American School of Correspondence

“ As the property which opposes the flow of electric current in an electrical circuit is called resistance, so the property which opposes the flow of magnetic lines of force in a magnetic circuit is called reluctance. In the case of the electric circuit, the resistance is the reciprocal of the conductivity; in the case of the magnetic circuit, the reluctance is the reciprocal of the permeability. As in the case of an electrical circuit, the amount of flow of current is equal to the electromotive force divided by the resistance ”

“ There is no industrial art in which the advancement of the times has been followed more closely by practical application than in telephony. Commercial speech by telephone is possible by means of currents which so far are practically unmeasurable. In other words, it is possible to speak clearly and satisfactorily over a line by means of currents which cannot be read, with certainty as to their amount, by any electrical measuring device so far known. In this regard, telephony is less well fortified than are any of the arts utilizing electrical power in larger quantities. ”

“ When a current flows through the convolutions of an ordinary helix, the helix will exhibit the properties of a magnet even though the substance forming the core of the helix is of non-magnetic material, such as air, or wood, or brass. If, however, a mass of iron, such as a rod or a bundle of soft iron wires, for instance, is substituted as a core, the magnetic properties will be enormously increased. The reason for this is, that a given magnetizing force will set up in iron a vastly greater number of lines of magnetic force than in air or in any other non-magnetic material. ”

“ The user of the telephone and the telegraph forgets the tremendousness of the feat in the simplicity of its accomplishment; but the man who has made the feat possible knows that its very simplicity is due to the complexity of the principles and appliances involved; and he realizes his need of a practical, working understanding of each principle and its application. The Cyclopedia of Telephony and Telegraphy presents a comprehensive and authoritative treatment of the whole art of the electrical transmission of intelligence. ”

“ The physical thing which we call an electromagnet, consisting of a coil or helix of wire, the turns of which are insulated from each other, and within which is usually included an iron core, is by far the most useful of all the so-called translating devices employed in telephony. In performing the ordinary functions of an electromagnet it translates the energy of an electrical current into the energy of mechanical motion. An almost equally important function is the converse of this, that is, the translation of the energy of mechanical motion into that of an electrical current. ”

“ The term battery is frequently applied to a single voltaic cell, but this term is more properly used to designate a plurality of cells joined together in series, or in multiple, or in series multiple so as to combine their actions in causing current to flow through an external circuit. We may therefore refer to a battery of so many cells. It has, however, become common, though technically improper, to refer to a single cell as a battery, so that the term battery, as indicating necessarily more than one cell, has largely lost its significance. ”

“ Human Voice. Human beings communicate with each other in various ways. The chief method is by speech. Voice is sound vibration produced by the vocal cords, these being two ligaments in the larynx. The vocal cords in man are actuated by the air from the lungs. The size and tension of the vocal cords and the volume and the velocity of the air from the lungs control the tones of the voice. The more tightly the vocal cords be drawn, other things being equal, the higher will be the pitch of the sound; that is, the higher the frequency of vibration produced by the voice. ”

“ To distinguish the two kinds of reactance, that due to the capacity is called capacity reactance and that due to inductance is called inductive reactance.
All the foregoing leads to the generalization that the higher the frequency, the greater the opposition of an inductance to an alternating current. If the frequency be zero, the reactance is zero, i.e., the circuit conducts direct current as mere resistance. If the frequency be infinite, the reactance is infinite, i.e., the circuit is "open" to the alternating current and that current cannot pass through it. ”

“ All telephone systems are exposed to certain electrical hazards. When these hazards become actively operative as causes, harmful results ensue. The harmful results are of two kinds: those causing damage to property and those causing damage to persons. The damage to persons may be so serious as to result in death. Damage to property may destroy the usefulness of a piece of apparatus or of some portion of the wire plant. Or the property damage may initiate itself as a harm to apparatus or wiring and may result in greater and extending damage by starting a fire. ”

“ The two things principally affecting the manufacture of electromagnets for telephone purposes are the number of turns in a winding and the resistance of the wound wire. The latter governs the amount of current which may flow through the coil with a given difference of potential at its end, while the former control the amount of magnetism produced in the core by the current flowing. While a coil is being wound, it is a simple matter to count the turns by any simple form of revolution counter. When the coil has been completed it is a simple matter to measure its resistance. ”

“ The art of telephony in its present form has for its problem so to relate two diaphragms and an electrical system that one diaphragm will respond to all the fundamental and harmonic vibrations beating upon it and cause the other to vibrate in exact consonance, producing just such vibrations, which beat upon an ear. ”

“ Discovery may need to intervene, to enable invention to restimulate.
In both telegraphy and telephony, the longer the line the weaker the current which is received at the distant end. In both telegraphy and telephony, there is a length of line with a given kind and size of wire and method of construction over which it is just possible to send intelligible speech or intelligible signals. A repeater, in telegraphy, is a device in the form of a relay which is adapted to receive these highly attenuated signal impulses and to re-transmit them with fresh power over a new length of line. ”

