BPO Dials, Automatic, No's 1 - 9
|Lettering on dials
Some of the text below is taken from:-
C MARKETING, INSTALLATION, 3 Internal, M0015, Issue 1, Sept 1971.
DIALS, AUTOMATIC, No's 1 to 9
The second letter and any following number refer to the type of pulse wheel, shown below.
Prior to the introduction of the BPO No. 8 and 10 dials, each supplier had its own pattern and these were given numbers by the BPO as follows:-
History of early dial progression (by Jack Ryan)
In 1920 there were several trial exchanges operating on the public telephone network. Each of these exchanges made use of proprietary subscriber equipment which had been accepted for use by the GPO.
In November, 1922, the Engineer-in-Chief of the Post Office, Colonel Purves, recommended the adoption of the Step-by-Step system with the addition of the director for use by the Post Office.
Once adopted, this recommendation would standardise all switching equipment on the public network. The requirement of the GPO for standard interchangeable and interworking equipment was now clearly documented.
By this time the Standard Telephone was fairly well established but each exchange type used its own dial.
The GPO required a Standard Dial for logistical if for no other reason. None of the existing dials could be made to emulate the operation of all the others so a new Standard Dial was required.
This Standard Dial was to supersede the four dials listed above and to achieve that, the following was required:-
The first Standard Dial, the Dial No. 8, was released circa. 1921 on the Telephones No's105 and 124. The Telephone No. 124 superseded the Telephone No. 72 (ATM), the Telephone No. 76 (BWE), the Telephone No. 82 (Siemens Brothers) and others so that all manufacturers could now use the same pattern telephone and dial. It featured changeable pulse cams and an adjustable finger stop location.
Technical issues necessitated another change to the Standard Dial. To support telephone circuit changes designed to address problems of contact arcing, coherence and acoustic shock, the Dial No 10 was released. The only change from the Dial No 8 to the Dial No 10 was the number and configuration of the off normal contacts.
The Telephone No. 150 fitted with Dial No 10 was released circa. 1924.
With the introduction of the No. 10, most suppliers adopted this for telephones supplied to all customers, not just the BPO. ATM, however, favoured the Type 24 dial designed by Automatic Electric Inc of Chicago (introduced 1926) and supplied this to Hull Corporation and other private customers well into the 1960s. A variant of this was the dimple dial, using the same mechanism but a plastic finger wheel having moulded dimples instead of proper finger holes. These ATM dials were never adopted by the BPO, although many type 24 dials, taken from Hull Corporation Telephones 162, were brassed up in the 1970's and fitted to otherwise genuine BPO Telephone 150's.
Dial No. 1
The first dial used in this country (known as Dial, Auto., No. 1) is shown below. This was used on the ATM Co's Auto switching system. A minimum pause is not provided in this dial. The impulse springs normally rest in contact due to their own resilience instead of being pressed into contact by an impulse wheel. Impulsing is effected by causing the wings of a fibre butterfly cam to pass between the springs, so separating them. When the wing of the cam leaves the springs they fall together to form the “make” portion of the impulse. A ratchet, which can be seen in the side view, prevents the rotation of the cam while the finger plate is being pulled. The return motion of the dial is effected by a spiral spring (i.e., not a clock spring). It will be seen from fig. 31 that the method of operating the switching spring combination is not so robust as that of Dial, Auto., No. 10 and it was the undesirability of operating more than one spring set in this way which precluded the use of an instrument circuit such as the present standard (Telephone No. 150).
Adjustment instructions for these dials are contained in circular A.T. 6 dated July, 1919.
Dial No. 3
The Dial No. 3, which provides minimum pause and which was used with Messrs. Siemens earlier equipments at Grimsby, Stockport and Southampton. This dial resembles the No. 1 dial in that a fibre cam is used to interrupt the impulse circuit and also in the method of operation of the switching springs. Used on Telephones No's 77 or 82.
Adjustment instructions for these dials are contained in a mimeographed circular dated March, 1924.
Dials No's 5 & 6
Dial, Auto., No. 6 used in association with the S. T. & C. Company’s rotary system at Dudley and Darlington. The mechanism is totally enclosed. Externally the dial is unique in that the number ring rotates with the finger plate. This dial is similar to No. 5 which it superseded.
|Dial No. 5|
|Dial 5 - Front view||Dial 5 - Rear view|
|Dial No. 6 Front||Dial No. 6 Rear|
Dial No. 7
Similar to the No. 3 dial but was modified to give an impulse ratio suitable for the A.T.M. Company’s system, and was enclosed to exclude gas. It was intended for use in mines, or where petrol was stored. Obsolete by 1928.
Dial No's 8 & 9
Dial No. 8
This Dial is the prototype of Dial, Auto., No. 10 and its construction is essentially the same. The necessity for the introduction of the No. 10 dial arose when an improved telephone instrument circuit was developed (see T.I. XXV, Part 3). This circuit, which is the present standard, involves the use of two pairs of off normal springs instead of the single change over which may be seen in the illustration of the No. 8 dial shown below.
This dial has a British Patent in the name of Siemens Brothers.
The adjustments of Dial, Auto., No. 10 and the instructions regarding the replacement of parts are applicable to No. 8 except in respect of the switching springs for which the following requirements are specified. The three switching springs must never be simultaneously in contact. The contact opening must not he less than 10 mils and the pressure exerted by the springs must be sufficient to ensure a distinct follow on either side.
Dial No. 9
Until the introduction of the No. 10 dial the telephone used with pre-payment type coin collecting boxes was equipped with Dial, Auto., No. 9 (Telephone No. 119). This dial was the immediate forerunner of No. 11, described in para. 26. Dial, Auto., No. 9 differs from No. 11 in those particulars which distinguish the No. 8 dial from No. 10 and also differs slightly in the design of the auxiliary cam springs and impulse control cam.
The adjustments specified for Dial, Auto., No. 9 are the same as those for No. 8, and in addition the same specification is laid down for the auxiliary springs as for those on No. 11.
Should it be necessary to replace the main spring, remove the securing ring, celluloid label protector, and instruction card from the finger plate and loosen the screw in the gear wheel to release the tension in the main spring. In addition the finger stop, finger plate, securing ring for number ring, and number ring should be removed. The following parts should then be removed from the back of time dial in the order named:-
Dial No. 8 rear view
Parts for Dials No's 8 & 9
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