Telegraph Many different styles of sounders are in use. A well-known form is illustrated in Fig. 6. In this B, B are the binding posts; 1 represents the electromag nets; 2 the lever, pivoted on the support 4; a spring adjustable by the screw 5 gives the left end of the lever a tendency to rest on the upper stop 6. Magnetism in the cores draws the armature down, and with it, the lever, until the stop 7 meets the metal support & The base is designed to give resonant effects. When used on primary battery circuits these sounders are wound to 4 ohms; when em ployed on dynamo circuits they are often wound to 20 and 40 ohms.
Telegraph With the object of obtaining a simple and speedier method of transmitting the characters of his alphabet than the manually operated key method, Morse devised several mechanical arrange ments. One of these consisted of a plate of ebonite or wood on the surface of which were placed in a vertical row short and long strips of metal, corresponding to the letters of his alphabet. These strips were all me tallically connected under the plate by a wire which led to the battery, relay and ground. The line wire was connected by an insulated wire to a metal stylus. This stylus was held In the hand of the operator and in transmit ting a message he would run the point of the stylus over the metal strips representing the given letters, thereby closing and opening the circuit, in a manner corresponding to the manual transmission of the letter. Another somewhat similar device, also due to Morse, consisted of a metal cylinder on the surface of which the characters of the Morse alphabet were arranged in a practically similar way. A keyboard was arranged over this cylinder and the depression of a key brought a metal brush into contact with the cylinder. At the same time the cylinder was caused to make a partial revolution. The characters on the cyl inder being connected to the line, and the brush to the earth, the foregoing described actions resulted in the transmission of Morse charac ters. These transmitters did not go into wide use at that time, but within the past four or five years a keyboard transmitter, termed the oYet manl' transmitter, which is a much improved form of the Morse cylinder transmitter, has been largely adopted by telegraph operators in this country. The employment of this device was no doubt encouraged by the recent em ployment by operators of the typewriters as a means of transcribing received messages, which gave the receiving operator an advantage over the sending operator who could only send from 30 to 40 words per minute at best speed. By
means of the keyboard transmitter a speed of transmission approaching the speed of tran scription by the typewriter is obtained. This use of the typewriter was accompanied by a new arrangement of the sounder, which is now placed in a box, or resonator, mounted on an adjustable rod that brings the sounder in close proximity to the ear of the operator.
Lightning Lightning discharges tend to follow telegraph wires into cables across rivers, and into the telegraph offices, causing damage to the cables and instruments. Damage is sometimes caused also by the con tact of telegraph wires with electric light and power wires. To prevent as far as possible damage from these causes, lightning arresters and fuses are placed at points just outside of cables and at points where the wires enter offices. A combination lightning arrester and fuse wire is shown in Fig. 7. F is a small fuse wire carried on a strip of mica m which is tipped with metal at each end, and held by metal clips te n'. C represents two small blocks of carbon separated by a thin sheet of mica. and held in position by metal tension springs n n. The left-hand block is connected to ground at the post G ; the right-hand block is connected to the screw A from which a wire leads to the apparatus. The line wire is con nected to the screw L. A strong current pass ing through the fine wire F will fuse it; a lightning discharge will jump to ground by way of the carbon blocks; in either case pro tecting the cable or instruments.
Automatic Telegraph The function of a Morse automatic repeater is to take, as it were, the message from one wire and °relay' it to another wire without the in tervention of an operator. Some of the rea sons that render repeaters necessary are, first, that the resistance of the wire increases di rectly with its length, which tends to a diminu tion of current strength; second, the escape of current from the line wire at points where it makes contact with trees is greater on a wire of high resistance than one of low resistance (see RELAYS) ; third, the speed of signaling decreases as the electrostatic capacity is in creased and this capacity increases with the length of the wire. Hence it is found desirable in practice to limit the direct length of a tele graph circuit to a maximum of about 500 miles in this country.