Volume Ii Part 10 (1/2)
The line switches of one hundred lines are all a.s.sociated to form a single unit of apparatus, which, besides the individual line switches, includes certain other apparatus common to those lines. Such a group of one hundred line switches and a.s.sociated common apparatus is called a _line-switch unit_, or frequently, a _Keith unit_. Confusion is likely to arise in the mind of the reader between the individual line switch and the line-switch unit, and to avoid this we will refer to the piece of apparatus individual to the line as the line switch, and to the complete unit formed of one hundred of these devices as a line-switch unit.
_Line and Trunk Contacts._ Each line switch has its own bank of contacts arranged in the arc of a circle, and in this same arc are also placed the contacts of each of the ten individual trunks which it is possible for that line to appropriate. The contacts individual to the subscriber's line in the line switch are all multipled together, the arrangement being such that if a wedge or plunger is inserted at any point, the line contacts will be squeezed out of their normal position so as to engage the contacts of the trunk corresponding to the particular position in the arc at which the wedge or plunger is inserted. A small plunger individual to each line is so arranged that it may be thrust in between the contact springs in the line-switch bank in such manner as to connect any one of the trunks with the line terminals represented in that row, the particular trunk so connected depending on the portion of the arc toward which the plunger is pointed at the time it is thrust in the contacts.
These banks of lines and trunk contacts are horizontally arranged, and piled in vertical columns of twenty-five line switches each. The ten trunk contacts are multipled vertically through the line-switch banks, so that the same ten trunks are available to each of the twenty-five lines. We thus have, in effect, an old style, Western Union, cross-bar switchboard, the line contacts being represented in horizontal rows and the trunk contacts in vertical rows, the connection between any line and any trunk being completed by inserting a plunger at the point of intersection of the horizontal and the vertical rows corresponding to that line and trunk.
_Trunk Selection._ The plungers by which the lines and trunks are connected are, as has been said, individual to the line, and all of the twenty-five plungers in a vertical row are mounted in such manner as to be normally held in the same vertical plane, and this vertical plane is made to oscillate back and forth by an oscillating shaft so as always _to point the plungers toward a vertical row of trunk contacts that represent a trunk that is not in use at the time_. The to-and-fro movement of this oscillating shaft, called the _master bar_, is controlled by a master switch and the function of this master switch is always to keep the plungers pointed toward the row of contacts of an idle trunk. The thrusting movement of the individual plungers into the contact bank is controlled by magnets individual to the line and under control of the subscriber in initiating a call. As soon as the plunger of a line has been thus thrust into the contact bank so as to connect the terminals of that line with a given trunk, the plunger is no longer controlled by the master bar and remains stationary. The master bar then at once moves all of the other plungers that are not in use so that they will point to the terminals of another trunk that is not in use. The plungers of all the line switches in a group of twenty-five are, therefore, subject to the oscillating movements of the master bar when the line is not connected to a first selector trunk. As soon as a call is originated on a line, the corresponding plunger is forced into the bank and is held stationary in maintaining the connection to a first selector trunk, and all of the other plungers not so engaged, move on so as to be ready to engage another idle trunk.
_Trunk Ratio._ The a.s.signment of ten trunks to twenty-five lines would be a greater ratio of trunks than ordinary traffic conditions require.
This ratio of trunks to lines is, however, readily varied by multipling the trunk contacts of several twenty-five line groups together. Thus, ten trunks may be made available to one hundred subscribers' lines by multipling the trunks of four twenty-five line switch groups together.
In this case the four master bars corresponding to the four groups of twenty-five line switches are all mechanically connected together so as to move in unison under the control of a single master switch. If more than ten and less than twenty-one trunks are a.s.signed to one hundred lines, then each set of ten trunks is multipled to the trunk contacts of fifty line switches, the two master bars of these switches being connected together and controlled by a common master switch.
