Volume I Part 31 (1/2)
[Ill.u.s.tration: Fig. 273. Circuit of Simple Magneto Switchboard]
Little need be said concerning this circuit in view of what has already been said in connection with Figs. 240 to 245. It will be seen in the particular sub-station circuit here represented, that the talking apparatus is arranged in the usual manner and that the ringer and generator are so arranged that when the generator is operated the ringer will be cut out of circuit, while the generator will be placed across the circuit; while, when the generator is idle, the ringer is bridged across the circuit and the generator is cut out.
The line terminates in each case in the tip and sleeve contacts of the jack, and in the normal condition of the jack the line drop is bridged across the line. The arrangement by which the drop is restored and at the same time cut out of circuit when the operator plugs in the jack, is obvious from the diagrammatic ill.u.s.tration. The cord circuit is the same as that already discussed, with the exception that two ringing keys are provided, one in connection with the calling plug, as is universal practice, and the other in connection with the answering plug as is sometimes practiced in order that the operator may, when occasion requires, ring back the calling subscriber without the necessity of changing the plug in the jack. The outer contacts of these two ringing keys are connected to the terminals of the ringing generator and, when either key is operated, the connection between the plug, on which the ringing is to be done, and the rest of the cord circuit will be broken, while the generator will be connected with the terminals of the plug.
The listening key and talking apparatus need no further explanation, it being obvious that when the key is operated the subscriber's telephone set will be bridged across the cord circuit and, therefore, connected with either or both of the talking subscribers.
[Ill.u.s.tration: Fig. 274. Night-Alarm Circuit]
Night-Alarm Circuits. The circuit of Fig. 273, while referred to as a complete circuit, is not quite that. The night-alarm circuit is not shown. In order to clearly indicate how a single battery and bell, or buzzer, may serve in connecting a number of line drops, reference is made to Fig. 274 which shows the connection between three different line drops and the night-alarm circuit. The night-alarm apparatus consists in the battery _1_ and the buzzer, or bell, _2_. A switch _3_ adapted to be manually operated is connected in the circuit with the battery and the buzzer so as to open this circuit when the night alarm is not needed, thus making it inoperative. During the portions of the day when the operator is needed constantly at the board it is customary to leave this switch _3_ open, but during the night period when she is not required constantly at the board this switch is closed so that an audible signal will be given whenever a drop falls. The night-alarm contact _4_ on each of the drops will be closed whenever a shutter falls, and as the two members of this contact, in the case of each drop, are connected respectively with the two sides of the night-alarm circuit, any one shutter falling will complete the necessary conditions for causing the buzzer to sound, a.s.suming of course that the switch _3_ is closed.
_Night Alarm with Relay._ A good deal of trouble has been caused in the past by uncertainty in the closure of the night-alarm circuit at the drop contact. Some of the companies have employed the form of circuit shown in Fig. 275 to overcome this. Instead of the night-alarm buzzer being placed directly in the circuit that is closed by the drop, a relay _5_ and a high-voltage battery _6_ are placed in this circuit. The buzzer and the battery for operating it are placed in a local circuit controlled by this relay. It will be seen by reference to Fig. 275 that when the shutter falls, it will, by closing the contact _4_, complete the circuit from the battery _6_ through the relay _5_--a.s.suming switch _3_ to be closed--and thus cause the operation of the relay. The relay, in turn, by pulling up its armature, will close the circuit of the buzzer _2_ through the battery _7_ and cause the buzzer to sound.
[Ill.u.s.tration: Fig. 275. Night-Alarm Circuit with Relay]
The advantage of this method over the direct method of operating the buzzer is that any imperfection in the night-alarm contact at the drop is much less likely to prevent the flow of current of the high-voltage battery _6_ than of the low-voltage battery _1_, shown in connection with Fig. 274. This is because the higher voltage is much more likely to break down any very thin bit of insulation, such as might be caused by a minute particle of dust or oxide between contacts that are supposed to be closed by the falling of the shutter. It has been common to employ for battery _6_ a dry-cell battery giving about 20 or 24 volts, and for the operation of the buzzer itself, a similar battery of about two cells giving approximately 3 volts.
_Night-Alarm Contacts._ The night-alarm contact _4_ of the drop shown diagrammatically in Figs. 274 and 275 would, if taken literally, indicate that the shutter itself actually forms one terminal of the circuit and the contact against which it falls, the other. This has not been found to be a reliable way of closing the night-alarm contacts and this method is indicated in these figures and in other figures in this work merely as a convenient way of representing the matter diagrammatically. As a matter of fact the night-alarm contacts are ordinarily closed by having the shutter fall against one spring, which is thereby pressed into engagement with another spring or contact, as shown in Fig. 249. This method employs the shutter only as a means for mechanically causing the one spring to press against the other, the shutter itself forming no part of the circuit. The reason why it is not a good plan to have the shutter itself act as one terminal of the circuit is that this necessitates the circuit connections being led to the shutter through the trunnions on which the shutter is pivoted. This is bad because, obviously, the shutter must be loosely supported on its trunnions in order to give it sufficiently free movement, and, as is well known, loose connections are not conducive to good electrical contacts.
Grounded-and Metallic-Circuit Lines. When grounded circuits were the rule rather than the exception, many of the switchboards were particularly adapted for their use and could not be used with metallic-circuit lines. These grounded-circuit switchboards provided but a single contact in the jack and a single contact on the plug, the cords having but a single strand reaching from one plug to the other.
The ringing keys and listening keys were likewise single-contact keys rather than double. The clearing-out drop and the operator's talking circuit and the ringing generator were connected between the single strand of the cord and the ground as was required.
