The cars are suspended from pulleys running on one or the other of the conductors. A train of such cars are connected and the current is taken in near one end and leaves near the other end of the train. These current connections are so distant, their distance being regulated by the length of the train, that they are, for all but an instant at the time of pa.s.sing each of the poles, in connection with segments of the line which are of opposite potential. To carry out this principle the distance between contacts is equal to the distance between poles. Owing to the crossing over of the lines the contacts are in connection as described and thereby the actuating current is caused to go through the motors.

Cars running in one direction go on the electric conductors on the one side, those running in the other direction go on the other conductor.

A great many refinements have been introduced, but the system has been very little used.

Telephone.

An instrument for the transmission of articulate speech by the electric current. The current is defined as of the undulatory type. (See Current, Undulatory.)

The cut shows what may be termed the fundamental telephone circuit. A line wire is shown terminating in ground plates and with a telephone in circuit at each end. The latter consists of a magnet N S with a coil of insulated wire H surrounding one end. Facing the pole of the magnet is a soft iron diaphragm D, held in a frame or mouthpiece T. Any change of current in the line affects the magnetism of the magnet, causing it to attract the diaphragm more or less. The magnet and diaphragm really const.i.tute a little electric motor, the diaphragm vibrating back and forth through an exceedingly short range, for changes in the magnetic attraction.

The principle of the reversibility of the dynamo applies here. If the magnet is subjected to no change in magnetism, and if the diaphragm is moved or vibrated in front of its poles, currents will be induced in the wire bobbin which surrounds its end. If two such magnets with bobbins and diaphragms are arranged as shown, vibrations imparted to one diaphragm will send currents through the line which, affecting the magnetism of the distant magnet, will cause its diaphragm to vibrate in exact accordance with the motions of the first or motor diaphragm. In the combination one telephone represents a dynamo, the other a motor.

If the vibrations of the diaphragm are imparted by the voice, the voice with all its modulations will be reproduced by the telephone at the distant end of the line.

523 STANDARD ELECTRICAL DICTIONARY.

Fig. 324. DIAGRAM OF BELL TELEPHONES AND LINE WITH EARTH CONNECTIONS.

Fig. 325. SECTION OF BELL TELEPHONE.

The above gives the essential features of the Bell telephone. In practice the telephone is used only as the receiver. As transmitter a microphone is employed. To give the current a battery, generally of the open circuit type, is used, and the current in the line is an induced or secondary one.

The microphone which is talked to, and which is the seat of the current variations which reproduce original sound, is termed the transmitter, the telephone in which the sounds are produced at the distant end of the line is termed the receiver.

Fig. 325 shows the construction of the Bell telephone in universal use in this country as the receiver. M is a bar magnet, in a case L L. B B is a bobbin or coil of insulated wire surrounding one end of the magnet.

D is the diaphragm of soft iron plate (ferrotype metal), and E is the mouthpiece. The terminals of the coil B B connect with the binding screws C C. The wire in the coil is No. 36, A. W. G., and is wound to a resistance of about 80 ohms.

524 STANDARD ELECTRICAL DICTIONARY.

As typical transmitter the Blake instrument may be cited. It is a carbon microphone. It is shown in section in the cut; a is the mouthpiece and e is a diaphragm of iron plate, although other substances could be used; f is a steel spring, with a platinum contact piece at its end. One end bears against the diaphragm, the other against a carbon block k. The latter is carried by a bra.s.s block p, and pressure is maintained between these contacts by the spring g and weight of the piece c, which by gravity tends to press all together. The current pa.s.ses by way of the spring f, carbon b.u.t.ton k and spring g through the circuit indicated.

A battery is in circuit with these parts. If a telephone is also in circuit, and the transmitter is spoken against, the diaphragm vibrating affects the resistance of the carbon-platinum contact, without even breaking the contact, and the telephone reproduces the sound. The heavy piece of metal C acts by its inertia to prevent breaking of the contact.

The position of this piece c, which is carried by the bra.s.s plate m, is adjusted by the screw n.

Fig. 326. SECTION OF BLAKE TRANSMITTER.

