Cyclopedia of Telephony and Telegraphy Volume Ii Part 14

Total Time Consumed by Operator in Handling Calls on Automanual System

+-----------------------------------------------------------------+ First 100 Calls +-----------------------------------------------------------------+ Longest Individual Period 12.40 seconds Average five longest Individual Periods 7.44 seconds Average ten longest Individual Periods 6.34 seconds Shortest Individual Period 1.60 seconds Average five shortest Individual Periods 1.92 seconds Average ten shortest Individual Periods 1.96 seconds Average Entire 100 Calls 3.396 seconds Hourly Rate at which calls were being handled 1060 +-----------------------------------------------------------------+

+-----------------------------------------------------------------+ Second 100 Calls +-----------------------------------------------------------------+ Longest Individual Period 7.60 seconds Average five longest Individual Periods 5.52 seconds Average ten longest Individual Periods 5.34 seconds Shortest Individual Period 2.00 seconds Average five shortest Individual Periods 2.04 seconds Average ten shortest Individual Periods 2.18 seconds Average Entire 100 Calls 3.374 seconds Hourly Rate at which calls were being handled 1067 +-----------------------------------------------------------------+

+-----------------------------------------------------------------+ Third 100 Calls +-----------------------------------------------------------------+ Longest Individual Period 5.40 seconds Average five longest Individual Periods 5.32 seconds Average ten longest Individual Periods 4.44 seconds Shortest Individual Period 1.60 seconds Average five shortest Individual Periods 1.65 seconds Average ten shortest Individual Periods 1.80 seconds Average Entire 100 Calls 3.160 seconds Hourly Rate at which calls were being handled 1139 +-----------------------------------------------------------------+

Owing to the difficulty of securing accurate traffic data by means of a stop watch, an automatic, electrical timing device, capable of registering seconds and hundredths of a second, has been used in studying the performance of this system in regular operation at Ashtabula Harbor. The operators were not informed that the records were being taken, and the data tabulated represents the work of two operators in handling regular subscribers' calls. The figures in Table XI are given by C. H. North as representing the total time consumed by the operator from the time her line lamp was lighted until her work in connection with the call was finished, and it included, therefore, the pressing of the listening b.u.t.ton, the receiving of the number from the subscriber, repeating it back to him, setting up the connection on the keys, and pressing the starting key.

It will be seen that the average time for each 100 calls is quite uniform and is slightly over three seconds. The considerable variation in the individual calls, ranging from a maximum of 12.40 seconds down to a minimum of 1.60 seconds, is due almost entirely to the difference between the subscribers in the speed with which they can give their numbers. These figures indicate that, in each of the tests, calls were being handled at the rate of more than one thousand per hour by each operator.

The test of the subscriber's waiting time, _i. e._, the time that he waited for the operator to answer, for one hundred calls made without the knowledge of the operator, showed the results as given in Table XII, in which a split second stop watch was used in making the observations.

TABLE XII

Subscribers' Waiting Time

+----------------------------------------------------------+ Number of Calls Tested 100 Longest Individual Period 5.20 seconds Average 5 Longest Individual Periods 4.64 seconds Average 10 Longest Individual Periods 3.80 seconds Shortest Individual Period 1.00 seconds Average 5 Shortest Individual Periods 1.28 seconds Average 10 Shortest Individual Periods 1.34 seconds Average Entire 100 Calls 2.07 seconds +----------------------------------------------------------+

The length of time which the subscriber has to wait before receiving an answer from the operator is, of course, one of the factors that enters into the giving of good telephone service, and the times shown by this test are considerably shorter than ordinarily maintained in manual practice. The waiting time of the subscriber is not, of course, a part of the time that is consumed by the operator, and the real economy so far as the operator's time is concerned is shown in the tests recorded in Table XI.

CHAPTER x.x.xII

POWER PLANTS

The power plant is an organization of devices to furnish to a telephone system the several kinds of current, at proper pressures, for the performance of the several general electrical tasks within the exchange.

