Electrophone System |
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The Electrophone system supplies connection from Theatres, Churches etc to private residential homes or offices via telephone wires. In the UK in 1880 and 1881 the Marr transmitter was used by Moseley and Sons, a provider of private telephone lines in Manchester, to broadcast music. Music from Manchester theatres was received on ordinary telephones. The Theatrophone system was developed in Paris and was first exhibited at the 1'expostion d'electricite in 1881. Coin in the slot machines were installed in public places, such as bars and for one Franc, listeners could hear live performances from a Parish theatre. The poster to the right was designed by Jules Cheret in 1890 for the Compagnie du Theatrophone. This system used two transmitters, one stage left and the other stage right. The two transmitters used two lines and connected to the left and right of the listeners headset - effectively stereo. Music was also transmitted on telephone lines in Frankfurt, Munich and Vienna. In the UK the National Telephone Company trailed long distance performances in 1891 and this was between London and Birmingham. Further tests were completed from Birmingham, Manchester and Liverpool to a room in the Crystal Palace. Visitors to the Crystal Palace were charged three pence (before 18.00) and six pence (after 18.00) for a 10 minute period. Within six months approximately 60,000 people used this service. Because of the above successes the Electrophone Company was formed in 1894 and went into service in 1895, operating until 1926. The National Telephone Company supplied the lines and the Postmaster-General issued a licence for the service. The system relayed live theatre and music hall shows and, on Sundays, live sermons from churches. This was a subscription service and users would firstly ask the operator, by using their normal phone line, to connect them to Electrophone. The Electrophone switchboard operator would ask them which theatre they wanted to connect to. For opera, they would be connected to the Royal Opera House at Covent Garden. Other theatres were also available, The Palace Theatre, The Apollo, The Gaiety, The Garrick, The Lyric, The Palace, The Pavilion, The Empire, The Tivoli, The Savoy and The Adelphi to name a few. In 1906 an Electrophone advert advised that there were 14 theatres and 15 churches connected to the system. Listeners would receive sound on a hand held receiver (sometimes called a "Lorgnette" Headset) which was a rod with an inverted double earpiece headset fixed to the end. At the customers premises a small table was provided and the headsets would be hung on hooks around the edge of the table when not in use. To use the service the subscriber would call the local exchange and ask for the Electrophone Service. The operator would connect them to the Electrophone operator who would offer the caller a number of services (up to 30 at times). Once a service was chosen the subscriber would operate an "intermediate switch" which was installed on the subscribers line and this would switch the telephone line from the telephone to the Electrophone apparatus. During the service the subscriber could speak to the Electrophone operator by means of a microphone to change the service. They would then use the supplied receivers to listen to the service. Users would invite friends around to listen and in one of the pictures below there is a stand with four receivers. There were 600 subscribers by 1908. The Electrophone Company was based at Pelican House in Gerrard Street at the heart of London's Soho - an ideal location close to to the heart of theatre land. In 1895 the first years cost was £15 and £10 per year afterwards, this providing hearing for four persons. This was an expensive service, but later the cost was reduced to £5 a year and whilst this was not cheap, you were provided with two receivers. Extra receivers would cost an extra £1 each. Installation was free. The area covered was relatively large, some £5 customers being in Stratford and Sidcup. You could also pay via a coin-in-the-slot machine but if they wanted the service at home they needed to pay a subscription.
The system also had fairly good transmission quality and most subscribers thought it exceptional. But there were complaints and the GPO investigated a number of issues in 1908. Because none of the venues were the same the issues changed depending on the venue. Noise was caused by badly fitted boards, faintness by the varying distances the speakers were from the microphones. Microphones were even tested using 3 inch bell gongs as reflectors. Following the investigation the GPO compiled a specification for the maximum distance a performance could be reasonably heard. Investigation into better microphones was continued although it appears that the Angelini type was generally used. A special switching device was used to connect and disconnect the local batteries at the Theatre end. This device was operated from the central switchboard at the beginning and end of a performance. The Western Electrician of 21st November 1903, page 390,
reported the following:- Since the foregoing paragraph was put in
type the following explanatory note, from the London Electrician has
been received:- In 1906 the Queen requested an Electrophone service at Sandringham. The National Telephone Company was involved, along with the Post Office to supply a trunk line for testing purposes. The Trunk line went via Kings Lynn, Cambridge, Ware to TSX, where it was connected to Gerard trunk 23. In early 1907 the circuit was officially opened. The estimated line rental was £2 per annum. By 1908 the Electrophone service had around 600 subscribers and carried performances from 30 theatres and churches. During the First World War, recuperating servicemen were given free access to Electrophone services and there was a service in the Officers mess at the Knightsbridge barracks. Generally the service was fed over War Department emergency lines to Military Hospitals, as the War Department would not pay for any additional service lines. Even No. 10 Downing Street had a service.