The grounded-circuit switchboard has practically pa.s.sed out of existence, and while a few of them may be in use, they are not manufactured at present. The reason for this is that while many grounded circuits are still in use, there are very few places where there are not some metallic-circuit lines, and while the grounded-circuit switchboard will not serve for metallic-circuit lines, the metallic-circuit switchboard will serve equally well for either metallic-circuit or grounded lines, and will interconnect them with equal facility. This fact will be made clear by a consideration of Figs. 276, 277, and 278.
[Ill.u.s.tration: Fig. 276. Connection Between Metallic Lines]
[Ill.u.s.tration: Fig. 277. Connection Between Grounded Lines]
_Connection between Two Similar Lines._ In Fig. 276 a common magneto cord circuit is shown connecting two metallic-circuit lines; in Fig.
277 the same cord circuit is shown connecting two grounded lines. In this case the line wire _1_ of the left-hand line is, when the plugs are inserted, continued to the tip of the answering plug, thence through the tip strand of the cord circuit to the tip of the calling plug, then to the tip spring of the right-hand jack and out to the single conductor of that line. The entire sleeve portion of the cord circuit becomes grounded as soon as the plugs are inserted in the jacks of such a line. Hence, we see that the sleeve contacts of the plug and the sleeve conductor of the cord are connected to ground through the permanent ground connection of the sleeve conductors of the jack as soon as the plug is inserted into the jack. Thus, when the cord circuit of a metallic-circuit switchboard is used to connect two grounded circuits together, the tip strand of the cord is the connecting link between the two conductors, while the sleeve strand of the cord merely serves to ground one side of the clearing-out drop and one side each of the operator's telephone set and the ringing generator when their respective keys are operated.
_Connection between Dissimilar Lines._ Fig. 278 shows how the same cord circuit and the same arrangement of line equipment may be used for connecting a grounded line to a metallic-circuit line. The metallic circuit line is shown on the left and the grounded line on the right. When the two plugs are inserted into the respective jacks of this figure, the right-hand conductor of the metallic circuit shown on the left will be continued through the tip strand of the cord circuit to the line conductor of the grounded line shown on the right.
The left-hand conductor of the metallic-circuit line will be connected to ground because it will be continued through the sleeve strand of the cord circuit to the sleeve contact of the calling plug and thence to the sleeve contact of the jack of the grounded line, which sleeve contact is shown to be grounded. The talking circuit between the two connected lines in this case may be traced as follows: From the subscriber's station at the left through the right-hand limb of the metallic-circuit line, through the tip contact and tip conductor of the cord circuit, to the single limb of the grounded-circuit line, thence to the sub-station of that line and through the talking apparatus there to ground. The return path from the right-hand station is by way of ground to the ground connection at the central office, thence to the sleeve contact of the grounded line jack, through the sleeve conductor of the cord circuit, to the sleeve contact of the metallic-circuit line jack, and thence by the left-hand limb of the metallic-circuit line to the subscriber's station.
[Ill.u.s.tration: Fig. 278. Connection Between Dissimilar Lines]
A better way of connecting a metallic-circuit line to a grounded line is by the use of a special cord circuit involving a repeating coil, such a connection being shown in Fig. 279. The cord circuit in this case differs in no respect from those already shown except that a repeating coil is a.s.sociated with it in such a way as to conductively divide the answering side from the calling side. Obviously, whatever currents come over the line connected with the answering plug will pa.s.s through the windings _1_ and _2_ of this coil and will induce corresponding currents in the windings _3_ and _4_, which latter currents will pa.s.s out over the circuit of the line connected with the calling plug. When a grounded circuit is connected to a metallic circuit in this manner, no ground is thrown onto the metallic circuit.
The balance of the metallic circuit is, therefore, maintained.
To ground one side of a metallic circuit frequently so unbalances it as to cause it to become noisy, that is, to have currents flowing in it, by induction or from other causes, other than the currents which are supposed to be there for the purpose of conveying speech.
[Ill.u.s.tration: Fig. 279. Connection of Dissimilar Lines through Repeating Coil]
_Convertible Cord Circuits._ The consideration of Fig. 279 brings us to the subject of so-called convertible cord circuits. Some switchboards, serving a mixture of metallic and grounded lines, are provided with cord circuits which may be converted at will by the operator from the ordinary type shown in Fig. 276 to the type shown in Fig. 279. The advantage of this will be obvious from the following consideration. When a call originates on any line, either grounded or metallic, the operator does not know which kind of a line is to be called for. She, therefore, plugs into this line with any one of her answering plugs and completes the connection in the usual way. If the call is for the same kind of a circuit as that over which the call originated, she places the converting key in such a position as will connect the conductors of the cord circuit straight through; while if the connection is for a different kind of a line than that on which the call originated she throws the converting key into such a position as to include the repeating coil. A study of Fig. 280 will show that when the converting key, which is commonly referred to as the repeating-coil key, is in one position, the cord conductors will be cut straight through, the repeating coil being left open in both its windings; and when it is thrown to its other position, the connection between the answering and calling sides of the cord circuit will be severed and the repeating coil inserted so as to bring about the same effects and circuit arrangements as are shown in Fig. 279.
[Ill.u.s.tration: Fig. 280. Convertible Cord Circuit]
Cord-Circuit Considerations. _Simple Bridging Drop Type._ The matter of cord circuits in magneto switchboards is deserving of much attention. So far as talking requirements are concerned, the ordinary form of cord circuit with a clearing-out drop bridged across the two strands is adequate for nearly all conditions except those where a grounded-and a metallic-circuit line are connected together, in which case the inclusion of a repeating coil has some advantages.
[Ill.u.s.tration: Fig. 281. Bridging Drop-Cord Circuit]