Drawings of the various filament designs are shown below.
An early description of this lamp application is presented here in the words of Arthur Vaughan Abbott1:
"Many forms of luminous signals have been suggested but the only one in successful operation at present is the miniature incandescent lamp which has, on the whole, proven itself of the greatest value. So far as the writer can ascertain, the idea of employing incandescent lamps for signal purposes arose with Mr. J. J. O'Connell of the Chicago Telephone Co., who suggested in 1888, the employment of an incandescent lamp upon burglar alarm circuits in order to permit the legitimate occupant of a protected room to send to the alarm office an identification signal whereby his entrance to the premises could be made known.
"In the latter part of the summer of 1890 some of the trunk lines in use in Chicago were provided with disconnect signals using miniature incandescent lamps. At the receiving office the trunk lines terminated in the ordinary cords and plugs used upon a telephone switchboard, in front of each of which an incandescent lamp was located. So long as the line was in use a relay in conjunction therewith was excited by battery current flowing over the line, and its armature drawn up. As soon as conversation was completed the operator at the originating office withdrew the plug from the trunk line jack at that office, thus opening the battery circuit and causing the relay to be deenergized; the release of the relay armature completed a local battery circuit, illuminating the incandescent lamp. The operator at the receiving office noticing the illumination of the lamp was thereby informed that conversation was completed and instructed to remove the trunk line plug. As the local lamp circuit was carried through the operator's listening key, the placing of this key in the testing position extinguished the lamp. A very short experience served to demonstrate the superiority of incandescent lamps over any other form of signal, for the illuminated lamp was at once so positive and distinct as to instantly attract the attention of the operator, thus markedly accelerating switchboard work. As a result of this experience, most of the trunk lines in the main office of the Chicago Telephone Company were rapidly supplied with lamp signals.
"...As the service to which the incandescent lamp when used as a signal is subjected is peculiar, the employment of lamps upon telephone switchboards has afforded lamp manufacturers a new problem. In the early experiments ordinary miniature lamps of low voltage were employed. As these lamps were bulky and consumed a large amount of current, the first attempts at improvement were directed to obtaining a smaller and more economical lamp. The various changes through which the lamp has passed are illustrated in Fig. 7 (shown below), in which No. 1 is the original type of lamp employed (the Fig. has been "cleaned-up a bit" by erasing). Nos. 11 and 12 are those which experience has now demonstrated to be the most successful. Lamp No. 1 had a bulb about one inch in diameter and was about two inches long. Electrically it was designed for a four-volt circuit requiring over one-half ampere, and giving about two c. p. Lamps Nos. 11 and 12 are 13/4 inches long with a bulb 1/4 inch in diameter. They are designed for a 24-volt circuit, consume about .1 ampere and yield over 1/4 c. p. "
A second write-up of the early development of the telephone switchboard lamp is presented below and is due to Houston and Kennelly2.
"During the last few years small incandescent lamps have largely replaced electromagnetic indicators on central-station switchboards. The advantages of the incandescent lamp as an annunciator are that it very readily catches the eye of the operator, and, in fact, can be noticed from a considerable distance. Moreover, the space occupied on the surface of a switchboard by a lamp annunciator, or visual signal, is considerably less than the space taken by the ordinary drop signal, since the effective diameter of each lamp is only about 3/8". The lamp is noiseless in action, has no working contacts upon which dust can settle, is readily replaced when inoperative, and is cheaper than an electromagnetic annunciator. On the other hand, a lamp requires a relay to operate it, and also takes about 2.5 watts. The relays are, however, stowed away in dustproof cases behind, or away from, the switchboard, where the space is less expensive than at the switchboard surface, and the power is only supplied for a few seconds at a time to any one lamp.
"Telephone lamps are commonly made for 10 and 20 volts, the former being standardized at one quarter candle, and the latter at half candle. The tendency has gradually been shown to raise these pressures, which now sometimes reach 25 volts. The normal efficiency of these lamps at rated voltages is about 0.18 candle per watt. Not only are such very small incandescent lamps somewhat inefficient by reason of their short filaments, and the relatively large loss of heat by conduction from the ends of the filament, but they are designedly inefficient, in order to reduce the temperature of incandescence, and thereby prolong the lifetime, since the annoyance, which might be caused by frequent failure of these lamps, would outweigh the value of the relatively small amount of power saved in them. Figs.149 and 150, show different sizes and types of visual signal lamps.
