comes to the contact point by one spring and goes away by the other. A crooked back-lever b with an adjusting screw j serves to attain the proper initial pressure against the diaphragm. Another kind of transmitter, a modification of that of Hunnings' loudspeaking transmitter, is shown in Fig. 2896. The main feature of the Hunnings class of instruments is the use 6 of granulated coke carbon placed loosely so that the voice can act upon its particles while the current finds its way through the mass. The voice thus acts on all the loose contacts at once, while the numerous paths for the current permit a larger current to be used than is possible with single contacts. In the form shown in the cut the granules are placed in a kind of box with a thin bottom of metal or carbon, on which the granules rest, and which is capable of vibrating with the voice. A piece of metal or carbon presses lightly on the granules from above, and serves as the other electrode. Much stronger currents can be used with such transmitters than with those having single contacts.

Fig. 289a.

eeee

MOUTH-PIECE

Fig. 2896.

513. Telephone Circuits. The circuits required for using the transmitter and receiver, together with the call-bell and the automatic switch for throwing the battery out of circuit when not in use, are shown in diagram in Fig. 289c. The instruments here are a Blake's transmitter T and a Bell's receiver R. A single cell B1 serves as battery for the transmitter, the additional battery B2 being needed for ringing-up. [Frequently a small hand

dynamo or "magneto-ringer" is used instead for calling up.] P is the push which by connecting the line to the battery rings the bell at the other end of the line. The automatic switch is worked by the weight of the receiver, which, when not in use, is hung upon a hook at the end of the switch lever. In this position the line is disconnected from the telephonic instruments, but is in circuit with the bell. On taking down the receiver from its hook

the switch-lever falls and puts the line into circuit with the receiver and with the secondary wire of a small induction coil the primary of which is in the transmitter circuit. The object of this induction coil is to enable a single cell of battery to be used on the transmitter, the induction coil acting as a transformer to give to the currents the higher voltage required by the high resistance of the line. For a private line a similar arrangement of instruments is used at each end of the line.

For enabling a large number of subscribers to com

municate by telephone with one another, the lines from each subscriber's instrument are brought to a central office known as a telephone exchange. Here each line terminates on a switch-board which is so arranged that the operator can in an instant make a connexion from the line of any one subscriber to that of any other, so that these two can talk together. But it is impossible in a few words to give a technical description of the complicated details of a telephone switch-board. In the best arranged telephone exchanges the earth is not used as a return conductor, twin wires going to each subscriber. Only by the use of such metallic circuits can interference from stray currents and cross talk be prevented.

514. Hughes's Induction Balance. The extreme sensitiveness of Bell's receiver (Art. 510) to the feeblest currents has suggested its employment to detect currents

too weak to affect the most delicate galvanometer. The currents must be intermittent, or alternating, or they will not keep the disk of the telephone in vibration. Hughes applied this property of the telephone to an instrument named the Induction Balance (Fig. 289). A small battery B, connected with a microphone M, passes through two coils of wire P1, P2, wound on bobbins fixed on a suitable stand. Above each of these primary coils are

placed two secondary coils, S1, S2, of wire, of the same size, and of exactly equal numbers of turns of wire. The secondary coils are joined to a receiver T, and are wound in opposite directions. The result of this arrangement is that whenever a current either begins or stops flowing in the primary coils, P, induces a current in S1, and P, in S2. As S, and S, are wound in opposite ways, the two currents thus induced in the secondary wire neutralize one another, and, if they are of equal strength, balance one another so exactly that no sound is heard in the telephone. But a perfect balance cannot be obtained unless the resistances and the coefficients of mutual induction and of self-induction are alike. If a flat piece of silver or copper (such as a coin) be introduced between S, and P1, there will be less induction in S1 than in S, for part of the inductive action in P, is now spent on setting up currents in the mass of the metal (Art. 459), and a sound will again be heard in the telephone. But balance can be restored by moving S, farther away from P2, until the induction in S, is reduced to equality with S1, when the sounds in the telephone again cease. It is possible by this means to test the relative conductivity of different metals which are introduced into the coils. It is even possible to detect a counterfeit coin by the indication thus afforded of its conductivity. The induction balance has also been applied in surgery by Graham Bell to detect the presence of a bullet in a wound, for a lump of metal may disturb the induction when some inches distant from the coils.

CHAPTER XIV

ELECTRIC WAVES

LESSON LIV.— Oscillations and Waves

515. Electric Oscillations. If a charged condenser or Leyden jar is discharged slowly through a conductor of high resistance, such as a nearly dry linen thread, the charge simply dies away by a discharge which increases in strength at first, and then gradually dies away. If, however, the condenser is discharged through a coil of wire of one or more turns (the spark being taken between polished knobs to prevent premature partial discharges by winds or brushes) the effect is wholly different, for then the discharge consists of a number of excessively rapid oscillations or surgings. This is in consequence of the self-induction of the circuit, by reason of which (Art. 458) the current once set up tends to go on. The first

Fig. 290.

rush more than empties the condenser, and charges it the opposite way; then follows a reverse discharge, which also overdoes the discharge, and charges the condenser the same way as at first, and so forth. Each successive oscil

lation is feebler than the preceding, so that after a number of oscillations the discharge dies away as in Fig. 290. The