6. Traffic Distribution.—By means of selecting mechanism the
line of a calling subscriber is automatically connected to a
disengaged operator's position. This avoids the overloading of any
operator and at the same time gives all operators a fair load during
busy periods.
7. Ancillary Jacks.—Additional calling lamp signals and answering jacks associated therewith placed at different parts of the switchboard. When a subscriber calls, signals are displayed at two or more operators' positions, thus increasing the number of operators who may answer. The object of the arrangement is to provide for the more even distribution of traffic and thus to reduce the answering time of the operators during the rush periods.
Automatic Systems.—The idea of automatic telephony is to substitute for the operator of the manual exchange an electromechanical or other switching system, which, controlled in its movement by the action of the subscriber, will automatically select, connect and disconnect circuits as desired. The process of machine switching consists of successive group selection. Exchange switching machines are provided and are placed under the control of subscribers or operators. Considering only the former case, the machines are operated by impulses originating in a subscriber's telephone. The standard impulse now in general use is a disconnexion of the subscriber's loop (the loop having been closed in the first instance by the lifting of the subscriber's receiver). The train of impulses corresponds to the digit signalled. For instance, the signalling of the digit 7 would constitute a series of seven disconnexions of the subscriber's telephone loop as indicated in fig. 1. It will be seen that a complete impulse is 57% of the total cycle.
Fig. 1
When the impulses are passing at the standard rate of 10 per second, this gives an electrical condition at the exchange of sufficient duration to effect reliable operation of the equipment.
On the base of the automatic telephone instrument a circular device, known as the dial, is provided for transmitting the impulses. Fig. 2 shows a full view of the face of a dial that has been designed for use in large cities. It will be seen that there are 10 holes each of which corresponds to a digit, also that eight of the holes contain letters. Dealing first with the digits, the method of operation is for the subscriber to lift his receiver and then if he requires to call “7146” he will operate these digits in turn. To call 7 he will place his finger in the hole corresponding to that digit, rotate the front plate of the dial which is free to move in a clockwise direction until his finger and the finger hole for 7 reach the finger stop shown on the right-hand side of fig. 2. He will then withdraw his finger and the free plate of the dial will return to normal under the influence of a main spring, in doing which the mechanism of the dial will disconnect the subscriber's telephone loop seven times. He will now insert his finger in the hole corresponding to the digit 1 and proceed as before. The impulses passing from the dial will, as has been stated, be transmitted at the rate of 10 a second, and, as will be seen from fig. 1, the short current between the impulses, is only 43% of the cycle. An appreciable interval, by comparison, must occur between the trains corresponding to each digit because the process of manipulating the dial cannot be performed rapidly enough for it to be otherwise.
Fig. 2
In practically all automatic systems the impulse circuit is as indicated in fig. 3, from which it will be seen that the operation of the switching mechanism is controlled by the falling back of a relay armature at the exchange.
Fig. 3
The mechanism at the exchange is so designed that by the provision of a sluggish relay or equivalent device certain controlling connexions remain unchanged throughout the whole of the train of impulses, but the long current corresponding to the period between two trains is utilized for effecting a change-over so that each train operates a separate switching device. Briefly the effect is to produce successive selection as already indicated. The operation of the digit 7 in a typical case would operate the mechanism so that an idle outgoing connecting line of the 7th thousand is found. The following digit, 1, will pick out an idle outgoing connecting line to the 1st hundred in the 7th thousand being the 71st hundred. At this point it is common practice to give the subscriber connexion with a switch that can select any one of the 100 lines in the group, so that the next two digits 4 and 6 will call line 46 in the 71st hundred, viz. line 7146.
Automatic systems may in the main be roughly placed in two classes: (a) direct impulse systems; (b) stored impulse systems.
In the direct impulse system may be placed the systems of The Automatic Electric Co., Siemens Bros. & Co., The North Electric Co., and The Relay Automatic Telephone Co. (including licensees of the firms concerned).
The Automatic Electric Co. and Messrs. Siemens Bros. & Co. manufacture what is known as the “Strowger” system, the switches of which in operating utilize two motions, first, a vertical action in which the brush is not making contact followed by a rotary action in which the brush is either hunting for an idle connecting line in the case of a preliminary switch or moving towards the contact of the wanted subscriber's line in the case of a final switch. Two magnets are involved in this operation. There is a third magnet used for effecting the release at the end of the conversation, the operation of release consisting in the restoration of the moving element to normal, first in a rotary direction by means of a main spring and, secondly, in a vertical direction under the influence of gravity.
Fig. 4 shows the elementary circuit connexions for one of these systems. The calling subscriber on lifting his receiver operates relay “LR” which in turn operates “RR.” The impulses are received on “LR” whose armature falls back for each impulse placing an earth connexion intermittently on the circuit of “S” and “VM.” Both “RR” and “S” are sluggish relays that release slowly so that the succession of “breaks” and “makes” from the impulses do not affect them. When the long current between two trains of impulses occurs the circuit of “S” is disconnected for a period which is long enough for its operation, and “S” electromagnetically operates the switch “SS” so that “VM” is thrown out of circuit and “RM” is substituted. “VM” is the magnet which operates the vertical stepping by means of a ratchet and pawl device. “RM” similarly effects the rotary stepping. The next train of impulses will clearly operate “RM” after which the change-over devices depending on “S” are made to cut out the accessory mechanical apparatus, leav-