A London Hospital in 1917 with injured servicemen listening to the Electrophone service In 1913 England and France collaborated and the Paris Opera could be heard in the UK and the Royal Opera House in Paris. By 1919 there were over 1000 subscribers and just over 2000 in 1923. But by November 1924, due to the introduction of wireless radio, the number of subscribers had fallen to around 1000. For the year ending December, 1920 the Electrophone Company had an income of £11,868, against operating expenses of £5866 and a royalty payment to the Post Office of £496. Each Theatre received an annual royalty of 10 Shillings a year per connected subscriber. The Electrophone company was paying very high line rentals to the Post Office (which had bought out the National Telephone Company) and faced with declining audiences in the new era of free to air broadcasting, the Electrophone exchange closed on 30th June 1925, after more than 30 years of broadcasting. What must be remembered was all this was achieved without the need for amplification. After the 1st World War the GPO investigated using valve amplifiers to extend and improve the service, but with the new radio services taking many of the customers this idea was dropped. On the 8th May 1921 Madame Melba was singing at the Albert Hal in London. With the help of amplification the Electrophone System connected 600 subscribers to the concert. In the UK, Bailey and Sons, were issued with a licence in 1903 to install Electrophone exchanges in Bournemouth. Although only supplying 62 subscribers by 1924 the system continued until 1938 when the last two subscribers left the service. At that point there were no Electrophone subscribers in the UK and the service was officially closed. The fore runners of radio and the internet, these systems became redundant after the introduction of broadcast radio except in Holland and Switzerland where some radio programmes were relayed over telephone wires. This practice continued until the 1950's. The 1928 GPO Rate Book describes the following equipment:- Tables, Electrophone, NT No. 10 The table comprises of the following component parts:- A Hook No. 17 would be supplied for each Receiver NT No. 5. Transmitters No. 16 From The Electrical Engineer, September 10th, 1897, pages 343-344 THE ELECTROPHONE BY J. WRIGHT
It
is necessary, in order prevent loss of battery power over the
transmitters while idle during the day, to have a means of cutting
them out of circuit until required. This is effected by a species
of automatic switch, a diagrammatic representation of which is
shown in Fig. 2, where A is the active electromagnet, B is a
species of horizontal pivoted lever, provided at one end with a
counterweight, and at the opposite, or working extremity, with a
loosely-hinged pawl, C. The latter when depressed by the
lever, B, under the influence of the armature of the electromagnet
to which it is attached, actuates a compound ratchet wheel, D,
composed of two discs - one of brass and the other of
ebonite. These two discs are clamped together in such a
relative position that their teeth alternate. Contact is thus made
and broken with a small vertical brass spring, E, which acts as a
contact brush. These switches are as a rule connected on the
first line - viz., that in connection with the first transmitter,
and are actuated by a current from a battery of E.C.C. dry cells,
situated at the central exchange or switch-room. Now as
regards the switching arrangements. Now as to that part of the apparatus
situate at the subscriber's house, to wit, the receivers. These
are also of the Ader pattern, and have double poles, being very
similar, in fact, to the small receivers attached to the
instruments of the National Telephone Company. There are
two of these to each person, and they are conveniently arranged
on a light metal frame, A (Fig. 3), semi-circular in form and of
a convenient width to fit over the ears. The receivers are
indicated in Fig. 3 by the letter B, and they are attached to
the frame by the medium of a swivel joint behind, which allows
of a certain lateral motion in order that they may be
comfortably adjusted to the ears. A metal rod, C, attached to
the frame ends in a leather-covered handle, D, by which the
instrument is held when in use. The connections to the
receivers are made by means of a flexible cord, which runs up
through the centre of the handle and rod and subdivides to the
receivers. A small flat table of polished wood is provided
to carry the contact jacks. This can be stood on a
convenient table and connected with the system by a wall socket
and plug and a length of twin cord. The receivers are for
an ordinary subscriber four in number, but extra ones can be
fitted if desired. The connections from the receivers to
the line are connected via a 4 wire cord. This divides the
two receivers onto two loops
supplying respectively the right or left receivers of each set,
so as in the event of one failing, the other should still
operate correctly.
