complete a connection between the line of any of the group of subscribers directly served by her and the line of any other subscriber connected to the exchange in one operation. This method does away with the transfer of calls from one switchboard to another, and makes the work of establishing connections much simpler and quicker than it was with the original divided switchboards.

In a multiple switchboard each section is, as a rule, somewhat less than 6 feet long, and contains three operators' positions. In each section there is a connecting spring jack for every line in the exchange. Therefore, as regards the vertical part of the switchboard, each section is a duplicate or multiple of every other; hence the name of multiple

FIG. 24.

SERIES MULTIPLE SWITCHBOARD.

switchboard. In the earliest forms of multiple switchboard, only one set of jacks was pro vided at each section, but it was quickly found that it was a considerable strain on the operator to trace the relation between an indicator and any one of a thousand or more spring jacks. This arrangement was therefore modified by the introduction at each section of a certain number of special spring jacks, which form the terminals of the lines to be served at that section. These were called "answering jacks," as they were used exclusively for answering the calls of subscribers. There being at each section, or at each operator's position, a small group of signals and a small group of answering jacks, it was comparatively a simple matter for the operator to trace the relation, by means of the corresponding numbers, between any signal and any answering jack.

The horizontal portion of the switchboard, generally termed the key-board, serves as a support for the plugs and cords by means of

which the operator effects temporary connection between the lines, and for the keys and switches by means of which the operator connects her own telephone set to the line of any subscriber as may be required, and connects the ringing machine to a line on which it is desired to signal for the attention of a subscriber. In modern types of switchboard some of the signals are also placed in the key-board.

At first the multiple switchboard was thought to be too complicated, and it was some little time before it was put into practical use on any scale. A very short experience, however, showed it to be so superior in ease of manipulation, in accuracy, and in speed, that within a very few years the multiple switchboard displaced all types of transfer and divided switchboards. Of late years there has been a tendency to return to a divided type of switchboard in which the calls of subscribers are answered at one switchboard and completed at another; but, as will be described later, these special switchboards are used to cope with special conditions which a close study of the telephone traffic of large cities reveals, or else are due to theories which do not always turn out well in practice.

In tracing the evolution of the multiple switchboard, we find that it has gone through numerous modifications, arising from improvements both in mechanical and electrical conditions. In the early days of the multiple switchboard a switchboard for a thousand lines was considered a large piece of work. For many years a switchboard for 5.000 lines was the largest in existence. At that time, owing to the size of the spring jacks and the relatively small number of lines which could be worked under the prevailing methods of operation at cne section, such a switchboard consisted of some 44 sections. With improvements in manufacture, and with improvements in methods of signalling and methods of operating, it has become possible to work many more lines from one operator's position and to put many more spring jacks in one section. Consequently, switchboards for 10,000 lines are now made of practically the same length as a switchboard for 5,000 lines would have occupied a few years ago. It is practicable, indeed, to build multiple switchboards for even much larger numbers of lines than 10,000, and there are several multiple switchboards in existence or building designed to accommodate from 15,000 to 18,000 lines.

As described above, the separation of the two

springs of the spring jack when the plug was inserted, was utilised in the early switchboards for the temporary disconnection of the indicator from the line during the time that a subscriber's line was placed in connection with another. In the early telephone exchanges all the lines were single wire and the circuit through the switchboard was also a single wire circuit, the spring jacks being inserted in the line in series, the line going into one jack, so to speak, by one spring and coming out by the other, the pressure between the two springs maintaining a contact which was broken by the insertion of the plug. This gave a ready means of disconnecting the line signal whenever the line was in use, and of automatically restoring the signal to connection with the line when the plug was withdrawn. A few years' experience showed that it was necessary for good service to make all the lines of a city telephone system metallic circuit, as it was found that single wire circuits connected to earth picked up so many foreign currents by leakage and induction that communication was often impossible. Making the lines metallic circuit involved making the switchboard circuits also metallic. In the earliest metallic circuit switchboards the arrangement of the jacks in series was retained, but it was found that in large switchboards the series jack gave rise to many difficulties. With a large number of jacks in series on one line-and, as has been shown, there might be in a large switchboard as many as 45 jacks in each line-a sufficient number of bad contacts, due either to the presence of dust or to separation of the springs of the jacks, might arise to cause bad service. Also, in a large switchboard there would often be an unbalanced loop of wire running through the switchboard and back to the jack at which the connection was effected, which would be sufficient to throw the circuit out of balance and cause cross-talk or inductive action between one circuit and another.

In the single-wire switchboard the clearingout indicator, which was temporarily associated with two lines in use by means of the connecting cord, was legged on to earth. In the metallic circuit switchboard it became necessary to bridge the clearing-out indicator between the two strands of the connecting cord, using an indicator coil of high resistance and impedance so as not to shunt the telephonic currents. The use of the clearing-out drop bridged across the line led naturally to the idea of bridging the line signal across the

self-restoring, thus saving the operator the work of restoring the indicator shutter every time that a signal was given by a subscriber. The self-restoring indicator marked a distinct advance in telephone switchboard operating, and the bridging method of connecting the indicators to the circuits marked a distinct advance in the electrical design of the telephone switchboard.

In the bridging switchboard the circuit throughout the board is a balanced metallic circuit, free from series contacts. Each line is joined to one spring of each spring jack, and different parts of the connecting plug, when inserted in the spring jack, make contact with each of these springs. The line signal is, as has already been said, bridged permanently across the line, and the indicator shutter is controlled by a restoring magnet inserted in a third wire which is local to the switchboard.