In practice the transmitter and battery are usually on a local circuit, which includes the primary of an induction coil. The line and distant receiving telephone are in circuit with the secondary of the induction coil, without any battery.

Telephone, Bi-.

A pair of telephones carried at the ends of a curved bar or spring so that they fit the head of a person using them. One telephone is held against each ear without the use of the hands.

525 STANDARD ELECTRICAL DICTIONARY.

Telephone, Capillary.

A telephone utilizing electro-capillarity for the production of telephonic effects. The following describes the invention of Antoine Breguet.

The point of a gla.s.s tube, drawn out at its lower end to a capillary opening dips vertically into a vessel. This vessel is partly filled with mercury, over which is a layer of dilute sulphuric acid. The end of the immersed tube dips into the acid, but does not reach the mercury. One line contact is with mercury in the tube, the other with the mercury in the vessel. The arrangement of tube and vessel is duplicated, giving one set for each end of the line. On introducing a battery in the circuit the level of the mercury is affected by electro-capillarity. The tubes are closed by plates or diaphragms at their tops, so as to enclose a column of air. It is evident that the pressure of this air will depend upon the level of the mercury in the tube, and this depends on the electro-motive force. On speaking against the diaphragm the sound waves affect the air pressure, and consequently the level, enough to cause potential differences which reproduce the sound in the other instrument.

Fig. 327. BREGURT'S CAPILLARY TELEPHONE.

Telephone, Carbon.

A telephone transmitter based on the use of carbon as a material whose resistance is varied by the degree of pressure brought to bear upon it.

Undoubtedly the surface contact between the carbon and the other conducting material has much to do with the action. Many carbon telephones have been invented. Under Telephone the Blake transmitter is described, which is a carbon telephone transmitter. The Edison carbon transmitter is shown in section in the cut. E is the mouth piece and D the diaphragm. I is a carbon disc with adjusting screw V. A platinum plate B B, with ivory b.u.t.ton b, is attached to the upper surface of the carbon disc. C C is an insulating ring. The wire connections shown bring the disc into circuit. It is connected like a Blake transmitter. It is now but little used.

Fig. 328. SECTION OF EDISON CARBON TRANSMITTER.

526 STANDARD ELECTRICAL DICTIONARY.

Telephone, Chemical.

A telephone utilizing chemical or electrolytic action in transmitting or receiving. The electro-motograph is an example of a chemical receiver.

(See Electro-motograph.)

Telephone, Electrostatic.

A telephone utilizing electrostatic disturbances for reproduction of the voice. In the cut D and C are highly charged electrophori. The diaphragms A and B when spoken to affect the potential of the electrophorus so as to produce current variations which will reproduce the sound. Dolbear and others have invented other forms of transmitters based on electrostatic action. Receivers have also been constructed. A simple condenser may be made to reproduce sound by being connected with a powerful telephone current.

Fig. 329. DIAGRAM OF EDISON'S ELECTROSTATIC TELEPHONE.

Telephone Induction Coil.

The induction coil used in telephone circuits for inducing current on the main line. It is simply a small coil wound with two separate circuits of insulated wire. In the Edison telephone the primary coil, in circuit with the transmitter, is of No. 18 to 24 wire and of 3 to 4 ohms resistance. The secondary in circuit with the line and receiving instrument is of No. 36 wire and of 250 ohms resistance. The Bell telephone induction coil has its primary of No. 18 to 24 wire wound to a resistance of 1/2 ohm, and its secondary of No. 36 wire, and of 80 ohms resistance.

527 STANDARD ELECTRICAL DICTIONARY.

Telephone, Reaction.

A form of telephone containing two coils of insulated wire, one of which is mounted on the disc, and the other on the magnet pole in the usual way. These coils react upon each other so as to strengthen the effect.

Telephone, Thermo-electric.

A telephone transmitter including a thermo-electric battery, placed in circuit with the line. A plate of vulcanite faces it. When the sound waves strike the vulcanite they move it backward and forward. These movements, owing to the elasticity of the vulcanite, produce minute changes of temperature in it, which affecting the thermo-electric pile produce in the circuit currents, which pa.s.sing through a Bell telephone cause it to speak. This type of instrument has never been adopted in practice.