The Autodial

Almost 80 years ago, the GPO offered a device to store and automatically dial up to 50 numbers at the press of a lever.
Rather than all being done at the touch of a button, this device relied upon clockwork and 'castellated discs', which had to be modified using 'Tool, Instrument, No. 273'.

I recall fitting something similar (albeit battery powered) for a disabled customer in Canterbury in the mid seventies. The mechanism was similar, but the batteries came in a box the size of a small suitcase.

All rather a Heath-Robinson affair.

The 'hand micro telephone' referred to in the article is by today's standards anything but micro. This was one of the first bakelite models, and was hefty enough even without its seperate bellset.

This has been taken from Herbert & Procter's 'Telephony' Volume II, so best read in your best Cholmondley-Warner informative voice.

Most telephone subscribers have a small circle of business associates and friends with whom they most frequently communicate; with the object of reducing the time spent by the caller in the purely mechanical operation of dialling in such cases, the Autodial has recently been introduced.

It is a mechanical sender which can transmit trains of impulses corresponding to 25 or 50 predetermined numbers, dependent upon the size of the instrument. It is available to subscribers on a rental basis. The smaller size is illustrated below.



The mechanism is contained in a black japanned case having two labels fitted on the top on either side of the slider. In front of the case is a lever which is depressed to provide the motive power for driving the mechanism. To call a number, the name of the required subscriber is first located by sliding the pointer, which is positioned by a ball click engaging with the notches in the locating rack (see second illustration). The lever is then depressed and permitted to return. During the return motion, the impulse trains required by the number selected are sent out.








The sending mechanism is controlled by a series of 25 or 50 castellated discs previously prepared in accordance with the numbers of the subscribers whom it is desired to reach.

As supplied, the discs have 120 castellations; the disc is prepared for use by cutting out eight teeth between the sets of teeth corresponding to the called subscriber's number; thus, the number 3621 is prepared by leaving one tooth, cutting away eight teeth, leaving two teeth, cutting away eight teeth, leaving six teeth, cutting away eight teeth, leaving three teeth, and cutting away the remainder of the teeth.

The cutting is done by means of a special pair of pliers (Tool, Instrument, No. 273). The 25, or 50, sets of discs are then replaced in the instrument in correct order as determined by the label numbering.

The purpose of cutting away eight teeth between each digit is to provide for an inter digit pause of 800 mS.

The prepared discs are seen in position in the internal illustrations. Turning to the one on the left, it will be seen that the motive power is provided by a small clockwork motor, the speed of return being controlled by a governor.

The selector carriage contains the impulse masking springs which are connected to the wiring through two slide bars and wipers.


At the rear of the mechanism (left) is the off normal spring assembly. The internal connections of the instrument are brought out to a connection strip and are shown below.

Associated with the start lever is a device for centring the drum of discs on its return to normal, to prevent any possibility of the drum coming to rest in a position where the impulsing cam is opening the impulsing springs, as in these circumstances the line would be disconnected.

Having selected the required number, and so placed the impulse masking springs in position against the required disc, the depression of the start lever winds up the clockwork motor.

On release, the mechanism is driven round one revolution. So soon as the drum of discs moves away from its normal position, the off normal springs change over and remain in this position until the revolution has been completed.

The impulsing cam is driven round under the control of the governor and standard impulses are generated at the impulse springs. Until the impulse masking pawl is raised by a tooth or series of teeth, however, the impulse springs are short circuited by the impulse masking springs and no impulses are sent out.

So soon as the masking pawl is raised, this short circuit is removed. The mechanism is so geared that as each tooth passes the masking pawl, one impulse is sent out from the impulse springs.

The masking pawl is so shaped that in passing over the space between two consecutive teeth, as in sending out any digit greater than 1, although the pawl moves towards the disc, it does not move far enough to close the masking springs.
Thus, the number of impulses corresponding to the number of castellations left on the disc are sent out.



The connexions of the Autodial when used with a hand micro telephone (Telephone No. 162) are shown below. Calls to subscribers not represented on the Autodial are obtained by using the normal dial associated with the telephone.