Augustin Privat-Deschanel

“ The electro-magnet of the relay attracts an armature, the contact of which with the magnet completes the circuit of the local battery, in which circuit the coil belonging to the indicator is included. The armature of the indicator is thus compelled to follow the movements of the armature of the relay.
Relays are used when the currents which arrive are too much enfeebled to give clear indications by direct action. They are also frequently introduced at intermediate points in long lines which could not otherwise be worked through from end to end. ”

“ Morse's Apparatus, with Relay.
called a relay; that is to say, an apparatus which, on receiving a feeble current from a distance, sends on a much stronger current from a battery on the spot. The key (B) being up, a current arriving by the line-wire passes through the key from c to a, thence through another wire to the coil of the electro-magnet belonging to the relay, and through this coil to earth. ”

“ There is permanent connection between a and the negative pole of the battery through the spring s, and between b and the positive pole through the spring s' . In the position represented in the figure, a serves to connect the negative pole of the battery with the earth, and b serves to connect the positive pole with the spring t', down which the current passes from the point of contact of the pin, and then through the coil to the line-wire at L. The needle of the sending station is thus deflected to the same side as that of the receiving station. ”

“ The wheel, which is urged by a clock-movement, thus advances 1⁄26 of a revolution; and the hand on the dial, being attached to the wheel, moves forward one letter. When the current ceases, the pallet moves back to the remote side, and the hand is advanced another letter. If the hand is initially at the cross, it will be advanced to any required letter by so arranging matters that the number of currents plus the number of interruptions shall be equal to the number denoting the place of the letter in the alphabet. ”

“ The needle instrument is also, to a great extent, being superseded by Morse's instrument.
Telegraphs in which the ordinary letters of the alphabet are ranged round the circumference of a dial, and are pointed at by a revolving hand, are specially convenient for those who are not professional telegraphists. They are constructed on the principle of step-by-step motion, the hand being advanced by successive steps, each representing one current sent or stopped. ”

“ There are 26 notches cut in the edge of the dial, in which a pin attached to the movable arm catches; and the arm is allowed sufficient play to and from the face of the dial to admit of this pin being easily released or inserted. When the pin is in one of the notches, the instrument is in position for transmitting the corresponding letter. The action is as follows: A toothed or rather undulated wheel is fixed on the same axis as the revolving arm, and turns with it. ”

“ There is less leakage of electricity from subterranean than from air lines, but their cost is greater, and they are less suited for rapid signalling on account of the retardation caused by the inductive action between the wire and the conducting earth, which is similar to that between the two coatings of a Leyden jar. ”

“ It is not necessary, however, that it should be strong enough to produce an actual reversal, but merely that it should weaken the induced magnetism of the cores sufficiently to enable the opposing spring to overpower them. ”

“ The wires for land-telegraphs are commonly of what is called galvanized iron, that is, iron coated with zinc, supported on posts by means of glass or porcelain insulators, so contrived that some part of the porcelain surface is sheltered from rain, and insulates the wires from the posts, even in wet weather. ”

“ The copper cell is filled with a saturated solution of sulphate of copper, and crystals of this salt are placed at the bottom. The expenditure in sulphate of copper is about a pound and a half for each cell per annum. ”

“ Instead of a series of parallel lines, the styles may be made to trace the successive convolutions of a fine helix, the two sheets being bent round two cylinders, which revolve in equal times, and also advance longitudinally.
↑ Abridged from Deschanel's "Natural Philosophy," Part III.: "Electricity and Magnetism." ”