tacts. The lever is furnished with a spring, which presses upon the contact a, by which, when the key is lifted up, the contact with a is interrupted an appreciable time after that with b.

This is necessary, because if they were both interrupted at the same instant it is evident that no induction current could arise in the secondary coil, its circuit being broken. By the contact a continuing an instant longer than b, however, the induction current which follows the interruption at b has time to pass over a and through the line.

The Morse telegraph has been worked by this system of induction currents to a considerable extent on the lines in Russia, Bavaria, and Hanover. Schellen says that messages have been sent direct, without translation, by this method, on a line of two hundred German miles, equal to nearly one thousand English.

Compared with the methods of working the Morse telegraph by voltaic electricity, that of induction currents offers many advantages; the line batteries are opened, and spaces between

the signals are given by reversed circuits, which always work cleaner than those given by making and breaking the same

current.

The polarised relay above described is also profitably employed on lines worked only with galvanic currents, with

which it is found to be far more delicate than the relays with springs. It is, however, necessary to give the armature o (Fig. 37) a bias on the side D', which is done by advancing

Fig. 64.

the soft-iron continuation of the pole, N', of the electro-magnet a little nearer to the armature than N, by which, when no current passes, the tongue is held against the insulated contact, and the distances may be so finely adjusted that a very weak current suffices to move it.

93. The Magneto-Induction Key.-Instead of the Morse key, induction coil, and local battery, Siemens and Halske use also an instrument arranged in the form of a key, by which a coil of wire, wound on a soft-iron armature, is oscillated between the poles of a permanent magnet, and develops alternate currents for working the polarised relay.

The magneto-induction key is shown in Fig. 64, in perspective. s and N are two rows of permanent bar-magnets; the upper ones with their north ends, and the lower ones with their south ends, in contact with a stout plate, P, of soft iron,

which acts as a neutral block for all the magnets forming the battery. Between the poles of this system, and oscillated in an angle of a few degrees by means of a handle, н, in the frame between two screw points, is the soft-iron armature, as long as the magnet system is wide, cut in deep longitudinal grooves on opposite sides, as is shown by the sectional sketch, Fig. 65. In these grooves the coil,

c, of fine insulated wire is wound. The play of the handle is limited by two adjusting screws in the frame, a. When at rest, the handle is held against the upper screw by a spiral spring, s, stretched between the handle and front of the triangular piece, D, on the top.

One end of the coil of wire on the armature is attached to the screw, k, on the terminal, K, from which one connection goes to line and another to the screw w, at the foot of the frame, A. The other end of the coil is connected with the metal frame supporting the armature, and through the axisƒ, to the upright support Q, from which a leading wire goes to terminal, t, and earth.

When a current arrives while the instrument is in circuit with the line, it goes from L over R, w, upper adjusting screw in A, through handle н, axis ƒ, q, t, earth, without traversing the coil. This is the purpose of the connection between R and w.

When the handle is pressed down, the polarity of the armature is reversed, and a positive magneto-electric current induced in the coil, which circulates also in the line wire, deflects the tongue of the polarised relay at the receiving station from the insulated point, and closes the local circuit so long as the key is held down, and no negative current induced by letting it go back to its position of rest.

94. Intermediate-Station Commutators.-Where intermediate stations occur, which are supplied each with only one Morse instrument, it becomes necessary to employ a commutator or current director to put the apparatus at pleasure in the circuit of the up or down line, in order to meet the requirements of the service. At such a station the apparatus must be so arranged as to be able to assume either of these three positions :—

(1.) When the intermediate station is entirely cut out of circuit, and the end or distant stations on opposite sides correspond directly through the line.

(2.) When two end or distant stations on opposite sides

correspond with each other, and the intermediate station receives the despatch, at the same time.

(3.) When the intermediate station wishes to communicate with a station up or down the line whilst it has notice of currents arriving from the other side.

To avoid the inconvenience of altering continually the connections to suit these various positions of the apparatus, commutators are employed. Various forms of these instruments are given by Nottebohm, Borggreve, Siemens, and others.

95. One of the completest is that of Nottebohm. It consists of six bars of metal screwed on to a wooden base, cut out in seven holes to receive contact pegs between them, so as to bring them in metallic contact with each other.