Volume I Part 24 (2/2)
Since only one bell and one relay are in circuit at any one time, it is obvious that all of the current that pa.s.ses over the line is effective in operating a single bell or relay only. There is no splitting up of the current among a large number of bells as in the bridging system of operating step-by-step devices, which method sometimes so greatly reduces the effective current for each bell that it is with great difficulty made to respond. All the energy available is applied directly to the piece of apparatus at the time it is being operated. This has a tendency toward greater surety of action, and the adjustment of the various pieces of apparatus may be made with less delicacy than is required where many pieces of apparatus, each having considerable work to do, must necessarily be operated in multiple.
The method of unlatching the relays has been briefly referred to.
After a connection has been established with a station in the manner already described, the operator may clear the line when it is proper to do so by sending impulses of such a nature as to cause the line relays of the stations beyond the one chosen to operate, thus continuing the circuit to the end of the line. The operation of the line relay at the last station brings into circuit the coil _8_, Figs.
196 and 197, of a grounding device. This is similar to the line relay, but it holds its operating spring in a normally latched position so as to maintain the two limbs of the line disconnected from the ground.
The next impulse following over the metallic circuit pa.s.ses through the coil _8_ and causes the operation of this grounding device which, by becoming unlatched, grounds the limb _L_ of the line through the coil _8_. This temporary ground at the end of the line makes it possible to send an unlocking or restoring current from the central office over the limb _L_, which current pa.s.ses through all of the unlocking coils _7_, shown in Figs. 194, 196, and 197, thus causing the simultaneous unlocking of all of the line relays and the restoration of the line to its normal condition, as shown in Fig. 196.
[Ill.u.s.tration: Fig. 198. Details of Latching Relay Connections]
As has been stated, the windings _7_ on the line relays are the unlatching windings. In Figs. 196 and 197, for the purpose of simplicity, these windings are not shown connected, but as a matter of fact each of them is included in series in the continuous limb _L_ of the line. This would introduce a highly objectionable feature from the standpoint of talking over the line were it not for the balancing coils _7_^{1}, each wound on the same core as the corresponding winding _7_, and each included in series in the limb _R_ of the line, and in such direction as to be differential thereto with respect to currents pa.s.sing in series over the two limbs of the line.
The windings _7_ are the true unlocking windings, while the windings _7_^{1} have no other function than to neutralize the inductive effects of these unlocking windings necessarily placed in series in the talking circuit. All of these windings are of low ohmic resistance, a construction which, as has previously been noted, brings about the desired effect without introducing any self-induction in the line, and without producing any appreciable effect upon the transmission. A study of Fig. 198 will make clear the connections of these unlocking and balancing windings at each station.
The statement of operation so far given discloses the general method of building up the line in sections in order to choose any party and of again breaking it up into sections when the conversation is finished. It has been stated that the same operation which selects the party wanted also serves to give that party the use of the line and to lock the others off. That this is true will be understood when it is stated that the ringer is of such construction that when operated to ring the subscriber wanted, it also operates to unlatch a set of springs similar to those shown in Fig. 194, this unlatching causing the proper connection of the subscriber's talking circuit across the limbs of the line, and also closing the local circuit through his transmitter. The very first motion of the bell armature performs this unlatching operation after which the bell behaves exactly as an ordinary polarized biased ringer.
[Ill.u.s.tration: Fig. 199. Broken-Back Ringer]
The construction of this ringer is interesting and is shown in its two possible positions in Fig. 199. The group of springs carried on its frame is entirely independent of the movement of the armature during the ringing operation. With reversed currents, however, the armature is moved in the opposite direction from that necessary to ring the bells, and this causes the latching of the springs into their normal position. In order that this device may perform the double function of ringer and relay the tapper rod of the bell is hinged on the armature so as to partake of the movements of the armature in one direction only. This has been called by the inventor and engineers of the Roberts system a _broken-back ringer_, a name suggestive of the movable relation between the armature and the tapper rod. The construction of the ringer is of the same nature as that of the standard polarized ringer universally employed, but a hinge action between the armature and the tapper rod, of such nature as to make the tapper partake positively of the movements of the armature in one direction, but to remain perfectly quiescent when the armature moves in the other direction, is provided.
[Ill.u.s.tration: Fig. 200. Details of Ringer Connection]
How this broken-back ringer controls the talking and the locking-out conditions may best be understood in connection with Fig. 200. The ringer springs are normally latched at all stations. Under these conditions the receiver is short-circuited by the engagement of springs _10_ and _11_, the receiver circuit is open between springs _10_ and _12_, and the local-battery circuit is open between springs _9_ and _12_. The subscribers whose ringers are latched are, therefore, locked out in more ways than one.
When the bell is rung, the first stroke it makes unlatches the springs, which a.s.sume the position shown in the right-hand cut of Fig. 199, and this, it will be seen from Fig. 200, establishes proper conditions for enabling the subscriber to transmit and to receive speech.
The hook switch breaks both transmitter and receiver circuits when down and in raising it establishes a momentary circuit between the ground and the limb _L_ of the line, both upper and lower hook contacts engaging the hook lever simultaneously during the rising of the hook.