=Kinds of Currents Employed.= Sources of both direct and alternating current are required and a single exchange may employ these for one or more of the following purposes:

_Direct Current._ Current which flows always in one direction whether steady or varying, is referred to as direct current, and may be required for transmitters, for relays, for line, supervisory, and auxiliary signals, for busy tests, for automatic switches, for call registers, for telegraphy, and in the form of pulsating current for the ringing of biased bells.

_Alternating Current._ Sources of alternating current are required for the ringing of bells, for busy-back and other automatic signals to subscribers, for howler signals to attract the attention of subscribers who have left their receivers off their hooks, and for signaling over composite lines.

=Types of Power Plants.= Clearly the requirements for current supply differ greatly for magneto and common-battery systems. There is, however, no great difference between the power plants required for the automatic and the manual common-battery systems.

In the simplest form of telephone system--two magneto telephones on a private line--the power plant at each station consists of two elements: one, the magneto generator, which is a translating device for turning hand power into alternating current for ringing the bell of the distant station; and the other, a primary battery which furnishes current to energize the transmitter. In such a system, therefore, each telephone has its own power plant. The term power plant, however, as commonly employed in telephone work, refers more particularly to the organization of devices at the central office for furnis.h.i.+ng the required kinds of current, and it is to power plants in this sense that this chapter is devoted.

_Magneto Systems._ If magneto lines be connected to a switchboard, the current for throwing the drop at the switchboard is furnished by the subscriber's generator, and the current for energizing the subscriber's transmitter is furnished by the local battery at his station; but sources of current must be provided for enabling the central-office operator to signal or talk to the subscribers. These are about the only needs for which current must be furnished in an ordinary magneto central office. If a multiple board is employed, direct current is also needed for the purpose of the busy test and also for operating the drop restoring circuits, if the electrical method of restoring the drops is employed.

_Common-Battery Systems._ In common-battery systems the requirements are very much more extensive. The subscribers' telephones have no power plants of their own, but are provided with a common source of direct current located at the central office for supplying the talking current, and for operating the central-office signals, and the operators are provided with one or more common sources of alternating or pulsating current for ringing the subscribers' bells. Common-battery equipment requires the use of currents of different kinds for a greater number of auxiliary purposes than does magneto equipment. These facts make the power plant in a common-battery office much more important than in a magneto office.

=Operators' Transmitter Supply.= In a small magneto exchange, the transmitter current may be had from primary batteries, a separate battery being employed for each operator's set. When there are more than three or four operators, however, it is usual, even in magneto offices, to obtain the transmitter current from a common storage battery. A storage battery has the fortunate quality of very low internal resistance, therefore a number of operators' transmitters may be actuated by one source without introducing cross-talk. In other words, a storage battery is a current-furnis.h.i.+ng device of good regulation, the variation of consumption in one circuit leading from it causing slight variation in the currents of other circuits leading from it. If this were not so, cross-talk would exist between the telephones of the operators' positions connected to the same battery. This regulating quality enables the multiple feeding of telephone circuits to be carried further than the mere supplying of operators' sets and is the quality which makes possible the successful use of a storage battery as the single source of transmitter current for common-battery central-office equipment.

In furnis.h.i.+ng a plurality of operators' transmitters from a common battery, the importance of low resistance and inductance in the portion of the path that is common to all of the circuits must not be overlooked. Not only is a battery of extremely low resistance required, but also conductors leading from it that are common to two or more of the circuits should be of very low resistance and consequently large in cross-section and as short as possible. In common-battery offices there is obviously no need of employing a separate battery for the operators'

transmitters, since they may readily be supplied from the common storage battery which supplies direct current to the subscribers' lines.

=Ringing-Current Supply.= _Magneto Generators._ As a central-office equipment is required to ring many subscribers' bells, only the small ones find it convenient to ring them by means of hand-operated magneto generators. Small magneto switchboards are usually equipped so that each operator is provided with a hand-generator, but even where such is the case some source of ringing current not manually operated is desirable.