I may here state that the Electrophone Company, whose chairman is
Lord Loftus, are working in unison with the National Telephone
Company, so that it is necessary for a subscriber to be connected
with the telephone system in order that he may be able to
communicate with the switch-room. The electrophone is
generally fitted as an extension to the telephone apparatus, being
put in circuit by means of a switch after having obtained a
connection with the required theatre.  Electrophone Circuit
Theatre Equipment Addendum
The original Ader transmitters were superseded by the Angelini Transmitter and until the closure of the service, the GPO was constantly on the look out for, and testing of, different types of transmitters that gave better quality transmission. The Angelini was superseded by the Transmitter, Insert, No. 9. The Angelini transmitter is of a
similar principle to the Ader transmitter. The principle
feature of this transmitter is the microphone cell which is
formed on the back of the carbon diaphragm A - A, by a round rim
of paper, B - B, cemented to A - A. This cell is closed by
a disc of Mica, C - C, which is clamped to the end of an
insulated screw, D - D, between two metal discs, E - E, the
front one of which is formed with a semicircular rim of
platinum, F - F, the remaining part of the front being
insulated, so that only this rim comes into conducting contact
with the 3 grains weight of evenly sized carbon granules filled
into the recess thus formed. The Mica disc allows of a
very flexible connection and the pressure can be regulated at
will by the screw, D.
The transmitter is fitted to the
stand by means of a recessed sleeve, G, fitting into the collar,
H, of a separate stand, connection being made to the two
terminals, I and J, by the springs, K and L, the later being
insulated and bearing on the end of the adjusting screw, D.
The base, M, of the stand is of
thick metal and the weight of the upper part, N, and the
transmitter is borne on three springs, S, S, S, the centres of
which are fastened to N and the near ends clamped by screws to
M, whilst the far ends are free to slide in recesses formed in
M. The springs absorb vibrations from the stage.
 Angelini Transmitter The National
Telephone Journal ELECTROPHONE WORKING; OR, MUSIC BY TELEPHONE The Telephone has been and is being applied to many purposes, though now mainly utilised for the transmission of speech; its first use, however, was for the transmission of music. It was about the year 1861 that the first record of a telephone concert occurs; this was accomplished by Philip Reis, a German inventor, who invented an apparatus which by the aid of an electric current would transmit music, but not articulate speech. Now with the hundreds of thousands of telephones in use at the present time, which have Become a business necessity, it has been a problem with the Traffic Department what to do with its plant, which becomes idle at certain periods of the clay while the cost of maintenance still goes on. It was this question which led up to the introduction of what we call the "Electrophone" (to distinguish it from our ordinary telephone service), whereby the Junctions and subscribers' lines, lying almost idle after the traffic of the day is done might be utilised. In addition to the regular telephone lines, private lines are
run direct into the ''Electrophone" exchange, also party lines
for non-telephone subscribers from which taps are taken to a
number of residences and to these places is transmitted from the
"Electrophone" exchange music, etc., from the various places of
amusement to
Electrophone System wiring It is about twelve years since the Company decided to introduce to its London subscribers the transmission of performances and music from the various theatres over its telephone lines. During the first year there were only 47 subscribers to the special service, today the total number is about 600. At present we have about 250 miles of church and theatre
lines, connected to no less than fifteen churches and the same
number of theatres and halls and others are now being added. Sometime ago, with the assistance of the Post Office officials, we very successfully transmitted, in addition to vocal and instrumental music, political speeches over a distance of 220 miles, when every word was taken down verbatim and reported by the Press, and papers were on the streets for sale a few minutes after the meetings. On these occasions special precautions were taken to guard against a breakdown of the transmitting apparatus. In front and at each side of the speaker were placed transmitters wired to induction coils, the secondary of which, together with one side of the primary, was connected to three-way plugs and cords, the other side of the primary being connected to a strip of multiple jacks, so that the primary and secondary were connected and disconnected at one and the same time. Constant supervision was given these lines by an attendant whose listening apparatus was bridged across the circuit with three condensers on each side wired up in series, so that should any one of the transmitters become faulty the plugs could he withdrawn one at a time until the faulty transmitter was located and another substituted in its place. Within the last few weeks Grand Opera at Covent Garden has been transmitted to their Majesties at Sandringham over 200 miles of wire, the hearings being most satisfactorily received. London Church Services were also listened to by Her Majesty on a Sunday when the sermons were received with perfect distinctness and without the loss of a word. The first hearing given their Majesties was rather premature, for it was whilst we were making tests to see if satisfactory service could be given to Sandringham that Sir Dighton Probyn ordered the apparatus to be taken into the library for the King to hear Madame Melba singing in La Boheme. This was no easy task, for the distance the apparatus had to be taken was about 200 yards. But it