wise to use. For, if the magnet is nearly saturated, very little change could be made in the field. Hence, in that case, the gain in making H large would be balanced by a consequent diminution of h for a given current. But this limit is not reached with ordinary steel magnets, hence their value in the instrument.

640. Edison's Carbon Transmitters.-The first carbon transmitter constructed by Edison in 1877 passed through various stages, and afterwards received the form shown in Fig. 106.

When using for telephony a carbon transmitter, a battery is needed to generate the current, and it is the

M

T

FIG. 106.-EDISON'S CARBON TRANSMITTER.

function of the transmitter, by the variation of its own resistance, to vary the current thus independently produced. How it does this may easily be seen from Fig. 106. The sound-waves pass through the mouthpiece M, fall upon the vibrating plate D, and set it in motion. This vibratory motion acts upon the rounded ivory button B and the adjoining platinum plate, which thus makes a variable. contact with the disc of carbon C. The electrical circuit through the instrument is from the terminal T through the spring S to the platinum plate, thence through the carbon and the case of the instrument to the second terminal T'. Hence a downward motion of the diaphragm improves the contact with the carbon, lessens the electrical

resistance, and therefore increases the current. An upward motion makes the carbon contact worse, increases its resistance, and hence decreases the current.

641. Suppose now that for sounds of a given intensity the change of resistance in the transmitter is one per cent of the total resistance in the circuit. Then a one per cent change in the current is produced and the telephone receiver affected accordingly. Next, let a longer line be used so

that the total resistance in the circuit becomes ten times its former value. Then if the same battery is used, and sounds of the same intensity fall upon the diaphragm of the transmitter, we have two changes to notice. First, the current is only one-tenth of its original value; and, second, the changes which the transmitter can produce are only about one-tenth per cent of that reduced current, because the changes in resistance of the carbon contact remain of the same absolute value as at first, while the total resistance is made tenfold. Thus to maintain the changes in current at the receiver the same when the total resistance in circuit is made tenfold, we need to increase the voltage of the battery to one-hundredfold. Or, to generalise: when the line changes in length in order to maintain unimpaired the efficiency of the circuit, we should need to make the voltage of the battery vary as the square of that length. And this would be quite out of the question in practice on a large scale.

642. To obviate this necessity Edison passed the current from the transmitter through the primary of an induction coil, whose secondary was connected to the line. The transmitter then acts in connection with a small resistance due to the battery, the transmitter itself, and the primary wire of the induction coil. Hence the variations of resistance of the transmitter have a considerable relative magnitude, and produce correspondingly large relative changes in the current through the primary of the induction coil. And these changes, by induction, give rise to corresponding

periodic currents in the secondary of the induction coil, their voltage (or E. M. F.) being very high. Hence the line may be fairly long without prejudice to the action of the circuit.

643. Hughes' Microphone.-In 1878 Professor Hughes introduced what he called the microphone, which is really a form of telephone transmitter. Its action depended on a loose contact whose resistance varied with the sounds incident upon it. The changes in resistance thus produced affected the currents derived from a battery whose circuit included both the microphone and a telephone receiver. One of the earliest forms of the apparatus consisted of two

nails laid side by side, but not in contact, across which a third nail was laid. The effect is, however, better when carbon pencils are used, and the apparatus employing carbon may be regarded as the standard type of the microphone, and is with modifications still retained. One form of it consists of a small pencil of gas carbon A, Fig. 107, with pointed ends resting lightly in small circular holes in the two pieces of carbon B, C, so that the pencil takes up a vertical position between them. The pieces B and C are fixed to a thin sounding board or diaphragm D, fitted into a frame and mounted on a solid base F F. A battery E and a telephone receiver R are included in the circuit with the pieces B and C. The instrument, though apparently so

rough, is of surprising delicacy, the movement of a fly on the diaphragm serving to produce in the receiver audible effects.

644. The true nature of the action at the loose contact is perhaps not yet fully understood, though of late years much research has been done upon the contacts in the case of the coherers used in wireless telegraphy. Possibly the phenomena in the two cases are somewhat analogous.

For the loose contacts of the microphone Shelford Bidwell considered carbon is the best material, because it is unoxidisable and infusible, a poor conductor, and has a lower resistance when heated.

The difference between Edison's carbon transmitter and Hughes' microphone does not seem great, but the Edison form has disappeared, and the carbon transmitters now generally in use may be regarded as modifications of Hughes' microphone.

.

The details of these various instruments and all the intricacies of current practice are outside the scope of this work. The reader interested in them should consult one of the technical treatises, such as the Manual of Telephony by Preece and Stubbs.

645. Trunks and Transformers.-We may, however, with advantage mention here the following devices used for long-distance telephony:-For transmission to any considerable distance, free from the disturbances due to induction, the double wire or metallic circuit is imperative. It is therefore invariably used for the so-called trunk lines connecting towns at any great distance apart. But suppose a subscriber at one end is on a single wire circuit only, and desires to communicate with the distant town reached by the metallic circuit of the trunk-line. To admit of this, transformers or translators are used at the ends of the trunk or double line between the two central stations. These translators are induction coils of special construction. The National Telephone Co. have used coils of 290 ohms

resistance for the trunk, and 140 ohms for the local sections. The core is of the softest iron, and the coils wound closely and regularly. In the most approved forms the cores are more than double the length of the coils, and after the completion of the winding, the projecting ends of the iron wires constituting the core are folded back over the windings of the coil. Since the use of a translator involves some loss of effect it is desirable to avoid having more than two in use at any one time in any given speaking circuit. Indeed, in some districts it has been the rule with the National Telephone Co., that of the two subscribers using a trunk wire one must have a metallic circuit, SO that only one translator is in use.

646. Vibrations of a Telephone-Membrane.-In 1902 R. Kempf-Hartmann1 experimented on the vibrations of a telephone-membrane, obtaining many interesting results. Vibration curves were photographically produced by the light reflected to a moving film from a mirror fixed on the membrane of a telephone receiver. It was thus shown that the membrane very quickly takes up the vibrations impressed upon it, one-thousandth of a second in some cases sufficing for the steady state to be reached. Curves are given in the original paper for vowels and consonants at different pitches and intensities. The effect of the frequency natural to the membrane is also shown in a special set of

curves.

647. Rhythm Electrically perceived.-Prof. M'Kendrick, pursuing the researches mentioned before (Arts. 626629), arranged the following combination of apparatus :To the glass plate diaphragm in the reproducer of the phonograph was attached a tube connecting it with a microphone transmitter. This transmitter was connected in series with a battery and the primary of an induction coil. The wires from the secondary of this induction coil

1 Ann. d. Physik. viii. 3. pp. 481-538, June 1902. 1903.

Science Abstracts, p. 27,