ATM MONOPHONE
TELEPHONES

ATM Monophone

Made in Liverpool by ATM, the design of the Monophone owed a lot to the equivalent design of ATM’s US partner, Automatic Electric Company of Chicago. The only major difference in the British Monophone was the BPO type 164 handset.  Originally introduced in 1925, ATM of Liverpool produced the telephone from 1933 onwards in the UK and this model is pictured to the right.  This telephone has an integral induction coil, unlike the American version in the article below.

Colours: Black, possibly others.

Users: The Monophone saw use on private (PAX, not PABX) systems installed by  Communication Systems Ltd (an ATM subsidiary) and was adopted for public use by Hull Corporation Telephone Department and the Anglo-Portuguese Telephone Company on their networks in Lisbon and Oporto.

Note: The same phone was made and used in Italy, with the American pattern of handset and carrying a nameplate saying (on the left) AUTELCO Milano and on the right, SATAP Roma.

A photo of a Monophone used by Hull Corporation Telephones is shown to the right.

There are also photos of a Monophone and the related Strowgerphone at www.phone-pages.org.uk (go to Old Phones and tab down to Strowgerphone).

MONOPHONE SETS

In many parts of Europe, a telephone set having the transmitter and receiver mounted on the opposite ends of a handle to form a unit, called a "microtelephone". has long been in general use. Since the use of such sets have chiefly been confined to the Old World. this style of instrument has become known in America. as the European type telephone. The micro-telephone has many advantages not inherent in American telephone sets. For example. in using the conventional desk stand. which is the one commonly employed in the United States, the use of both hands is necessary - one to remove the receiver and the other to bring the transmitter into the proper position for conversation. If the user is forced to stand while conversing, it is necessary to continually hold the heavy desk stand: also transmission is apt to be poor at times when the user turns his head while talking, or otherwise does not speak directly into the mouthpiece. In using the European microtelephone, these conditions are not encountered, as the use of only one hand is necessary, and the transmitter is naturally held in the correct position for conversation, regardless of whether the head is turned.

While the American public were cognizant of these advantages and favoured the European type instrument in preference to the types in vogue, American telephone companies retrained from adopting it because of the transmission qualities of the micro-telephone, which were unquestionably deficient, and therefore would not meet American standards.
Further, the construction of the instrument made it fragile.

Its adoption would therefore have already greatly increased maintenance costs.  Efforts on the part of American telephone engineers to overcome these obstacles were unsuccessful until the advent of a bakelite hand unit of the style illustrated. It was not until then that domestic telephone companies felt assured of a practical micro-telephone type instrument comparable in performance with the conventional desk sets.

These instruments fulfil along-felt want and have won public approval, in fact, the new telephones are rapidly supplanting the conventional telephone sets.

Hand Unit
A cross-sectional view of the Monophone hand unit is shown in the picture.  All exposed non-metal parts are of moulded bakelite. The receiver and the transmitter fits into recesses in opposite ends of the hand-piece, as indicated. A threaded
metal ring holds the transmitter and mouthpiece in place. The same construction however is not used for securing t he receiver and it's ear piece, as the earpiece diameter is larger than that of the metal ring. Instead a threaded metal cap clamps the receiver and diaphragm in place. The receiver earpiece is fastened to the metal cap by means of a metal bushing having an octagonal hole. Two embossments in the metal cap, fitting into two diametrically opposite holes of a series encircling the back of the bakelite earpiece, provide for the proper setting of the earpiece with relation to the handle and prevent it from being turned after it is locked in position by the bushing. A special tool is required to remove the bushing holding the earpiece in place.

The type of receiver used, depends upon the circuit of the Monophone set and may be either one of the two watch case patterns described under "Design of Apparatus". The transmitter is of the Monophone type previously described.

Plunger Spring Assembly
In both the American and European types of telephones employing a transmitter-receiver hand unit, the unit is placed on a support when not in use and the weight of the unit is usually utilised to operate contact springs which switch the transmitter and receiver in and out of the circuit. The method of of accomplishing this switching, however, differs radically in the two styles of instruments.

In the European telephone the support is a "cradle" which moves upward under spring tension when the hand unit is removed. The movement of the cradle accomplishes the switching.  In the Monophone type sets the support is a cradle which does not move, but is rigidly secured to the telephone base. The necessary switching is accomplished by employing a hard rubber plunger which rises from the centre of the fixed cradle when the handset is removed.

The manner in which the interior of the cradle houses the plunger switch assembly is shown. A top and a cross-sectional view of the assembly also shown, which shows the details of the switching mechanism. When the hand unit is placed in the cradle, the plunger is depressed. Its motion is in turn imparted to roller bushes which force the springs apart: when the hand unit is removed from the cradle the reverse action takes place.
The spring assembly is covered by a name plate held in place by a spanner nut. The springs are readily accessible for inspection and adjustment.

Types of Monophone Sets
Monophone sets are manufactured in four types - namely:

1. a desk set with a separate ringer box.
2. a desk set embodying a ringer.
3. a wall set.
4. a wall extension set without ringer and dial.

These various types are illustrated above. The hand unit and plunger spring assembly in all types are of the same design and construction.
The bases of both types of Monophone desk sets are of moulded bakelite. Metal inserts incorporated in the bakelite serve (or terminals. This construction provides a substantial base of lasting durability. The desk set with the round base requires a separate bell box. This bell box in appearance is the same as the bell box previously described, except that it contains a three-winding induction coil. The desk set embodying the ringer movement is provided with a separate bakelite connecting block for making the connection between the desk stand cord and the line wires.

A bell box with a dial mounted on its face is used for the base of the wall type Monophone set. The cradle is mounted on the top of the box.

The extension wall Monophone set illustrated is often referred to as a ‘bed-side set” because of its adaptability for hospital service, does not employ a cradle to hold the Monophone. The receiver end of the unit hangs on a metal fork similar to a receiver hook. The fork, however, does not move. Instead, the hand unit rests against a plunger which projects front the front of the cover. The plunger operates the switching device. Since this set is used for answering service only, in automatic telephone systems, it is not equipped with a dial. The base, which has a bakelite cover, contains sufficient space for mounting the switching mechanism, an induction coil, a condenser and the necessary terminals.
As explained under "Automatic Telephone Circuits" two fundamental circuits are standard for Monophone sets. They are both of the anti side tone booster- battery type. In one circuit, the anti-sidetone booster principle as incorporated in a three-winding induction coil operating in conjunction with a permanent-magnet receiver. In the other circuit, this principle is incorporated in the induction-coil receiver.
Since the three-winding induction coil located in the bell box if the round-base Monophone desk set it is not possible to use the round base desk stand in conjunction with a bell box incorporating an ordinary two-winding induction coil when the permanent magnet receiver circuit is employed. If it is desired to replace an existing desk stand with the round-base Monophone desk stand and retain the old bell box, the induction coil receiver circuit Monophone desk stand must be used, as it only requires that the bell box contain a ringer and a condenser. If the existing bell box contains an induction coil it must be omitted from the circuit: i.e. the bell box should be rewired to conform with the circuit diagram for the telephone involved.

Taken from Automatic Electric Company Ltd, Engineering Bulletin 302E, 1928.

Last revised September 03, 2006

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