"'The carbon glow lamp had not been invented many months before a holder was found to be necessary. It is natural that Edison should have been the first to design and patent such an arrangement, for from the beginning, long before a practical lamp had been discovered, he was arranging for mains, meters, switches and a score of details by which electric light might be introduced into daily life.' Thus The Electrician of London, in 1893, paid tribute to Edison's sagacity and wisdom in laying out his plans first and then materializing them.
"After the incandescent lamp had been invented, Edison gave his attention to the problem of devising some means by which the lamp could be easily connected with the electrical circuit. The successful laboratory lamps were all glass, with wires leading out to the electrical supply. He was well aware at the time that the lamp base, and the lamp holder must be essentially simple in all their details, so that no skill would be required in placing or replacing the lamp; or in turning the light on or off.
"Edison's first experimental socket, used at the Menlo Park Exhibition of 1879, was what a present-day radio fan would have designated as being based on the "jack" principle—the lamp was pushed straight into the socket. The base was constructed by simply soldering each of the leading out copper wires to a strip of foil copper, and securing them opposite to each other on the outside of the stem of the lamp. The socket was turned down from wood, hollowed out at the top while its base contained a screw threaded aperture by which it could be fitted to any gas fixture.
"In the interior of the hollowed out part, Edison placed two metal strips opposite to each other and screwed them at their top ends. On the outside of this socket and in front of one of the strips, but insulated from it he placed a metal bushing having a thread through which a thumb screw could play. This bushing and the other strip were connected to two outside screws to which terminals of the current carrying wires were connected. Thus, when the thumb screw was turned in or out, it made or disconnected the electrical contact with the strip. The lamp base was simply inserted into the socket so that its foil strips pressed against those of the socket. But Edison soon began to devise another form that would obviate certain disadvantages, particularly the possibility of the lamp falling out when used in an inverted position.
Inspiration From an Oil Can
"One night in the early part of 1880, while Edison was talking upon this subject to some of his assistants, he noticed a kerosene can on a shelf near where he was sitting. Taking it up and unscrewing its cover, he studied the combination for a while and then exclaimed: 'This certainly must make a bang up socket for the lamp.'
"From this idea sprang the Menlo Park experimental socket that was used at the second exhibition of Edison's system towards the end of 1880. It consisted of a turned and hollowed out wooden base, at the inner top of which a metal screw shell was placed to serve as one of the contacts. Below it three inverted U spring shaped strips of metal, at equal distance from one another in a circle, formed the other contact. As in his first types, Edison made sockets with and without keys, and it may be mentioned that the key he designed for the second type was the first one in history that was constructed on a snap principle.
"The second Edison socket permitted lamps to be placed in any position. The lamp was provided with a wooden cup-like base into which its stem was secured by plaster of Paris mixed with gum tragacanth, while one of its leading out wires was soldered to a metal screw shell and the other to a cylindrical flat ring that made contact at the U shaped springs in the socket.
"When Edison moved to New York City in 1881 to introduce his system, he worked out and developed his first commercial socket at the Bergmann and Company's shop in Wooster Stree. This third form or type, having also a wooden base encased in brass sheathing, contained a bevel metal ring at its top and a screw-shell below it for contacts. The plaster of Paris lamp base was similarly constructed to fit the socket. The first Edison plants and the Paris exhibition in 1881 were fitted out with this type, but it was soon found that the lamp base had a radical defect; when it was screwed in tight the tensile strain on the plaster of Paris base often broke the screw and bevel ring apart. This situation created a flurry at the time and after Edison had spent a couple of nights at Bergmann's shops trying various other forms, he evolved a base with a screw shell at the top and a bottom or soleplate below it and the bevel ring in plaster was discontinued."