The mechanism at the central office by which selection of the proper station is made in a rapid manner is shown in Fig. 201. It has already been stated that the selection of the proper subscriber is brought about by the sending of a predetermined number of impulses from the central office, these impulses pa.s.sing in one direction only and over the metallic circuit. After the proper party has been reached, the ringing current is put on in the reverse direction.
[Ill.u.s.tration: Fig. 201. Central-Office Impulse Transmitter]
The operator establishes the number of impulses to be sent by placing the pointer opposite the number on the dial corresponding to the station wanted. The ratchet wheel is stepped around automatically by each impulse of current from an ordinary pole changer such as is employed in ringing biased bells. When the required number of impulses has been sent, a projection, carried on a group of springs, drops into a notch on the drum of the selector shaft, which operation instantly stops the selecting current impulses and at the same time throws on the ringing current which consists of impulses in the reverse direction. So rapidly does this device operate that it will readily follow the impulses of an ordinary pole changer, even when this is adjusted to its maximum rate of vibration.
[Ill.u.s.tration: VIEW OF A LARGE FOREIGN MULTIPLE SWITCHBOARD]
_Operation._ s.p.a.ce will not permit a full discussion of the details of the central-office selective apparatus, but a general resume of the operation of the system may now be given, with the aid of Fig. 202, which shows a four-station line with the circuits of three of the stations somewhat simplified. In this figure Station A, Station B, and Station D are shown in their locked-out positions, A and B having been pa.s.sed by the selection and ringing of Station C, while Station D is inoperative because it was not reached in the selection and the line is still broken at Station C. Station C, therefore, has possession of the line.
When the subscriber at Station C raised his receiver in order to call central, a ”flash” contact was made as the hook moved up, which momentarily grounded the limb _L_ of the line. (See Fig. 200.) This ”flash” contact is produced by the arrangement of the hook which a.s.sures that the lower contact shall, by virtue of its flexibility, follow up the hook lever until the hook lever engages the upper contact, after which the lower contact breaks. This results in the momentary connection of both the upper and the lower contacts of the hook with the lever, and, therefore, the momentary grounding of the limb _L_ of the line. This limb always being continuous serves, when this ”flash” contact is made, to actuate the line signal at the central office.
[Ill.u.s.tration: Fig. 202. Circuits of Roberts Line]
Since, however, all parties on the line are normally locked out of talking circuits, some means must be provided whereby the operator may place the signaling party in talking connection and leave all the other instruments on the line in their normally locked-out position.
In fact, the operator must be able automatically to pick out the station that signaled in, and operate the ringer to unlatch the springs controlling the talking circuit of that station. Accordingly the operator sends impulses on the line, from a grounded battery, which are in the direction to operate the line relays and to continue the line circuit to the station calling. When, after a sufficient number of impulses, this current reaches that station it finds a path to ground from the limb _L_. This path is made possible by the fact that the subscriber's receiver is off its hook at that station. In order to understand just how this ground connection is made, it must be remembered that each of the ringer magnets is energized with each selecting impulse, but in such a direction as not to ring the bells, it being understood that all of the ringer mechanisms are normally latched. When the selecting impulse for Station C arrives, it pa.s.ses through the ringer and the selecting relay coils at that station and starts to operate the remainder of the ringers sufficiently to cause the spring _12_ to engage the spring _13_. This establishes the ground connection from the limb _L_ of the line, the circuit being traced through limb _L_ through the upper contact of the switch, thence through springs _12_ and _13_ to ground, and this, before the line relay has time to latch, operates the quick-acting relay at the central office, which acts to cut off further impulses, and thus automatically stops at the calling station. Ringing current in the opposite direction is then sent to line; this unlatches the ringer springs and places the calling subscriber in talking circuit. When the operator has communicated with the calling subscriber, and found, for example, that another party on another similar line is desired, she turns the dial pointer on the selector to the number corresponding to the called-for party's number on that line, and presses the signal key. Pressing this key causes impulses to ”run down the line,”
selecting the proper party and ringing his bell in the manner already described. The connection between the two parties is then established, and no one else can in any possible way, except by permission of the operator, obtain access to the line.
It is obvious that some means must be provided for restoring the selecting relays to normal after a conversation is finished. By referring to Fig. 194 it will be seen that the upper end of the latch spring _5_ is bent over in such a manner that when the armature is attracted by current flowing through the coil _7_, the k.n.o.b on the left-hand end of the armature on rising engages with the bent cam surface and forces back the latch, permitting spring _2_ to return to its normal position.
To restore the line the operator sends out sufficient additional selective impulses to extend the circuit to the end of the line, and thus brings the grounder into circuit. The winding of the grounder is connected in such a manner that the next pa.s.sing impulse throws off its latch, permitting the long spring to contact with the ground spring. The operator now sends a grounded impulse over the continuous limb _L_ of the line which pa.s.ses through the restoring coils _7_ at all the stations and through the right-hand coil of the grounding device to ground. The selecting relays are, therefore, simultaneously restored to normal. The grounder is also energized and restored to its normal position by the same current.
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