was not a question of "can it be done?" but "do it, and do it well"; so the necessary wire was quickly run, the apparatus connected up, and his Majesty was listening to the famous opera in about three minutes after the command was received. The delight expressed by His Majesty and his guests at the distinctness of the hearing settled the question as to whether or not a satisfactory service could be given. On the following Sunday morning, as I was making further tests, you may imagine my surprise when certain members of the household, who were listening at the time, quickly laid down their receivers and retired, leaving me alone with Her Majesty the Queen and Princess Victoria who had just entered and desired to listen to some of the Church Services. At the "Electrophone Central Office" there are employed about one dozen female operators whose duty it is to connect outlying exchanges and subscribers' lines working direct into the "Electrophone" exchange to theatre lines when desired. Constant supervision is given to these lines by the operators, so that should a breakdown occur at any time in any portion of the circuit, the line would he immediately changed before any inconvenience was experienced by the subscribers. At outlying exchanges where we get more subscribers than we have junctions available for "Electrophone" working, distributing boards are fitted at which several subscribers' lines can be connected to one music junction. At these exchanges the local operator attends direct to the subscribers' wants, tapping the line from time to time so that if one of the subscribers' lines goes out of order during the hearing, arrangements can be made to disconnect that line and thus avoid upsetting the hearing of other subscribers connected on the same distribution. With regard to the service, we have limited and unlimited service for private residences connected to our system. Special rates are charged for residential flats, clubs, cafés, hotels, and for temporary hearings. Party line service for non-telephone subscribers is now being given, whereby the use of condensers or a specially designed repeating coil enables twenty or more subscribers, working on one line with not less than two receivers each, to be served from one theatre line. The condensers or repeating coils fitted at each subscriber's station permit transmission through the main circuit without interruption should trouble occur at any one station working on the circuit. The equipment at the theatre for each line consists of one "Electrophone" transmitter, which is constructed with two carbon buttons supported on springs mounted on an ebonite button. The carbon buttons are surmounted and insulated from each other with soft wool to hold the granular carbon, which does not impede the movements of the diaphragm. The carbon buttons are made adjustable by means of the screw-threaded ebonite ring on which they are mounted. To prevent packing, the transmitter is mounted on a metal support having two springs at the back which make sliding contact on the metal rings projecting from the centre of the ebonite, by means of which the transmitter can be manually turned round, and the whole parts being rotated, the granular carbon is stirred up and falls into proper position in their containing chambers. Then we have an induction coil with a primary winding of 0.5 ohms resistance and a secondary winding of 150 ohms resistance, and a battery, consisting of two type T11 storage cells, from which several transmitters are worked. Throwing the battery in and out of circuit is done from the exchange by sending momentarily over the switch-line a strong current. The induction coil on the first line is brought into use for this purpose, and acts both as an automatic switch and an induction coil. It is fitted with a specially constructed armature which upon a current being sent over the lines through the secondary of the coil is drawn smartly down, and striking between the teeth of the ratchet wheel, causes it to revolve at every stroke of the armature to an "on" or "off" position, every other tooth of the wheel being insulated. The first transmitter used for "Electrophone" purposes was the "Blake"; then a multiple electrode transmitter, called the "Ader", with a thin piece of pinewood for its diaphragm. Whilst the quality of these transmitters was good, they lacked power, and trouble was experienced when they were subjected to loud music, as then they would rattle or entirely break the circuit. It was therefore decided to use a granular carbon transmitter, and tests were made with transmitters manufactured by all the well-known makers, but not until we constructed one of our own did we get the satisfaction desired. With this transmitter little trouble has been experienced, whilst others gave out quickly owing to the heat generated by the current (which is constantly on for about four hours each night) and the heat from the footlights, near which the transmitters are placed. It might be mentioned that experiments have been made with a view to centralising the power, and removing the batteries from the theatres, thus saving cost of battery maintenance at these places, but this course was found unsatisfactory for "Electrophone" working. The question of charging accumulators at the theatres direct from the electric light service was considered, but found to be more expensive than our present system of charging them at the exchange and carting them to the theatres as required. All theatre lines coming into the "Electrophone" exchange terminate on break jacks, and are multipled at every other panel throughout the switchboard. When it is necessary to connect several subscribers to one theatre line, connection is made from the theatre jack to a set of distributing jacks with a double flexible cord having plugs at each end (see picture below). The distributing jacks are connected up in multiple with a 2 microfarad condenser connected on the "A" and " B" side of each jack.
