Page:The American Cyclopædia (1879) Volume XV.djvu/644

 616 TELEGRAPH magneto-electricity, thus dispensing with vol- taic batteries, the use of which involves much care and expense. The apparatus is remarka- bly compact, without clockwork or complicated movements such as are common in other tele- graphs. Though used double, with two sets of magnets, with a wire from each connecting with two needles upon the dial at the oppo- site station, the whole apparatus, including the tablet or dial, occupies but a few inches of space, and is always ready for instant use, however long it may have remained inactive. The magnets, of horse-shoe form, about 12 in number for each set, are 15 in. long and 1 in. broad. They are laid one upon another in two piles near together, and fastened down to the table by screws. Opposite the ends of each pile, placed upon a rotating axis, is the soft iron armature, consisting of two cylinders wound around with long coils of fine copper wire covered with cotton. The wire of the two coils is connected together, and one end of each passes in a spiral through the axle to the platform upon which the apparatus rests. One end is thence carried into the earth, and the other goes to the electro-magnet of its own dial, thence to the distant station, and through the instrument there into the earth. The same arrangement is repeated with the other set. The axis of each armature extends toward the operator, and is provided with a crank handle by which each is turned to gen- erate the electric current. The effect is seen in the movement of the two needles placed upon the dial over the magnets. It is asserted that this telegraph is worked with the greatest economy, that it cannot be disturbed by electric storms in the atmosphere, and that its average celerity has been found to be 27 words a min- ute, with a maximum of 37. In the United States the dial telegraph is largely used where operators are supposed to have but moderate skill, as in police and private telegraphy. The instruments are worked with a small battery. Primary signals are given by bells, and the let- ters are pointed out by the revolving needle. The transmitting part is the usual circuit wheel, which breaks and closes the circuit and pro- duces the rotating movement of the needle of the distant instrument. This circuit wheel is arrested, in the process of telegraphing, by a series of pins, one of which is placed opposite each letter. When the A pin of the trans- mitter is pressed down, the circuit wheel is arrested just as it has caused the needle of the other instrument to rotate to A. Con- struction of Telegraph Lines. Telegraph wires are usually carried over the surface of the country upon poles standing from 25 to 30 ft. above the ground, and placed from 80 to 100 yards apart. As poles are objectionable in cit- ies, many plans have been devised for carrying the wires under ground. In London they are covered with gutta percha and tape and put into lead or iron pipes, which are laid under the sidewalk, or into creosoted wooden troughs filled with bitumen, which are buried in trench- es beside the roadway. In Paris the wires are carried in lead pipes through the sewers and catacombs. The "American Compound "Wire Company " have introduced a wire, consisting of a core of steel and envelope of copper, with a tinned surface, which, with equal conduc- tivity and greater strength, weighs less and requires fewer supports. Another insulated wire, called "kerite wire," the invention of Mr. A. G. Day of New York, has a covering compounded of rubber and hydrocarbons. It is said to offer great resistance to oxidation, and that it may be exposed in the air or buried in the earth for years without serious injury. As, with batteries of the same intensity, the conductivity increases with the cross section of the wire, large wires are to be preferred to small ones upon long circuits. In work- ing direct, a distance of over 400 or 500 m., the line is usually divided at some intermedi- ate point into two distinct circuits, which are connected by means of a "repeater." If the circuit be broken on either side of the re- peater, it will break the circuit on the other side also. The combined circuits can thus be operated from either end as if they were one continuous wire, while the current of each battery has to pass only half the distance be- tween the terminal stations. A line can thus be extended indefinitely. Copper wire is a much better conductor than one of iron of the same size, and will carry the current from five to six times as far ; but want of strength, and frequent breakage from its greater ex- pansion and contraction by the changes of temperature, prevent its use except on impor- tant submarine lines. The insulation of the wires upon the posts is a matter of much im- portance, and is not easily effected, for any non-conductor interposed between the wire and the post becomes a conductor when its surface is wet with rain. Glass knobs with grooves around them for securing the wire have been made in a great variety of forms, and secured to the posts, or to the cross bars where there are several wires, by pins of wood or iron. A great improvement upon this is a glass cap exactly fitting over a wooden pin 1J in. in diameter, and having an outer covering of wood, saturated like the pin with coal tar and pitch, to which the wire is fastened, and which, projecting below and entirely covering the glass, keeps it dry and makes the insula- tion complete. Batchelder's vulcanite insu- lators have been very extensively applied in the United States. In Europe, insulators of earthenware and porcelain are used. In for- ests the wires should be allowed to pass loosely through the supports, so that in case of a tree falling upon them they need not be broken; but in an open country they are usually fast- ened to each post. On some telegraph lines in Europe and in Asia, the wires, instead of being supported upon poles, are buried beneath the ground. Their first cost is always heavy,