In larger switchboards the hand generators are entirely dispensed with.

The magneto generator may be driven by a belt from any convenient constantly moving pulley, and the early telephone exchanges were often equipped with such generators having better bearings and more current capacity than those in magneto telephones. These were adapted to be run constantly from some source of power, delivering ringing current to the operators' keyboards at from 16 to 20 cycles per second.

_Pole Changers._ Vibrating pole changers were also used in the early exchanges, but pa.s.sed out of use, partly because of poor design, but more because of the absence of good forms of primary batteries for vibrating them and for furnis.h.i.+ng the direct currents to be transformed into alternating line current for ringing the bells. The pole changer was redesigned after the beginning of the great spread of telephony in the United States in 1893. Today it is firmly established as an element of good telephone practice. Fig. 411 ill.u.s.trates the principle upon which one of the well-known pole changers--the Warner--operates. In this _1_ is an electromagnet supplied by a constant-current battery _2_ to keep the vibratory system continually in motion. This motor magnet and its battery work in a local circuit and cause vibration in exactly the same manner as the armature of an ordinary electric door bell is caused to vibrate. The battery from which the ringing current is derived is indicated at _3_, and the poles of this are connected, respectively, to the vibrating contacts _4_ and _5_. These contacts are merely the moving members of a pole changing switch, and a study of the action will readily show that when these moving parts engage the right-hand contacts, current will flow to the line supposed to be connected to the terminals _6_ and _7_ in one direction, while, when these parts engage the left-hand contacts, current will flow to the line in the reverse direction. The circuit of the condenser shown is controlled by the armature of the relay _8_.

The winding of this relay is put directly in the circuit of the main battery _3_, so that whenever current is drawn from this battery to ring a distant bell, this relay will be operated and will bridge the condenser across the circuit of the line. The purpose of the condenser is to make the impulses flowing from the pole changer less abrupt, and the reason for having its bridged circuit normally broken is to prevent a waste of current from the battery _3_, due to the energy which would otherwise be consumed by the condenser if it were left permanently across the line.

[Ill.u.s.tration: Fig. 411. Warner Pole Changer]

[Ill.u.s.tration: Fig. 412. Pole Changers for Harmonic Ringing]

Pole changers for ringing bells of harmonic party lines are required to produce alternating currents of practically constant frequencies. The ideal arrangement is to cause the direct currents from a storage battery to be alternated by means of the pole changers, and then transformed into higher voltages required for ringing purposes, the transformer also serving to smooth the current wave, making it more suitable for ringing purposes. In Fig. 412 such an arrangement, adapted to develop currents for harmonic ringing on party lines, is shown. The regular common battery of the central office is indicated at _1_, _2_ being an auxiliary battery of dry cells, the purpose of which will be presently referred to. At the right of the battery _1_ there is shown the calling plug with its a.s.sociated party-line ringing keys adapted to impress the several frequencies on the subscribers' lines. The method by which the current from the main storage battery pa.s.ses through the motor magnets of the several vibrators, and by which the primary currents through the transformers are made to alternate at the respective frequencies of these vibrators, will be obvious from the drawing. It is also clear that the secondary currents developed in these transformers are led to the several ringing keys so as to be available for connection with the subscribers' lines at the will of the operator. The condensers are bridged across the primary windings of the transformers for the purpose of aiding in smoothing out the current waves. The use of the auxiliary battery _2_ and the r.e.t.a.r.dation coil _3_ in the main supply lead is for the purpose of preventing the pulsating currents drawn from the main battery _1_ from making the battery "noisy." These two batteries have like poles connected to the supply lead, and the auxiliary battery furnishes no current to the system except when the electromotive force of the impulse flowing from the main battery is choked down by the impedance coil and the deficiency is then momentarily supplied for each wave by the auxiliary battery. This is the method developed by the Dean Electric Company for preventing the pole-changer system from causing disturbances on lines supplied from the same main battery.