_Structure of Line Switch._ The details of the parts of a line switch that are individual to the line are shown in Fig. 385, the line and trunk contact bank being shown in the lower portion of this figure and also in a separate view in the detached figure at the right. A detailed group of several such line switches with the oscillating master bar is shown in Fig. 386. This figure shows quite clearly the relative arrangement of the line and trunk contact banks, the plungers for each bank, and the master bar.
[Ill.u.s.tration: Fig. 385. Line Switch]
In practice, four groups of twenty-five line switches each are mounted on a single framework and the group of one hundred line switches, together with certain other portions of the apparatus that will be referred to later, form a line-switch unit. A front view of such a unit is shown in Fig. 387. In order to give access to all portions of the wiring and apparatus, the framework supporting each column of fifty line switches is hinged so as to open up the interior of the device as a whole. A line-switch unit thus opened out is shown in Fig. 388.
[Ill.u.s.tration: Fig. 386. Portion of Line-Switch Unit]
_Circuit Operation._ The mode of operation of the line switch may be best understood in connection with Fig. 389, which shows in a schematic way the parts of a line switch that are individual to a subscriber's line, and also those that are common to a group of fifty or one hundred lines. Those portions of Fig. 389 which are individual to the line are shown below the dotted line extending across the page. The task of understanding the line switch will be made somewhat easier if Figs. 385 and 389 are considered together. The individual parts of the line switch are shown in the same relation to each other in these two figures with the exception that the bank of line and trunk springs in the lower right-hand corner of Fig. 389 have been turned around edgewise so as to make an understanding of their circuit connections possible.
[Ill.u.s.tration: Fig. 387. Line-Switch Unit]
[Ill.u.s.tration: Fig. 388. Line-Switch Unit]
[Ill.u.s.tration: Fig. 389. Circuits of Line-Switch Unit]
The vertical and rotary sides of the subscriber's line are shown entering at the lower left-hand corner of this figure, and they pa.s.s to the springs of the contact bank. Immediately adjacent to these springs are the trunk contacts from which the vertical and the rotary limbs of the first selector trunk proceed. The plunger is indicated at _1_, it being in the form of a wheel of insulating material. It is carried on the rod _2_ pivoted on a lever _3_, which, in turn, is pivoted at _4_ in a stationary portion of the framework. A spring _5_, secured to the underside of the lever _3_ and projecting to the left beyond the pivot _4_ of this lever, serves always to press the right-hand portion of the lever _3_ forward in such direction as to tend to thrust it into the contact bank. The plunger is normally held out of the contact bank by means of the latch _6_ carried on the armature _7_ of the trip magnet.
When the trip magnet is energized it pulls the armature _7_ to the left and thus releases the plunger and allows it to enter the contact bank.
[Ill.u.s.tration: POWER SWITCHBOARD FOR MEDIUM-SIZED OFFICE Mercury Arc Rectifier Panel and Transformer at Right.]
The master bar is shown at _8_, and a feather on this bar engages a notch in the segment attached to the rear end of the plunger rod _2_.
This master bar is common to all of the plunger rods and by its oscillatory movement, under the influence of the master switch, it always keeps all of the idle plunger bars pointed toward the contacts of an idle trunk. As soon, however, as the trip magnet is operated to cause the insertion of a plunger into the contact bank, the feather on the master bar is disengaged by the notch in the segment of the plunger rod, and the plunger rod is, therefore, no longer subject to the oscillating movement of the master bar.
When the release magnet is energized, it attracts its armature _9_ and this lifts the armature _7_ of the trip magnet so that the latch _6_ rides on top of the left-hand end of the lever _3_. Then, when the release magnet is de-energized, the spring _5_, which was put under tension by the latch, moves the entire structure of levers back to its normal position, withdrawing the plunger from the bank of contacts. The notch on the edge of the segment of the plunger rod, when thus released, will probably not strike the feather on the master bar, and the plunger rod will thus not come under the control of the master bar until the master bar has moved, in its oscillation, so that the feather registers with the notch, after which this bar will move with all the others.
If, while the plunger is waiting to be picked up by the master bar, the same subscriber should call again, his line will be connected with the same trunk as before. There is no danger in this, however, that the trunk will be found busy, because the master bar will not have occupied a position which would make it possible for any of the lines to appropriate this trunk during the intervening time.
