Page:The American Cyclopædia (1879) Volume XV.djvu/647

 TELEGRAPH 619 in Ireland, and Rye, New Hampshire, by the way of Nova Scotia. In spite of many obsta- cles and delays, the cable was put in position between Rye Beach and Torbay, N. S., and be- tween Torbay and Newfoundland, also between Cahirciveen island and a point 200 m. E. of Newfoundland, before rough weather put an end to the work. The final splice of 200 m. was made early in the summer of 1875. In 1873 a cable was laid between Lisbon and Madeira; in 1874 Madeira was connected by cable with St. Vincent, one of the Cape Verd islands (1,200 m.), and St. Vincent with Per- nambuco (1,845 m). In 1875 cables were laid between Jamaica and Porto Rico, Constanti- nople and Odessa, Zante and Otranto, and Bar- celona and Marseilles. In all, more than 200 cables have been laid, with a length of about 50,000 m. The interval of time which must elapse between the sending of successive sig- nals through similar cables increases as the square of their lengths; and in different ca- bles of equal length, this time is the least when the thickness of the insulating coating is one third of the diameter of the compound conductor. With the improved transmitting apparatus of Thomson and Varley, eight words can be sent in the time otherwise required for one. Seventeen words a minute have been sent through the French Atlantic cable. Thomson's syphon recorder quadrupled the speed of cable telegraphy. The current from the cable passes into a coil of wire suspend- ed between the poles of magnets. The coil turns round in a direction depending upon the direction of the current. The motion of the coil is communicated by means of a thread and lever to a glass syphon which feeds it- self with ink from a basin. The ink is elec- trified and spurts out against a moving strip of paper, and draws an undulating curve which ' indicates the letters of the message. The speed of working with this recorder is about the same as with the reflecting galvanometer ; and in either case it is much greater than could be attained by the moving armature, which re- quires that the current should rise and fall by large differences ; and this would take more time. Telegraphic Disturbance. The offices and operators of air lines of telegraph are ex- posed to accidents from lightning, either from the direct stroke or the induced electricity when a discharge occurs between two clouds. A great many lightning guards have been de- vised. Sabine mentions eleven. In lines which follow the undulations of mountainous regions (as between Vienna and Milan), there is so great disturbance from atmospheric currents, even under a blue sky, that it is impossible to send messages at certain hours. The aurora sometimes acts powerfully upon the wires, in- terfering with the battery currents. On such occasions, if the battery be taken off, the mes- sages may be sent by means of the current (induced by the aurora. The action of cable lines is disturbed by earth currents. Gen- erally, the difference of electric potential be- tween different parts of the earth is small ; but it is subject to sudden and capricious changes, and amounts sometimes to that of a battery of 140 of the Daniell elements. The direction of these earth currents is such as to derange particularly the Atlantic lines. The instru- ments are protected by the use of the con- densers of Varley and others. These earth currents must not be confounded with those excited when plates of zinc and copper are buried in the earth, which Kemp, Fox, and Reich made the subject of numerous experi- ments, and which Bain, Palagi, and others put into the harness to work the telegraph. Vari- ous Uses of the Telegraph. The electric tele- graph has been applied to uses never contem- plated by its originators. In 1852 Channing and Farmer of Boston devised a system of telegraphic fire alarms, which was adopted in the city of Boston. Five so-called signal cir- cuits were extended from the city hall to dif- ferent parts of the city, and in connection with these were stationed 50 signal boxes attached to buildings at convenient points. The door of a box being opened, a crank is seen with directions for the number of times it is to be turned to convey to the central office the num- ber of the station and district. From the central station five wires called alarm circuits connect with the different fire bells throughout the city, the hammers of which, run by weights, are set in action by the telegraph itself and strike the number of the district and station of the alarm. The electric current is excited by a magneto-electric machine which is set in mo- tion by the pressure of the water with which the city is supplied, and the same power is employed to wind up the weights that move the bell hammers. The bells have been rung, as an experiment, from Portland through the telegraph wires extending to that place. The fire alarm also affords an incidental protection to the city from lightning. Large metallic surfaces being placed near the wires at all the stations and connected with the ground, a stroke of lightning upon the wires will leap across to these conductors, and pass harmlessly to the ground, while the artificial current pos- sesses too little intensity ever to overcome the intervening space, and continues in the circuit. Similar arrangements are provided upon many telegraph lines. The telegraphic fire alarm has now been introduced into all the larger cities. The fire alarm telegraph of Boston is employed to designate the exact noon by a single stroke upon the bell of the Old South church, an ex- act chronometer being placed in the circuit and arranged so as to pass the current at 12 o'clock precisely. By a similar arrangement in London a large ball is made to drop exactly at 12 o'clock from a pole erected in the Strand by the action of a current from the royal ob- servatory. The same thing is also done at Nelson's monument, Edinburgh. In Paris a cannon is fired upon a similar plan. Chro-