Popular Science Monthly/Volume 70/May 1907/Notes on the Development of Telephone Service VI
|NOTES ON THE DEVELOPMENT OF TELEPHONE SERVICE|
IN November, 1876, Graham Bell perceived the value and efficiency of the metallic circuit and advised its adoption for telephone service to overcome the inductive annoyances. On February 1, 1878, the parent Bell company recommended the use of metallic circuits for
exchange service, although only three telephone companies had exchanges in operation that month. But many of the local companies could only perceive that the introduction of a metallic-circuit system meant double the cross-arm space, stronger poles, double the terminal equipment, the rapid displacement of open wires with cable, etc. Then, in August, 1877, Graham Bell showed the advantage of twisting 'the direct and return wires around one another, so they should be absolutely equidistant from the disturbing wires' in order to neutralize the effect of the inductive current and eliminate the noise.
Many experiments were made to invent an improved transmitter that would overcome the inductive effect and yet retain the marvelous simplicity of the hand telephone, with its entailed low cost of maintenance. But eventually it was perceived that the displacement of the magnets in the simple self-contained telephone was possible only
through the introduction of a battery current and the employment of much mechanism that has always carried relatively heavy maintenance charges.
In the winter of 1878-9, the more progressive companies began to install the Blake transmitter in combination with the rubber-encased Bell receiver and the magneto bell. At the close of 1878, 246 Blake transmitters were in service, and by July 1, 1879, the number had increased to 7,000. On noisy circuits this change afforded a marked improvement in service that was highly appreciated by local subscribers. Several modifications in the form of these telephone sets (Fig. 30) were sent out before a standard type was selected. Even then, as there were several factories licensed to manufacture under Bell patents, the output of each, while not essentially different, bore distinctive trade-marks. In each the battery wires were led into the bell box so that the battery circuit might be under the control of and closed and opened by the telephone hook switch. Interior and sectional views of the Blake transmitter are shown in Fig. 31. With each of these early Blake instruments a circular was sent stating that it
It is a fact that no modern transmitter exceeds the Blake in clearly and distinctly reproducing the articulation of the subscriber. But owing to the mechanism employed in its single contact form, it proved deficient in volume or power required on noisy and on long suburban lines. Again, its first cost was comparatively low, and the Blake and similar types of transmitters possess the striking advantage over the old hand telephone of being placed on a local circuit, thus removing their varying resistances from the line circuit, to the improvement of the qualities of transmission. The old hand telephone and the early box magneto telephone formed a part of the main-line circuit, thus materially increasing its resistance.
The first Blake instruments were larger in whole and in part than the transmitter so familiar to all telephone users, while the screw that controlled the proper adjustment of the electrodes projected through the box, thus making it possible for the subscriber to adjust the instrument for long circuits or short lines, regardless of the mood he might be in. It only required a little experience to teach the local companies that the wiser plan was to have trained telephone inspectors do the necessary adjusting. So the adjusting screw was put inside the box and the door fastened with lock and key.e
There is a wide difference between the underlying principles of Bell's self-contained transmitter and his variable resistance transmitter, both of which were exhibited at the Centennial. The microphone, or carbon, or battery transmitter, now in use on nearly all telephone lines, belongs to the variable resistance type. Unlike the early hand transmitter, it does not generate current, but serves as a voice-governed mechanical regulator of the flow of current chemically generated in a battery.