_Master Switch._ a.s.sociated with each master bar there is a master switch which determines the position in which the master bar shall stop in order that the idle plungers may be pointed always to the contacts of an idle trunk. The arm _10_ of this switch is attached to the master bar and oscillates with it and serves to connect the segment _11_ successively with the contacts _12_, which are connected respectively to the third, or release wire of each first selector trunk. In the figure the arm _10_ is shown resting on the sixth contact of the switch and this sixth contact is connected to a spring _13_ in the line-switch contact bank that has not yet been referred to. As soon as the plunger is inserted into the contact bank, the spring _14_ will be pressed into engagement with the spring _13_, and this spring _14_ is connected with the live side of the battery through the release magnet winding.
The contact strip _11_ on the master switch is thus connected through the release magnet to the battery and from this current flows through the left-hand winding of the master-switch relay. This energizes this relay and causes the closure of the circuit of the locking magnet which magnet unlocks the master bar to permit its further rotation. The unlocking of the master bar brings the spring _15_ into engagement with _16_ and thus energizes the master magnet, the armature of which vibrates back and forth after the manner of an electric-bell armature, and steps the wheel _17_ around. The wheel _17_ is mechanically connected to the master bar so that each complete revolution of the wheel will cause one complete oscillation of the master bar. The master bar will thus be moved so as to cause all the idle plungers to sweep through an arc and this movement will stop as soon as the master-switch arm _10_ connects the arc _11_ with one of the contacts _12_ that is not connected to the live side of the battery through the springs _13_ and _14_ of some other line switch. It is by this means that the plungers of the line switches are always kept pointing at the contacts of an idle trunk. The way in which this feature has been worked out must demand admiration and accounts for the marvelous quickness of this line switch.
The fact that the plungers are pointed in the right direction before the time comes for their use, leaves only the simple thrusting motion of the plunger to accomplish the desired connection immediately upon the initiation of a call by the subscriber.
_Locking Segment._ It will be understood that the locking segment _18_ and the master-switch contact finger _10_ are both rigidly connected with the master bar _8_ and move with it, the locking segment _18_ serving always to determine accurately the angular position at which the master bar and the master-switch arm are brought to rest.
_Bridge Cut-Off._ One important feature of automatic switching, particularly as exemplified in the system of the Automatic Electric Company, is the disconnection, after its use, of each operating magnet of each piece of apparatus involved in making a connection. Since these operating magnets are always bridged across the line at the time of their operation and then cut off after they have performed their function, this feature may be referred to as the _bridge cut-off_.
_Guarding Functions._ Still another feature of importance is the means for guarding a line or a piece of apparatus that has already been appropriated or made busy, so that it will not be appropriated or connected with for use in some other connection. For this latter purpose contacts and wires are a.s.sociated with each piece of apparatus, which are multipled to similar contacts on other pieces of apparatus in much the same way and for a similar purpose that the test thimbles in a multiple switchboard are multipled together. Such wires and contacts in the Automatic Electric Company's apparatus are called _private wires_ and _contacts_.
The bridge cut-off and guarding functions are provided for in the line switch by a bridge cut-off relay shown in Fig. 389 and also in Fig. 385, it being the upper one of the individual line relays in each of those figures. This bridge cut-off relay is operated as soon as the plunger of the line is thrust into the bank; the contacts _19_ and _20_, closed by the plunger, serving to complete the circuit of this relay. To make clear the bridge cut-off feature it will be noted that the trip magnet of a line switch is connected in a circuit traced from the rotary side of the line through the contacts _21_ and _22_ of the bridge cut-off relay, thence through the coil of the trip magnet to the common wire leading to the spring _23_ of the master-bar locking device and thence to the live side of the battery. Obviously, therefore, as soon as the bridge cut-off relay operates, the trip magnet becomes inoperative and can cause no further action of the line switch because its circuit is broken between the springs _21_ and _22_.