[Ill.u.s.tration: Fig. 413. Multi-Cyclic Generator Set]

_Ringing Dynamos._ Alternating and pulsating currents for ringing purposes are also largely furnished from alternating-current dynamos similar to those used in commercial power and lighting work, but specially designed to produce ringing currents of proper frequency and voltage. These are usually driven by electric motors deriving their current either from the commercial supply mains or from the central-office battery. In large exchanges harmonic ringers are usually operated by alternating-current generators driven by motors, a separate dynamo being provided to furnish the current of each frequency. Fig. 413 shows a set of four such generators directly connected to a common motor. As no source of commercial power for driving such generators is absolutely uniform, and since the frequency of the ringing current must remain very close to a constant predetermined rate, some means must be employed for holding the generators at a constant speed of revolution, and this is done by means of a governor shown at the right-hand end of the shaft in Fig. 413. The principle of this governor is shown in Fig.

414. A weighted spring acts, by centrifugal force, to make a contact against an adjustable screw, when the speed of the shaft rises a predetermined amount. This spring and its contact are connected to two collector rings _1_ and _2_ on the motor shaft, and connection is made with these by the brushes _3_ and _4_. The closing of the governor contact serves, therefore, merely to short-circuit the resistance _5_, which is normally included in the shunt field of the motor. This governor is based on the principle that weakening the field increases the speed. It acts to insert the resistance in series with the field winding when the speed falls, and this, in turn, results in restoring the speed to normal.

[Ill.u.s.tration: Fig. 414. Governor for Harmonic Ringing Generators]

=Auxiliary Signaling Currents.= Alternating currents, such as those employed for busy signals to subscribers in automatic systems, those for causing loud tones in receivers which have been left off the hook switch, and those for producing loud tones in calling receivers connected to composite lines, all need to be of much higher frequency than alternating current for ringing bells. The simplest way of producing such tones is by means of an interrupter like that of a vibrating bell; but this is not the most reliable way and it is usual to produce busy or "busy-back" currents by rotating commutators to interrupt a steady current at the required rate. As the usual busy-back signal is a series of recurrent tones about one-half second long, interspersed with periods of silence, the rapidly commuted direct current is required to be further commuted at a slow rate, and this is conveniently done by a.s.sociating a high-speed commutator with a low-speed one. Such an arrangement may be seen at the left-hand end of the multicyclic alternating machine shown in Fig. 413. This commuting device is usually a.s.sociated with the ringing machine because that is the one thing about a central office that is available for imparting continuous rotary motion.

=Primary Sources.= Most telephone power plants consume commercial electric power and deliver special electric current. Usually some translating device, such as a motor-generator or a mercury-arc rectifier, is employed to transform the commercial current into the specialized current required for the immediate uses of the exchange.

_Charging from Direct-Current Mains._ In some cases commercial direct current is used to charge the storage batteries without the intervention of the translating devices, resistances being used in series with the battery to regulate the amount of current. Commercial direct current usually is available at pressures from 110 volts and upward, while telephone power plants contain storage batteries rarely of pressures higher than 50 volts. To charge a 50-volt storage battery direct from 110-volt mains results in the loss of about half the energy purchased, this lost energy being set free in the form of heat generated in the resistance devices. Notwithstanding this, it is sometimes economical to charge directly from the commercial direct-current power mains, but only in small offices where the total amount of current consumed is not large and where the greatest simplicity in equipment is desirable. It is better, however, in nearly all cases, to convert the purchased power from the received voltage to the required voltage by some form of translating device, such as a rotary converter or a mercury-arc rectifier.

_Rotary Converters._ Broadly speaking, a rotary converter consists of a motor adapted to the voltage and kind of current received, mechanically coupled to a generator adapted to produce current of the required kind and voltage. The harmonic ringing machine shown in Fig. 413 is an example of this, this particular one being adapted to receive direct current at ordinary commercial pressure and to deliver four different alternating currents of suitable pressures and frequencies. It is to be understood, however, that the conversion may be from direct current to direct current, from alternating to direct, or from direct to alternating. Such a device where the motor is a separate and distinct machine from the generator or generators is called a _motor-generator_.