After Graham Bell had shown how to solve the problem of speech transmission, many other inventors were naturally quick to suggest commercial improvements. A few worked hand-in-hand with Graham Bell and gladly contributed to his success. Among this number was Francis Blake, Jr., who invented the transmitter bearing his name and which was the only transmitter used on a majority of the Bell lines prior to 1893. Mr. Blake was a Christmas present in 1850; graduated from the Brookline, Mass., High School in 1866; entered the United States Coast Survey, and during the next three years assisted in the transcontinental longitudinal determinations. Finding it necessary to make many experiments in determining the velocity of telegraphic time signals over long circuits, he made a thorough study of electricity. In 1869 and again in 1872 he was in Europe and made all the observations in the third and final determination of the difference of longitude between Greenwich, Paris and Cambridge. Subsequent observations by European astronomers confirmed his work. During 1874—6, he was preparing the results of his transatlantic work for publication, and during this period became acquainted with Graham Bell. On April 5, 1878, he tendered his resignation, which was accepted with the greatest reluctance to date April 15. During the four years that had elapsed since his return from Europe he had devoted all his leisure to experimental physics. It is recorded that in carrying on these experiments
Other workers were also successful in serviceably utilizing the 'loose contact' or microphonic principle in the telephonic transmitter. In January, 1877, Emile Berliner devised his well-known transmitter, for which he filed a caveat on April 14. It was referred to in the Washington Critic, May 18, and on June 4, 1877, he filed an application based on his caveat. The patent was issued January 15, 1878. On April 27, 1877, Thomas A. Edison filed his application for a patent on a battery transmitter; while in December, 1877, Professor Hughes commenced his now famous microphonic experiments, which were followed by Hunnings's employment of carbon granules. One of the first of the Berliner transmitters is illustrated in Fig. 32
Referring to some of these experiments with carbon electrodes, Sir William Thomson (now Lord Kelvin) wrote:
Bell's hand transmitter was not only a telephone complete in itself, but was a self-contained generator of the alternating-current type. It was operated by the voice creating sound waves that in turn generated electric waves through the movements of the diaphragm. These electrical waves were similar in form to the sound waves and were transmitted to the receiver and there changed back to sound waves. When in operation the flow of the current, and every variation in its strength, was dependent on the varying motions of a diaphragm moving in a magnetic field; that is, on the speed of an armature of a miniature dynamo driven by the spoken word. In other words, in the hand telephone respondent vibratory motion of a soft iron induction armature in a magnetic field was the essential element in the successful transmission of speech.
In the Blake and other forms of variable resistance transmitters, whether single or multi-contact, there is no electro-magnet and no armature. A battery, usually of the sal-ammoniac type, supplies a constant current, the flow of which is regulated by increasing or decreasing the pressure of the diaphragm against the carbon button, the changes in pressure being governed by the impact of the changing sound waves on the diaphragm. Thus a carbon transmitter is not so sensitive, nor does it possess that delicate responsiveness so noticeable in a magneto transmitter. It matters little what may be the nature or character of the diaphragm in a variable resistance transmitter, so long as it is sensitive enough to reciprocally respond to the sound waves produced by the vocal cords. But only a soft iron inductive diaphragm will serve in the magneto type of transmitter.
In the White or solid-back transmitter, now so familiar a part of Bell equipment, the single-contact feature of the Blake transmitter is succeeded by a multi-contact arrangement composed of two carbon electrodes made of the hardest of pure carbon separated by carbon granules. The selected granules insure a multitude of contacts, and talking qualities that are unexcelled.
In all these variable-contact transmitters the current is always knocking at the carbon gateway and seeping through. When the telephone is not in use, the carbon offers just sufficient resistance to prevent the current from forcing the gate wide open. When a person is talking, the vibrations of the diaphragm decrease the resistance of the carbon and enable the current to flow through the partially or wholly opened gateway.
Unlike the hand telephone in every respect, the Blake transmitter consisted of a small black-walnut box, nearly square in form and having a funnel-shaped hole cut in the door to serve as a mouthpiece. Within the box was a soft iron diaphragm and suspended parallel to its center was a polished button of pure carbon; between the two hung a German-silver spring bearing a pellet of platinum which barely touched the center of the carbon. When the Blake transmitter was in use, the impinging sound waves pressed the diaphragm against the platinum and forced it with varying pressure against the carbon button. This changing pressure varied the resistance offered to the flow of the battery current, which pulsated through the carbon and into the primary winding of an induction coil or transformer, where it was converted into an alternating current through the inductive effects of the secondary winding and passed out in undulating or wavelike form into the line or subscriber circuit, thence through the copper wire in the green-covered telephone cord attached to the receiver, and on into the wire wound on the electro-magnets. Energizing the latter varied the attractive or pulling power of the pole pieces, thus causing the receiver diaphragm to vibrate in a manner exactly reproducing the vibratory motion of the transmitting diaphragm and setting up a series of sound waves in the receiver exactly corresponding to those produced by the vocal cords of the speaker.