Switchboard Cross Connection The theatre line, operator's cord and subscribers' circuits
are shown in Figs. 4, 6 and 7 and the mode of operating is as
follows:- Should he desire to change the hearing or use the telephone, he signals the operator from the electrophone table, or from the telephone; if from the table he presses the spring of the hand transmitter which closes the primary circuit and puts the "A" and "B" line to earth. A current then flows from the 22 Volt battery at the exchange through the winding of relay, through the grid indicator to the tubular drop-out on to the "A" and "B" line to subscriber's station, through the secondary of induction coil to earth back to battery. This operation causes the grid indicator to drop and the relay to dose, throwing in circuit the supervisory lamp worked on a 4 volt battery.
Switchboard Wiring Diagram Should the subscriber signal from his telephone, be would alter turning his switch from point marked "Electrophone" to point marked "Exchange'', ring in the usual way, dropping the shutter of the tubular drop at the local switchboard.
Customer premises wiring For an ideal electrophone service the following requirements
should be considered:- In the operating promptness in
answering is requisite - for a delay might mean a subscriber's
missing a song which he particularly desired to hear.
So also is courtesy from operators,
for a little more attention is expected by subscribers when
using the electrophone than is desired when hustling for
business on the telephone.
Freedom from mistakes in connecting
the theatre desired is another requisite, as it must be very
annoying for a subscriber after listening for some time to
discover he has been connected to a Salvation Army meeting
instead of the Gaiety.
There must be proper supervision of
lines by the operator, to ascertain that no breakdown has
occurred, that the lines and junctions are kept free from noise
and that the hearing is satisfactory.
Inspectors should be quite familiar
with the apparatus they have to handle and be able to repair any
defect without delay.
Frequent inspections should be made,
when both receivers and signalling apparatus should be carefully
tested and apparatus kept in first class working order.
Before installing electrophone
apparatus, lines should he carefully tested to sec that they are
fit for electrophone working, and that no difficulty is
experienced owing to their running through tress etc. No
trouble is more detrimental to good electrophone working than
the "earth" or "Escape"; whilst it helps us in its proper place,
it becomes nuisance when it throws a little of its own in, where
it is not wanted, and which we can very well do without.
And last but not least, a loud
receiver or transmitter is to be desired that will enable us to
throw out in a room the music and dialogue so that everyone
might hear distinctly without having to place receivers to their
ears; also an invention capable of transmitting scenes from the
theatre to a distance. This as yet is only a dream, yet
after what we have seen we cannot but regard it as something
which will be realised sooner or later.
An extract from The Electrophone Theatre Arrangements When it is desired to connect up,
a momentary strong current is sent through the line and through
the secondary coils this magnetises the core, attracts the
armature and the ratchet moves the wheel one tooth round, so
that the spring makes a conducting contact which completes the
primary circuit in which is local battery and transmitter. This switch must not be too sensitive, or the armature and ratchet would be kept down by the local battery current through the primary coil.
Electrophone Combined Induction Coil and Connecting Switch
Carbon Blocks These form the two electrodes, separated from each of Electrophone Transmit tr. Scale other by a strip of felt, a ring of which also surrounds the two. The felt projects above the surface of the blocks, and the spaces formed between the blocks and a thin carbon diaphragm are about three-quarters filled with carbon granules. The diaphragm (which is insulated from the outer case) thus forms the connection between the granules in the two recesses. One of the blocks is connected to a central stud at the back, against which a screw on the end of a spring contact presses, and the other block is connected to a rim of the outer case and makes contact with two slotted springs, as per the picture below which shows how the instrument is mounted on a brass base, supported by a thick layer of felt, cemented to a fibre base. This arrangement protects the transmitter from vibration, and may be fitted on a bracket at the "wings" or placed on the stage of the theatre. A height of about 3 feet above the stage is found to be best for transmission.