It is usual to connect the motors and the generators together directly by a coupling having some flexibility, as shown in Fig. 413, so as to prevent undue friction in the bearings.

[Ill.u.s.tration: THE POWER AND WIRE CHIEF'S ROOM OF THE EXCHANGE AT WEBB CITY, MISSOURI]

As an alternative to the converting device made up of a motor coupled to a generator, both motor and generator windings may be combined on the same core and rotate within the same field. Such a rotary converter has been called a _dynamotor_. As a rule the dynamotor is only suitable for small power-plant work. It has the following objectionable features: (_a_) It is difficult to regulate its output, since the same field serves for both the motor and the dynamo windings. For this reason its main use is as a ringing machine where the regulation of the output is not an important factor. (_b_) Furthermore, the fact that the motor and dynamo armature windings are on the same core makes it difficult to guard against breakdowns of the insulation between the two windings, especially when the driving current is of high voltage.

_Charging Dynamos._ The dynamo for charging the storage battery is, of course, a direct-current machine and may be a part of a motor generator or it may derive its power from some other than an electric motor, such as a gas or steam engine. It should be able to develop a voltage slightly above that of the voltage of the storage battery when at its maximum charge, so as always to be able to deliver current to the charging battery regardless of the state of charge. A 30-volt generator, for example, can charge eleven cells in series economically; a 60-volt generator can charge twenty-five cells in series economically.

Battery-charging generators are controlled as to their output by varying a resistance in series with their fields. Such machines are usually shunt-wound. Sometimes they are compound-wound, but compounding is less important in telephone generators than in some other uses. A feature of great importance in the design of charging generators is smoothness of current. If it were possible to design generators to produce absolutely even or smooth current, the storage battery would not be such an essential feature to common-battery exchanges, because then the generator might deliver its current directly to the bus bars of the office without any storage-battery connection and without causing noise on the lines. Such generators have been built in small units. Even if these smooth current generators were commercially developed to a degree to produce absolutely no noise on the lines, the storage battery would still be used, since its action as a reservoir for electrical energy is important. It not only dispenses with the necessity of running the generators continuously, but it also affords a safeguard against breakdowns which is one of its important uses.

The ability to carry the load of a central office directly on the charging generator without the use of a storage battery is of no importance except in an emergency which takes the storage battery wholly out of service. Since the beginning of common-battery working such emergencies have happened a negligible number of times. Far more communities have lacked telephone service because of accidents beyond human control than because of storage-battery failures.

In power plants serving large offices, the demand upon the storage battery is great enough to require large plate areas in each cell. The internal resistance, therefore, is small and considerable fluctuations may exist in the charging current without their being heard in the talking circuits. The amount of noise to be heard depends also on the type of charging generator. Increasing the number of armature coils and commutator segments increases the smoothness of the charging current.

The shape of the generator pole pieces is also a factor in securing such smoothness.

If, with a given machine and storage battery, the talking circuits are disturbed by the charging current, relief may be obtained by inserting a large impedance in the charging circuit. This impedance requires to be of low resistance, because whatever heat is developed in it is lost energy. This means that the best conditions exist when the resistance is low and the inductance large. These conditions are satisfied by using in the impedance coil many turns of large wire and an ample iron core.

Dynamotors are not generally suitable for charging purposes. Not only is the difficulty in regulating their output a disadvantage, but the fact that the primary and secondary windings are so closely a.s.sociated on the armature core makes them carry into the charging current, not only the commutator noises of the generator end, but of the motor end as well.

_Mercury-Arc Rectifiers._ In common-battery offices serving a few hundred lines, and where the commercial supply is alternating current, it is good practice to transform it into direct-battery charging current by means of a mercury-arc rectifier. It is a device broadly similar to the mercury-arc lamp produced by Peter Cooper Hewitt. It contains no moving parts and operates at high efficiency without introducing noises into the telephone lines. It requires little care and has good length of life.