So sensitive is a properly adjusted telephone diaphragm that its vibrations may cause several hundred thousand variations a minute in pressure of platinum point on carbon button in the Blake transmitter, or between carbon granules in certain other microphonic forms Xaturally the amount of current thus passing through this carbon gateway is extremely small, depending principally on the pitch of the speaker's tone and the physical condition of the line. Under ordinary circumstances and with both telephones and a complete copper circuit in good condition, distinct transmission of speech only requires a maximum generation of about one tenth of a milliampere of current at any one period, or only a millionth part of the current required to light an incandescent lamp. Again, probably only one fourth or less of even this infinitely small amount of current reaches the electro-magnets in the receiver, the other portion being used up in overcoming resistances. Where the circuit is three or four hundred miles in length, it is probable 'that only about one one-hundredth of the original current produced at the transmitting station is finally utilized at the receiving station.'
Where operating companies desired a less expensive instrument than the standard Blake set, for use of small users of service, only willing to pay a low rate, a much cheaper set (Fig. 33) was supplied. This set was originally intended to be used only on private lines, or for educational purposes; that is, to gradually acquaint the subscriber with the convenience and value of having a telephone in the home. A glance at Fig. 33 shows that A is an electric tap-bell, B the hand telephone or receiver, as it is now called, C the Blake transmitter, D 'an automatic switch on which the telephone must be hung when not in use,' and E the signalling key.
Installing the regular Blake telephone sets in residences was not an easy task by reason of there being three separate parts to find location for, the magneto bell and receiver, the Blake transmitter, and the batteries (Fig. 30). So much opposition was encountered in handsome homes where the owners objected to the disfigurement of walls,
|Fig. 33.||Fig. 34.|
that immediate efforts were made to devise more compact forms. Finally the different parts were all merged into the oblong set or wall telephone (Fig. 34) so familiar to users of telephone service. An elaborate Gilliland set, designed for use in the better class of residences, is shown in Fig. 35. The battery was kept in one drawer, and pencil, memorandum book, etc., in the other. The Law set used in New York City in 1879-80 is shown in Fig. 36.
Some years ago it was asserted that all the credit for this serviceable arrangement belonged to a grocer in Denver, who, all unconscious of the value of the idea to telephone companies, fastened the magneto bell to a partition in his store, attached the Blake transmitter below the magneto, and screwed an empty soap-box underneath the transmitter. He placed the batteries in the box and made the top of the box serve as a desk on which to record orders received over the telephone. It is said that the partition suggested to an observant telephone man the back-board of the present telephone set, while the soap box suggested the usual battery-box. At any rate, about that time began the movement towards uniformity in equipment, economy in maintenance and artistic serviceability in installation. No matter how expert the installer, it was a difficult task to quickly and neatly install several parts of a telephone set, where each part had to be firmly attached to the wall, especially in handsome residences. Thus the more compact forms were welcomed innovations. But they had one
|Fig. 35.||Fig. 36.|
exasperating defect. The Blake transmitter, instead of being placed flush with the front of the bell box was set in so far as to lead to much vexation of spirit, through the subscriber's forehead coming in contact with the bell box.
In referring to the early telephone equipment, Mr. B. E. Sunny stated, in 1887, that
The parent Bell company perceived the wisdom of standardizing its equipment long before it decided on uniformity in line construction. With that end in view, as well as 'to obtain a permanent interest in the manufacture of telephones and switchboards,' in 1881, it purchased the factory and business of Charles Williams, Jr., of Boston, where Graham Bell had carried on his early experiments, and where the first several thousand telephones were made. It also bought an interest in the Western Electric Manufacturing Company of Chicago and merged the two into one organization, which, under the later name of Western Electric Company, has grown to be the largest industrial plant of its kind in the world, occupying more than seventy acres of floor space, employing more than twenty-five thousand persons, and with sales exceeding $70,000,000 annually.
In connection with the early selection of a permanent manufacturer, Mr. T. B. Doolittle, formerly an experienced manufacturer of metal goods, makes the following statement that indicates how easily the city of Bridgeport, Connecticut, might possibly have had a manufacturing establishment similar to the Western Electric Company: