(i.) The differential method.--In the figure, E and E' represent the two Morse instruments (or relays), the electro-magnet in each being wound with two equal coils. The line from the key K branches at a; one path is through the one coil of E, through the line through the one coil of the receiving instrument E', and so to earth; the other path is through the second coil of E,

through an adjustable resistance box r, and so direct to the other terminal of the battery. The same may be said with respect to the paths from the other key K'.

In order to work this system it is necessary that the two paths from either key offer equal resistances. In this case, since there are in each instrument two equal and opposite coils, the depression of the key K alone will not affect E, but will affect E'; while the depression of K' alone will not affect E', but will affect E. In the diagram the keys are shown in their normal position. This fact enables us to test for the desired equality of resistances; r and r being adjusted until the desired end is attained.

It is to be noticed that in the action on the Morse instruments or relays we need not consider the direction of the current.

Let us now suppose that, while A is sending a message to B by depressing the key K, the key K' is also depressed. This will destroy the balance previously existing. Through the one coil of E there will now flow less current than before, or zero current, or even a reverse current; this depending upon the relative E. M. F.s of the two opposed batteries. So long, then, as K and K' are both depressed, the instrument E is worked by this difference of

currents in its two coils; and, if K be released, E will be worked directly by the current from B alone. Hence, under both conditions E will attract its armature, or release it, in obedience to the movements of the key K' at the station B. The same holds good with respect to the instrument E'.

Thus, by means of a system in which checks to the currents sent are recorded as currents received, the problem of duplex telegraphy was solved.

(ii.) The Wheatstone's bridge method.—In another method a somewhat different principle is employed. Here each instrument is placed in a ‘bridge,' and is unaffected as long as the extremities of the bridge are at the same potential. When matters are properly adjusted, this state of equilibrium at the one station A is destroyed by the depression of the key at the other station B; and this is the case whether the key at A be worked or no. duplex working is rendered possible.

Hence

TELEPHONES.

§ 11. Telephones. Introductory.-Telephones are instruments by means of which it is possible to transmit between two stations, more or less remote from each other, musical notes or other sounds, and even articulate speech; an instrument at the one end is spoken to or sung to, and the instrument at the other end gives out the words spoken or the tune sung. [The same result can be obtained for comparatively short distances by means of speaking tubes; but to such a system, which is merely a case of reflexion of sound, the word telephone is not applied.]

The only telephones much used are those in which the transmission is effected electrically. There are mechanical telephones, sometimes used, in which mechanical impulses are transmitted along the connecting wire; but in electric telephones it is an undulatory electric current that passes.

In all electric telephones we have a transmitter into which the message is spoken, and a receiver which again utters the message: these two instruments may be identical or different in form.

In some telephones the speaking of the messages causes an undulatory current to spring into existence and this again causes the receiver to utter the message. In such, there is no external

source of current needed. In other telephones there is an external source of current, and the speaking of the messages causes this previously existing current to become undulatory.

$12. The Bell Telephone.-The most complete form of telephone, for general use, is that invented by Graham Bell, called, after his name, the Bell telephone. In this instrument no external source of current is needed, and it can act either as receiver or as transmitter.

In the figure we have a section of the instrument; from this the construction can be explained. E is the mouthpiece into which we speak; M is a bar-magnet of about four inches long; B is a long coil of very fine insulated wire surrounding the pole of the magnet; C C are the terminal screws; and finally, D is a vibrating membrane made of soft iron and very thin When we speak into

the mouthpiece, the soft iron diaphragm vibrates in exact accord with the air-waves impinging upon it. This acts inductively upon the steel magnet, and alters the number of marked lines of force piercing the coil of fine long wire. This gives rise to induced currents in the coil; these will be feeble, but of high E. M.F. Hence, when we speak into the transmitting instrument, we cause undulatory currents to pass along the line wire, these undulations answering to the air vibrations, and therefore answering to the words spoken.

These currents arrive at the receiving instrument, and there pass round the coil. This alters the magnetism of the magnet's pole, and this again causes the iron diaphragm to vibrate. Finally, this iron diaphragm, vibrating in exact accord with that of the transmitter, will cause the air to vibrate, and, when the ear is held close to the instrument, the words of the message will be heard.

It is to be noticed that the vibrations of the diaphragm of the transmitter to and fro will give rise to induced currents in opposite directions respectively; and that in the receiver these currents will strengthen or weaken the magnet pole, and therefore attract further or release somewhat the diaphragm of the transmitter respectively. Hence, the to and fro vibrations of the one diaphragm will be reproduced in the other.

Sensitiveness of the telephone.—This telephone is of remarkable sensitiveness. By it we can detect currents too faint to affect any ordinary galvanometer, provided that the currents are undulatory, or at least subject to abrupt variations in strength. Hence, in many cases the telephone can be used as a very delicate galvanoscope.

If a telephone (protected by a bridge when necessary) be in a secondary circuit such as that of a Ruhmkorff's coil, the note that it emits will indicate the number of induced currents passing per second; or the number of makes-and-breaks per second in the primary circuit. It has been used in investigations regarding the stratifications that appear in vacuum tubes; and it has thrown light upon the relation that these bear to the number per second of induced currents, and also to the steadiness of the contact maker in the primary.

In the circuit of a 'Gramme,' the telephone shows plainly, by emitting a note, that the current is really undulatory, though for many purposes it acts as if continuous.

Induction disturbances on telephonic lines.-The great sensitiveness of telephones is a source of difficulty in their practical use. If the return be made through the earth, conversation cannot as a rule be carried on at all. For, what with earth currents and leakage from other circuits, the instrument emits a continuous bubbling or frying sound that drowns the faint speech.' Return wires are therefore always used. But even then there is much induction if the wires run anywhere near ordinary telegraph wires; and it requires a very special method of laying the telephone wires to reduce the Babel of sounds, due to these induced currents, to comparative silence.

Details as to the precautions taken will be found in more technical works.

$ 13. Telephones with External Source of Current.-The Bell telephone is a wonderful instrument, certainly; it acts as transmitter or receiver, and it requires no battery to work it. But it must be remembered that the currents transmitted by it are very small indeed, and that consequently the message is delivered by the receiving instrument in a very faint way.

To obviate this difficulty, telephones have been invented which make use of strong currents driven by an external source, such as a Leclanché cell.

Edison's carbon transmitter is an example of this class of telephone. This instrument has a mouthpiece and a vibrating metallic diaphragm, but no magnet or coil. At the back of the diaphragm is a button of carbon, this resting against a piece of metal. As the diaphragm vibrates the carbon will make better or worse contact with the pieces of metal touching it on the two sides; better contact when it is pressed harder, worse when it is somewhat released. Now the two pieces of metal and the carbon between them form part of the circuit of a Leclanché or other cell. Hence, as the diaphragm vibrates, the current flowing in this circuit will vary in strength, the variations corresponding exactly to the original sound vibrations. Now this current forms the primary to a small induction coil, the secondary wire of which is in circuit with the line to the distant station. No current will flow in the secondary, and therefore none through the line, so long as the primary is steady. But when some one speaks to the transmitter and thus causes variations to occur in the primary, then will there be currents induced in the secondary, in exact accord with the air vibrations of the speech uttered to the transmitter. These currents traverse the line to the distant station, and the receiver (which we may suppose, e.g., to be a Bell telephone) will reproduce the original speech.

$ 14. Microphones.-Professor Hughes found that when in a circuit there is a loose contact, or still better a group of loose contacts, the current is exceedingly sensitive to even very slight mechanical disturbances, such, e.g., as those produced by soundwaves impinging upon the loose contact or upon the stand supporting the same. Undulations are produced in the current that is passing, and these undulations are so exactly in accord with the mechanical vibrations to which they are due, that a telephone