“ For the serving of a large traffic from a large number of lines, as is required in practically every city of the world, a very great contribution to the practical art was made by the development of the multiple switchboard. Such a switchboard is merely such a device as has been described for the simpler cases, with the further refinement that within reach of each operator in the central office appears every line which enters that office, and this without regard to what point in the switchboard the lines may terminate for the answering of calls. ”

“ Frequently electromagnetic coils have only the function of offering impedance, where no requirements exist for converting any part of the electric energy into mechanical work. Where this is the case, such coils are termed impedance, or retardation, or choke coils, since they are employed to impede or to retard or to choke back the flow of rapidly varying current. The distinction, therefore, between an impedance coil and the coil of an ordinary electromagnet is one of function, since structurally they may be the same, and the same principles of design and construction apply largely to each. ”

“ As lightning damage is caused by current and as it is current which destroys a fuse, a lightning discharge can open a circuit over which it passes by melting the fuse metal. But lightning may destroy a fuse and at the same discharge destroy apparatus in series with the fuse. There are two reasons for this: One is that lightning discharges act very quickly and may have destroyed apparatus before heating the fuse enough to melt it; the other reason is that when a fuse is operated with enough current even to vaporize it, the vapor serves as a conducting path for an instant after being formed. ”

“ High potentials in the secondary winding are admirably adapted to transmit currents in a high-resistance line, for exactly the same reason that long-distance power transmission meets with but one-quarter of one kind of loss when the sending potential is doubled, one-hundredth of that loss when it is raised tenfold, and similarly. ”

“ A fuse is a metal conductor of lower carrying capacity than the circuit with which it is in series at the time it is required to operate. Fuses in use in electrical circuits generally are composed of some alloy of lead, which melts at a reasonably low temperature. Alloys of lead have lower conductivity than copper. A small copper wire, however, may fuse at the same volume of current as a larger lead alloy wire. ”

“ Obviously, when placed in a circuit a condenser offers a complete barrier to the flow of direct current, since no conducting path exists between its terminals, the dielectric offering a very high insulation resistance. If, however, the condenser is connected across the terminals of a source of alternating current, this current flows first in one direction and then in the other, the electromotive force in the circuit increasing from zero to a maximum in one direction, and then decreasing back to zero and to a maximum in the other direction, and so on. ”

“ Of the ways in which artificial electricity may injure a telephone system, the entrance of current from an external electrical power system is a greater menace than an abnormal flow of current from a source belonging to the telephone system itself. Yet modern practice provides opportunities for a telephone system to inflict damage upon itself in that way. ”

“ For instance, in an open wire line of two wires, the electrostatic capacity depends upon the diameter of the wires, upon the length of the line, upon their distance apart, upon their distance above the earth, and upon the specific inductive capacity of the air. Air being so common an insulating medium, it is taken as a convenient material whose specific inductive capacity may be used as a basis of reference. Therefore, the specific inductive capacity of air is taken as unity. All solid matter has higher specific inductive capacity than air. ”

“ A great advantage of the harmonic party-line system is the simplicity of the apparatus at the subscriber's station. The harmonic bell is scarcely more complex than the ordinary polarized ringer, and the only difference between the harmonic-ringing telephone and the ordinary telephone is in the ringer itself. The absence of all relays and other mechanism and also the absence of the necessity for ground connections at the telephone are all points in favor of the harmonic system. ”

“ A very large number of step-by-step systems have been proposed and reduced to practice, but as yet they have not met with great success in commercial telephone work, and are nowhere near as commonly used as are the polarity and harmonic systems.
Principles. An idea of the general features of the step-by-step systems may be had by conceiving at each station on the line a ratchet wheel, having a pawl adapted to drive it one step at a time, this pawl being associated with the armature of an electromagnet which receives current impulses from the line circuit. ”

“ Electrical power is supplied to single-trolley railroads principally in the form of direct current. Usually all the trolley wires of a city are so connected to the generating units as to be positive to the rails. This causes current to flow from the cars toward the power stations, the return path being made up jointly of the rails, the earth itself, actual return wires which may supplement the rails, and also all other conducting things in the earth, these being principally lead-covered cables and other pipes. ”

“ Of all the materials available for the variable-resistance element in telephone transmitters, carbon is by far the most suitable, and its use is well nigh universal. Sometimes one of the rarer metals, such as platinum or gold, is to be found in commercial transmitters as part of the resistance-varying device, but, even when this is so, it is always used in combination with carbon in some form or other. Most of the transmitters in use, however, depend solely upon carbon as the conductive material of the variable-resistance element. ”