ARTHUR H. BELL
With the apparatus described in the last issue the amateur is enabled to establish communication between two points not more than a few hundred feet apart. In beginning this chapter we will briefly outline a circuit (Fig. 6) introducing a simple induction coil for increasing the talking efficiency, but the scope of this phone is, like its predecessor, limited by the signalling equipment; in fact, with a good transmitter and coil it matters little in the talking whether parties are a few yards or a few miles apart, line conditions being all right, for the transmitter utilizes but a small current in operation. From this we may deduce that the amount of battery current necessary in signalling over long distances would not be beneficial to a transmitter calling for 2 to 4 volts and considerably less than 1 ampere, also that the introduction of resistance in the transmitter circuit would be unsatisfactory as a current cut down.
This leads us to the more modern equipment often styled the "magneto system"; a magneto being an electrical generator of the alternating type, mounted in a small wooden box and capable of delivering a voltage ranging from 50 to 70 or more, according to its size and the speed of rotation applied at the hand crank. Such a device cannot be used with a battery call bell, but requires a polarized ringer designed to accompany the generator. When the ringer is connected by two wires to the magneto generator and the crank rotated briskly, the signal will be given even though the instruments are several miles apart. Thus, in using these devices, the number of cells of local battery need never be more than two at either end.
Relative to magneto equipments, the price of standard machines is so low that no similar contrivances could be completed at home for anywhere near the price, hence a description of both
bell and generator construction is not essential at this poiirt. Induction coils for the talking circuit may also be purchased at about wire prices.
Fig. 7 shows in detail a circuit for one end of abridging magnetio private line. In this diagram the bell and generator are shown separated one from the other, that the equipment may be fully understood but it, is generally the custom to purchase these set up in one box, the hook for the receiver being at one side, the crank handle on the other and the gongs and transmitter on the front, which is usually a cover to the box, and with two binding posts at the top to take the line wires. Batteries for the transmitter may be placed close at hand or stored some distance away and connected by insulated wires with the circuit.
In this diagram it will be noticed that with the receiver on the hook, a ring from the other end operates the signal bell. In ringing with the generator the pressure of the hand in rotation causes a circuit to be completed by the armature spindle bearing upon a spring and ringing your own bell as well as the distant one. When the receiver is removed from the hook the two lower contacts are made common with the hook, thus putting the receiver, transmitter, battery and coil upon the line for talking. Should this private line comprise more than two stations the bell coils of such instruments as are not in use are, of course, across the line, but owing to their form of construction and high resistance, they act as a choke to the voice currents and keep them on the line. Different manufacturers employ various methods of wiring bridging telephone sets, each having some special qualification. The one here shown is common to a number of good systems and should be closely studied by the amateur. Nearly all dealers supply customers with working diagrams when same are requested.
Wires for outside construction may be of No. 12 galvanized iron wire, and as conditions for line work vary in different localities, the subject of lines passing over other than one's own territory will be treated in another chapter. On one's own property wires may be run circuitously through trees to avoid setting poles, but care must be taken to insulate the wires from all substances likely to give trouble. Insulators of glass to fit on wooden pins and porcelain affixed with screws or spikes are commonly used.
In running such lines separate the wires a full foot from each other, using care that a sag in the one does not throw it against the other or against some conductor that will cause trouble.
Where wires enter buildings some certain means of protection must be provided. Carbon and fuse arrestors are for sale by dealers in telephone equipments generally, and may be depended upon to protect both instruments and property when well installed, from foreign currents upon the line. Lightning, which is oscillating in character and of enormous voltage, often strikes contiguous to telephone lines and a certain amount of the charge passes to the arresters, where it enters the ground without damage. No device has been provided that will ensure safety should lightning discharge directly upon the circuit near the instrument. The modern carbon arrestor and fuse block, however, may be depended upon in practically all cases except the foregoing, and as types and methods of installation vary, a diagram will not be presented here, that being usually supplied by the dealer.
All protecting devices, however, are placed just inside the building as near the point of entrance as possible. The outside line ends, usually on two glass or porcelain insulators, supported on wooden or iron brackets. The wiring from this terminal to the lightning arrestor inside the building should be heavily insulated with rubber and take separate holes where entering. The usual form of arrestor has five binding posts - two for the line, two for the instrument side, and one additional- to which is connected a heavy wire, known as a ground wire of value only as a discharging circuit to earth for foreign currents breaking in on the line and causing the protecting device to operate. At no time does this ground wire enter info the talking circuit.
In the next chapter will be given a diagram of a stiple interior outfit with several stations wired suitable for room to room communication.