Volume I Part 20 (1/2)

[Ill.u.s.tration: Fig. 157. Bell for Common-Battery Desk Set.]

The Kellogg Switchboard and Supply Company is one which places all the parts, except the polarized ringer and the a.s.sociated condenser, in the desk stand itself. In Fig. 156 is shown a bottom view of the desk stand with the bottom plate removed. In the upper portion of the circle of the base is shown a small condenser which is placed in the talking circuit in series with the receiver. In the right-hand portion of the circle of the base is shown a small impedance coil, which is placed in series with the transmitter but in shunt relation with the condenser and the receiver.

[Ill.u.s.tration: Fig. 158. Bell for Common-Battery Desk Set]

In Figs. 157 and 158 are shown two views of the type of bell box employed by the Kellogg Company in connection with the common-battery desk sets, this box being of pressed-steel construction and having a removable lid, as shown in Fig. 158, by which the working parts of the ringer are made readily accessible, as are also the terminals for the cord leading from the desk stand and for the wires of the line circuit. The condenser that is placed in series with the ringer is also mounted in this same box. By employing two condensers, one in the bell box large enough to transmit ringing currents and the other in the base of the desk stand large enough only to transmit voice currents, a duplication of condensers is involved, but it has the corresponding advantages of requiring only two strands to the flexible cord leading from the bell box to the desk stand proper.

[Ill.u.s.tration: Fig. 159. Microtelephone Set]

A form of desk-telephone set that is used largely abroad, but that has found very little use in this country, is shown in Fig. 159. In this the transmitter and the receiver are permanently attached together, the receiver being of the watch-case variety and so positioned relatively to the transmitter that when the receiver is held at the ear, the mouthpiece of the transmitter will be just in front of the lips of the user. In order to maintain the transmitter in a vertical position during use, this necessitates the use of a curved mouthpiece as shown. This transmitter and receiver so combined is commonly called, in this country, the _microtelephone set_, although there seems to be no logical reason for this name. The combined transmitter and receiver, instead of being supported on an ordinary form of hook switch, are supported on a forked bracket as shown, this bracket serving to operate the switch springs which are held in one position when the bracket is subjected to the weight of the microtelephone, and in the alternate position when relieved therefrom. This particular microtelephone set is the product of the L.M. Ericsson Telephone Manufacturing Company, of Buffalo, New York. The circuits of such sets do not differ materially from those of the ordinary desk telephone set.

[Ill.u.s.tration: Fig. 160. Kellogg Common-Battery Desk Set]

[Ill.u.s.tration: Fig. 161. Dean Common-Battery Set]

Circuits of Common-Battery Telephone Sets. The complete circuits of the Kellogg desk-stand arrangement are shown in Fig. 160, the desk-stand parts being shown at the left and the bell-box parts at the right. As is seen, but two conductors extend from the former to the latter. A simplified theoretical sketch is also shown in the upper right-hand corner of this figure.

The details of the common-battery telephone circuits of the Dean Electric Company are shown in Fig. 161. This involves the use of the balanced Wheatstone bridge. The only other thing about this circuit that needs description, in view of what has previously been said about it, is that the polarized bell is placed in series with a condenser so that the two sides of the circuit may be insulated from each other while the telephone is not in use, and yet permit the pa.s.sage of ringing current through the bell.

[Ill.u.s.tration: Fig. 162. Monarch Common-Battery Wall Set]

The use of the so-called direct-current receiver has brought about a great simplification in the common-battery telephone circuits of several of the manufacturing companies. By this use the transmitter and the receiver are placed in series across the line, this path being normally opened by the hook-switch contacts. The polarized bell and condenser are placed in another bridge path across the line, this path not being affected by the hook-switch contacts. All that there is to such a complete common-battery telephone set, therefore, is a receiver, transmitter, hook switch, bell, condenser, and cabinet, or other support.

The extreme simplicity of the circuits of such a set is ill.u.s.trated in Fig. 162, which shows how the Monarch Telephone Manufacturing Company connect up the various parts of their telephone set, using the direct-current receiver already described in connection with Fig. 54.

[Ill.u.s.tration: VENTILATING PLANT FOR LARGE TELEPHONE OFFICE BUILDING]

CHAPTER XV

NON-SELECTIVE PARTY-LINE SYSTEMS

A party line is a line that is for the joint use of several stations.

It is, therefore, a line that connects a central office with two or more subscribers' stations, or where no central office is involved, a line that connects three or more isolated stations with each other.

The distinguis.h.i.+ng feature of a party line, therefore, is that it serves more than two stations, counting the central office, if there is one, as a station.

Strictly speaking, the term _party_ line should be used in contradistinction to the term _private_ line. Companies operating telephone exchanges, however, frequently lease their wires to individuals for private use, with no central-office switchboard connections, and such lines are, by common usage, referred to as ”private lines.” Such lines may be used to connect two or more isolated stations. A _private_ line, in the parlance of telephone exchange working, may, therefore, be a _party_ line, as inconsistent as this may seem.

A telephone line that is connected with an exchange is an exchange line, and it is a party line if it has more than one station on it. It is an individual line or a single party line if it has but a single station on it. A line which has no central-office connection is called an ”isolated line,” and it is a party line if it has more than two stations on it.

The problem of mere speech transmission on party lines is comparatively easy, being scarcely more complex than that involved in private or single party lines. This is not true, however, of the problem of signaling the various stations. This is because the line is for the common use of all its patrons or subscribers, as they are termed, and the necessity therefore exists that the person sending a signal, whether operator or subscriber, shall be able in some way to inform a person at the desired station that the call is intended for that station. There are two general ways of accomplis.h.i.+ng this purpose.

(_1_) The first and simplest of these ways is to make no provision for ringing any one bell on the line to the exclusion of the others, and thus allow all bells to ring at once whenever any station on the line is wanted. Where this is done, in order to prevent all stations from answering, it is necessary, in some way, to convey to the desired station the information that the call is intended for that station, and to all of the other stations the information that the call is not intended for them. This is done on such lines by what is called ”code ringing,” the code consisting of various combinations of long and short rings.

(_2_) The other and more complex way is to arrange for selective ringing, so that the person sending the call may ring the bell at the station desired, allowing the bells at all the other stations to remain quiet.

[Ill.u.s.tration: Fig. 163. Grounded-Circuit Series Line]

These two general cla.s.ses of party-line systems may, therefore, be termed ”non-selective” and ”selective” systems. Non-selective party lines are largely used both on lines having connection with a central office, and through the central office the privilege of connection with other lines, and on isolated lines having no central-office connection. The greatest field of usefulness of non-selective lines is in rural districts and in connection with exchanges in serving rather spa.r.s.ely settled districts where the cost of individual lines or even lines serving but a few subscribers, is prohibitive.