the lamp and insert the neck of the lamp in the mould. Guides on the rack were lowered over the tip of the lamp, to align the lamp and base properly, and the plaster allowed to dry. The plaster of paris became fairly hard in about twenty minutes, when the mould was removed and the lamp with its base put in a heated enclosure to drive out the moisture from the plaster, a process requiring about 36 hours, The excess length of the leading-in wires was then cut off, their ends being soldered to the base.

Waterproof Base

The plaster of paris would absorb moisture when the lamp was used in exposed places, such as in outdoor signs. In 1900, porcelain insulation was used to hold the parts of the base together, the base being fastened to the bulb by a waterproof cement. This cement consisted of plaster of paris with a shellac solution which, when heated, made a hard waterproof cement through the evaporation of the alcohol in the shellac.

The tensile strength of the cement was later improved by substituting Portland cement, or in some cases marble dust, for the plaster of paris. Bakelite is now used and, in order to determine whether or not it is heated to the proper temperature, a green dye is mixed with the powdered bakelite, the dye decomposing at a certain temperature so that the green color disappears.

In 1901, glass was used to insulate and hold the terminals of the base, this being made possible by an invention by Alfred Swan, Of the General Electric Company. This greatly reduced the cost of the base, and all bases are now so made. A fine stream of molten glass is allowed to flow in a holder containing the screw shell and the end contact. When a sufficient amount of molten glass has been put in, a jet of air blows the stream of glass to one side—it cannot be shut off as it would otherwise freeze up in the orifice—and a plunger is inserted which shapes the glass, leaving a hole through which the leading-in wire can be inserted and soldered to the end of the base.

EDISON LAMP, 1901

Glass was used in the base for insulation in place of porcelain. This is the same as used today.

Other Bases

Soon after the commercial introduction of Edison's lamp, many other concerns began making lamps, each with an individual design of base. This required a corresponding socket to fit the base and no less than fourteen different designs were in use at one time or another.

In 1900, the more important designs in use were the Edison, which covered about 70 per cent of the total, the Westinghouse, 15 per cent and the Thomson-Houston 10, per cent. The remaining 5 per cent covered the other designs.

SOME OF THE VARIOUS BASES IN USE PRIOR TO 1900

A few of these had disappeared from use, the proportion in 1900 being 70 per cent Edison, 15 per cent Westinghouse, 10 per cent Thomson-Houston, and 5 per cent for the others remaining.

Standardizing the Edison Base

As the use of incandescent lamps became more general and the necessity arose for more types of lamps to meet individual specific requirements, together with the need of stocks at convenient distributing points throughout the country to supply the demand promptly, the existence of so many different lamp bases presented a situation which, if continued, would seriously retard the development of the electric lighting industry. The necessity for overcoming this condition seemed imperative and it was recognized that something must be done to simplify the lamp base problems.

The task seemed insurmountable. At this time, 1900, there was an aggregate of about fifty million sockets of various designs in use in the United States. It seemed desirable to standardize on the Edison base and socket because of the simplicity of its design and extent of its use. To change the sockets of the types other than Thomson-Houston and Westinghouse to the Edison type of socket was considered possible, but to replace every Thomson-Houston and Westinghouse socket with an Edison screw socket was thought impossible. It appeared, however, that adapters could be designed to enable existing Thomson-Houston and Westinghouse sockets to receive lamps fitted with the Edison screw base, but even this was considered by many to be impossible of accomplishment commercially. Nevertheless, believing that it should at least be tried, the adapters were designed and made, being sold at cost.

The campaign, which was started to effect the corresponding changes commercially, was so successful that in less than five years the demand for lamps in the United States with other than the Edison base practically ceased. At the present time, the five hundred million sockets now in use in this country on commercial lighting circuits are all of the Edison screw type.

ADAPTERS FOR EDISON BASE LAMPS

The adapter placed in the socket permitted the use of lamps fitted with Edison base.