"The lamps are inserted end on, in recesses in the switchboard provided for them, and a small circular opal cover or cap seals the lamp from view while diffusing the light yielded over a circular area. These opal glass covers are often tinted with different colors, or marked with distinctive signs in various ways to indicate visually the class of line, or to differentiate pilot lamps, supervisory lamps, and line lamps. A set of ten visual signals in a strip for insertion in a switchboard is shown in Fig. 151.
"The line lamps are generally inserted in the switchboard immediately beneath the answering jacks of the respective lines, so that the operator has only to point the answering plug at the line lamp lit up, in order to find the right answering jack. The supervisory lamp signals are also placed immediately in line with the pair of cords to which it belongs, so that either of these lamps lighting is a signal to pull out that particular pair of cords. In this way all the selection which the operator is called upon to perform is confined to executing the orders of the calling subscribers; i. e., trunking out the order if the subscriber is not directly wired to the switchboard, or calling the subscriber if he is connected to the switchboard."
In 1898 John W. Howell of the General Electric Company filed an application for an incandescent lamp designed for switchboard use. Quoting from his U. S. Patent No. 669,306, issued on 5 March 19013:
"My present invention relates to the construction of incandescent lamps of the type now commonly known in the art as 'telephone-lamps,' especially designed for use in telephone-switchboards, taking the place of other and more complicated forms of annunciators. The construction of these lamps has been peculiarly difficult. They are extremely minute, being only about a quarter of an inch in diameter. The old type, which my invention is designed to replace, was constructed of a small piece of glass tube having fitted to its sides a pair of copper terminals and provided with platinum lead-in wires for the filament, passing through the side of the lamp. As these lamps were designed to be used with small current and at low voltage and were provided with very attenuated filaments, the support of which was of platinum, and since a very high degree of skill was required in the construction, they were unduly expensive, costing several times as much as standard lamps of sixteen-candle power. As ordinarily constructed in the way just pointed out these lamps had no base, but were simply short pieces of glass tube having exterior terminals. They were peculiarly liable to breakage, and, since the seal between the platinum and the glass was so short, in a little time the contraction and expansion breaks the seal and impairs the vacuum. It is to obviate the difficulties attendant upon this style of lamp that I have devised my present invention.The Howell lamp structure is shown below from the first page of the patent.
"In my improved lamp in one form I provide a base, to which the terminals are attached, and use a filament of the ordinary arched form, having lead-in wires of platinum sealed through the end of the lamp with a considerable thickness of glass by methods commonly in use. With such an extremely small lamp it is inconvenient to use any of the common forms of base or socket. The ordinary telephone-switchboard has simply a pair of clips, forming opposite terminals of a circuit, and between these the lamp is slipped. The terminals, therefore, of my invention extend along each side of the lamp and furnish an efficient support for the fragile glass structure, so that breakage is negligible."
Some early, as well as present day, samples of telephone lamps are presented below.
The six lamps shown above are numbered from left to right for the purpose of discussion. The first three are from the 1908 Shelby salesman's kit shown below. Lamp No. 1 is a 24-volt design with a single curl filament. Lamp No. 2 is a 30-volt design, also with a single curl filament. Lamp No. 3 is a 48-volt design with a double curl filament. Lamps 4, 5 and 6 are of more recent designs. Lamp No. 4 is a 10-volt design with a C-2V filament. Lamp No. 5 is a 24-volt design with a C-2F filament. Lamp No. 6 is a 48-volt design with a C-5 filament.
The drawings below are from a GE Lamp Cataog, dated 19796. The telephone lamps at that time had one of the three base types, Slide #1, Slide #2 or Slide #5.
My sincere thanks go to Don Burgener for his help to make this topic possible.
References and Bibliography
1) Arthur Vaughn Abbott, "The Evolution of the Line Signal," Transactions of the American Institute of Electrical Engineers, Vol 15, 1899, pp 425-442. http://www.google.com/books?id=5S4SAAAAIAAJ&pg=PA425&dq=incandescent+lamp+telephone+switchboard#PPP13,M1
2) Edwin J. Houston and A. E. Kennelly, The Electric Telephone, 2nd edition, enlarged, McGraw Publishing Co., 114 Liberty St., New York, 1902, pp 406-411. http://www.google.com/books?id=PX1RAAAAMAAJ&pg=PA406&dq=incandescent+lamp+telephone+switchboard&lr=
3) John W. Howell, "Incandescent Lamp," U.S. 669,306, 5 Mar 1901.
4) H. J. Jaeger, "Incandescent Lamp," U.S. 703,791, 1 Jul 1902.
5) General Electric Miniature Lamps, Miniature Lamp Catalog No. 3-247.
6) General Electric Miniature & Sub-Miniature Lamps, Miniature/Subminiature Lamp Catalog No. 208-9165.