This wire is connected to a source of current and to contacts in the spring jack, so arranged that when the plug is inserted in answer to a call the circuit is completed and the restoring magnet energised, thus restoring the indicator shutter to its normal position. The clearing out indicator is similarly made self-restoring, the restoring magnet in this case being operated by a depression of the listening key.

The self-restoring switchboard indicator, which was brought out in the early nineties, was largely copied by means of mechanically restored indicators, in which the indicator shutter was replaced to its normal position by the insertion of the plug in the spring jack or by the replacement of the plug on the cord shelf. It is of course easy to make either of these operations perform a second operation at the same time, by means of causing the plug to actuate a lever or cam on entering the jack or by causing it to trip a lever connected to a shutter when it drops back into its place on the cord shelf. These mechanical restoring devices, however, involve the close association of the indicator and the spring jack or of the indicator and the plug socket on the cord shelf, methods of construction which other requirements of a telephone switchboard make it not always advantageous to adopt. Consequently, these mechanically restoring devices are seldom used except on switchboards of small size.

We now come to the era of lamp signals and common battery working. From the very earliest days of telephone exchange working it has been the object of telephone engineers to do away with the use of individual batteries at the subscribers' stations. The cost of the upkeep of these batteries is very great, and their unequal performance renders it almost impossible, except at enormous expense, to maintain in a large system a high average efficiency of service. There have been many methods and devices tried with the object of working all the telephones of an exchange from one central battery, and there have been various methods tried, from the very earliest days of telephone exchange working, of making the signals required at the beginning and end of a conversation automatic, that is, making the natural acts of taking the telephone off the hook and replacing it control the necessary signals at the exchange. The storage battery at first displaced the collections of primary batteries originally used for energising the operators' transmitters at the exchange, and later was used at subscribers' stations instead

With

The first appearance of a power plant in a telephone exchange (Fig. 27, p. 1080) was in connection with the bridging switchboard just described. In the original switchboards there was placed a hand magneto generator at each operator's position, to enable the operators to ring the bells at subscribers' stations. These individual generators were very quickly replaced by a machine-driven generator, from which a supply circuit was carried along the switchboard and made available at each operator's position by means of ringing keys. This was the beginning of the telephone exchange power plant. To operate the signals and circuits of the bridging switchboard a more elaborate power plant was required, comprising storage cells with a generator to charge them, and about this time there was evolved the motor generator for supplying ringing current. In the common battery system the power plant reaches much larger proportions, as it is called upon to do a much greater variety of work than its predecessor. It replaces the batteries and magneto generators at all the telephones of the system, as current is taken from the one central plant to operate all the telephone transmittors in the system as well as all the signals in the switchboard. The source of current supply is a battery of eleven large storage cells which are charged periodically by a motor-driven

generator of 30 kilowatts capacity. The ringing machines, also motor driven, have been greatly improved and elaborated, and supply not only alternating current for ringing purposes, but also current of special characteristics for giving the distinctive signals required in the varied operations of a large telephone service. For example, an operator at a distant exchange informs the operator at an originating exchange that a line wanted in the distant exchange is " engaged" not by word of mouth, as was formerly the practice, but by means of an electrical signal which gives a distinctive hum or buz on the wire. In the same way other forms of current, giving distinctive pulsations or vibrations, are used. for other special signals between operator an operator.

FIG. 27.

TERMINAL ROOM OF MODERN EXCHANGE, SHOWING POWER PLANT.

In the common battery switchboard, practically all of the signals employed are lamps, instead of indicators, and this gives the face of the switchboard an entirely different appearance from that of a switchboard of the older type, in which indicators were used as signals. The lamp has so many advantages over the electromagnetic indicator as a telephone switchboard signal that, although the early lamps and the relays for controlling them gave rise to much trouble, continuous efforts were made to reduce them to a practical and reliable state, with such success that within a very few years thoroughly efficient telephone relays and lamps were produced. This says a good deal for the telephone manufacturers, as the telephone lamp is necessarily extremely

small, and works under much more difficult conditions than the ordinary electric lamp, while the telephone relay to be really useful must work with certainty, often on a comparatively small margin of current, and often a very large number of times each day. That the difficulties in the production of reliable lamp signals and relays have been completely overcome is clear from the fact that telephone lamps and relays are now in use by the million, and that the telephone service of practically every city in America, and of many large cities in Europe, is conducted entirely by means of relays and lamps.

The lamp signal is not only more effective as a signal than the indicator, but it has the great advantage of being automatic or selfeffacing. It is also extremely compact, and as it has no working parts it may be placed in any position on the switchboard, vertically, horizontally, or at an angle; an indicator could naturally be placed only in one position. As a result of these qualities of the lamp signal, we see that it has been possible to effect many improvements in the telephone switchboard, which make greatly for its efficiency and economy. In the original series switchboard it was necessary to place the signals within reach of the operator, as the operator had to restore the indicator shutters by hand. In large switchboards this involved the use of a trough behind the keyboard (in which the indicators were placed), to permit the top rows of the multiple jacks to be within the reach of the operators. In the bridging switchboard, as the indicators were self-restoring, they were placed at the top of the switchboard, thus effecting a considerable gain in space in the face of the switchboard available for multiple jacks. But in both these arrangements it was necessary for the operator to trace the relation between any given indicator and its corresponding spring jack or cord, and this involved a certain mental effort. The compactness and simplicity of the lamp signal enabled it to be placed immediately adjacent to the particular spring jack or cord to which it is related. Thus, in the common battery switchboard we find each signal immediately above or below its corresponding answering jack, and each cord signal immediately alongside its corresponding cord. This arrangement makes greatly not only for the compactness of the switchboard but for accuracy in operating, as there is never any doubt as to the meaning of a signal or the particular part of the switchboard which has to be manipulated in response