Complete Electrophone Transmitter on Weighted Base Receivers The transmission lines terminate in jacks at the central station, and when more than one subscriber is connected through to a transmitter the connections are made through a condenser inserted in each leg of every line connected. This is done to equalise the volume of sound to long and short lines, and also to prevent an earth fault on any one line affecting the transmission to the other lines connected. An Electrophone Double Receiver is shown to the right.
Paris Theatre phone With reference to the picture
below, ten Ader transmitters, T, t, t, t, t, and T', t', etc.,
are fixed on the stage, as shown in the picture below. Each
transmitter is connected by separate line to the central office,
and can there be connected to sets of Ader receivers, as shown -
a pair of receivers being allotted to each listener, one of the
pair being connected to one transmitter, and the other to a
separate transmitter fitted on the opposite side of the
prompter's box. The figure shows the connection of two such
transmitters T, and r', to sixteen receivers arranged in pairs,
a b, a b. A listener using a pair of receivers would thus be
connected by the left-hand one a to the left-hand transmitter T,
and by the b receiver to the right-hand transmitter T'. With
such an arrangement the transmitted sounds will be most intense
from that one of the two transmitters which is nearest the
singer on the stage, so that a distinctly different result is
obtained from the two receivers of each pair. The effect
produced is somewhat analogous to the action of a stereoscope in
giving solidity to a double photographic picture. In the case of
the telephone it is difficult to imagine that the singer is not
close in front of the listener.
From The London Times, June 17th, 1925, page 18 ELECTROPHONE LTD LICENCE WITHDRAWN BY POSTMASTER-GENERAL The Electrophone, Ltd., the company which for many years has run
the Electrophone service last night issued the following notice to
subscribers:- Additional
Pictures
Adverts
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Last revised: September 01, 2023FM | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
From the very first telephone, music
has been transmitted over the wires. When Alexander Graham
Bell was lecturing and giving public demonstrations he regularly
included music. In front of Queen Victoria he demonstrated his
telephone which included a singer and band music.
The system operated with rows of microphones,
installed in the theatres in front of the footlights. When
installed in Churches the microphones were sometimes disguised
as bibles (see picture to the right).
These are of the well-known Ader pattern,
consisting of a series of carbon rods, A (Fig. 1), smaller in
diameter at their extremities than at the centre, loosely
supported in a horizontal position by the carbon blocks, B, which
are provided with cavities in the sides for their reception.
These carbon blocks are mounted by means of screws on the under
side of a thin wooden diaphragm of pine or other resonant wood, C,
and the whole is supported by a massive rectangular framework of
cast lead, D. The connections to this transmitter are made
by pieces of flexible cord, having thin copper tabs soldered to
their extremities and clamped under the small nuts or screws which
hold the carbon blocks, B, in position. These substantial
transmitters are susceptible to the softest strains of an
orchestra, and are equally as efficient when subjected to a full
chorus. They are, therefore, admirably adapted to the
purpose, and very seldom get out of order. They are as a
rule arranged in what is known theatrically as the "float"
i.e., the part of the stage just in front of or behind the footlights,
and are, on an average, from 12 to 24 in number, but this, of
course, depends a great deal on the size of the stage. They
are mounted in cavities of such a size as to allow a certain
amount of side and end play for freedom of vibration, and the
upper surfaces of their diaphragms are flush with the stage.
The mounting consists of a rubber ring suspension on stationary
wire hooks, this form of fixing having been found advantageous as
offering immunity from actual stage vibrations, and allowing them
plenty of freedom on all sides for their own individual action,
which is maintained uniform and steady by the weight of the leaden
frame.
The
receivers used are of the Ader type, a pair being mounted on the
ends of a spring fork, as shown in Fig. 499, each one being
fitted with a swivel joint, so that it can be adjusted to the
listener's ear. Each receiver is kept in a separate circuit, so
that 4-way cords and jacks are used fox connection. Several of
such pairs are connected up in two circuits, one receiver of
each pair being in each circuit, so that if anything should go
wrong with one of the circuits a receiver of each pair would
probably he still left working.
