Electric discharge

“ Lightning discharges are known to be of a highly oscillatory character, their frequency being much greater than that of the alternating currents developed in transmission systems. The self-induction of a lightning discharge in passing through one of these choke-coils is great, and the consequent tendency is to keep the discharge from passing through the choke-coil and into the station apparatus and thus to force the discharge to pass to earth through the lightning arrester. ”

“ In the arc discharge we get very large currents with comparatively small potential differences. We may get the arc discharge by taking a battery of cells large enough to give a potential difference of 60 to 80 volts, and connecting the cells with two carbon terminals, which are put in contact, so that a current of electricity flows round the circuit. If the terminals, while the current is on, are drawn apart, a bright discharge, which may carry a current of many amperes, passes from one to the other. ”

“ When, therefore, disruptive discharge has once commenced at the root of a brush, the electric force which has been accumulating in the conductor attached to the rod, finds a more ready discharge there than elsewhere, and will at once follow the course marked out as it were for it, thus leaving the conductor in a partially discharged state, and the air about the end of the wire in a charged condition ”

“ The Spark Discharge.—In practice the wire w is not actually used, but the air itself suddenly becomes a conductor and makes the connection. When the electric field at a point in the air exceeds a certain limiting strength, the air seems to break down and {85} suddenly become a conductor and remains one for a short time. This breaking down is accompanied by light and heat, and is known as the spark discharge or electric spark. ”

“ He has, in the first place, to make sure that, wherever the lightning can fall, it shall find an open and practically unobstructed path to traverse in its passage to the ground. He is quite sure that the electric discharge will confine itself to the track of a conductor, and will pass quietly and harmlessly along it, provided its dimensions are adequate to the task of transmission, and provided the inlets and outlets are sufficiently capacious for its unimpeded reception and escape. ”

“ When this is the case the back electromotive force is destroyed and the same potential difference will carry a much larger current. The arc discharge may be regarded as analogous to the discharge between incandescent terminals, the only difference being that in the arc the terminals are maintained in the state of incandescence by the current and not by external means. On this view the cathode is bombarded by positive ions which heat it to such a temperature that negative corpuscles sufficient to carry the current are emitted by it. ”

“ As soon as this circuit is provided, the opportunity is given to the iron to receive the oxygen. This it does, and in so doing develops electrical energy.
This energy is in the form of electric current which is then delivered by the battery on what is called the "discharge," and this current may be used for lighting lamps or for operating motors or other electrical devices.
The battery is said to be discharging as long as the iron is receiving oxygen from the nickel oxide. ”

“ The object of dividing the total air-gap in a lightning arrester for lines that carry alternating current up into a number of short gaps is to prevent the continuance of an arc by the regular generator or line current after the arc has been started by a lightning discharge. As soon as an electric spark leaps through air from metal to metal, a path of low electrical resistance is formed by the intensely heated air and metallic vapor. If the arc thus formed is, say, two inches long it will cool a certain amount as the passing current grows small and drops to zero. ”

“ These were invariably shattered by the passage of the spark on account of the accompanying elevation of temperature, which caused expansion. He also established the facts that the atmosphere adjacent to an electrified body acquires electrification of the same sign by abstracting electricity from the body, and that the air then parts with its electricity very slowly. He advanced the theory that there is a mutual repulsion between the particles of the electric fluid and those of air, and that a temporary vacuum is formed at the moment of the passage of a disruptive discharge or spark. ”

“ For such short sparks the electric intensity is very high. The electric intensity required to produce the discharge from a gas diminishes as the pressure of the gas diminishes, but not nearly so rapidly as the electric intensity for long sparks. Here again the discharge from a point is comparable with short sparks, which, as we have seen, are much less sensitive to pressure changes than longer ones. The minimum potential at which the electricity streams from the point does not depend upon the material of which the point is made ”

“ The spark in these early experiments was usually supplied from the outer coats of a pair of oppositely-charged small Leyden jars, whose knobs sparked into each other; the idea being to ascertain all the conditions pertaining to the feeble residue of a lighting discharge which is liable to be conducted by telegraph wires to a distance, and there cause some damage to sensitive instruments not suitably protected from sudden electric jerks, whose laws of flow are quite different from those proper to steady currents. ”

“ The discharge by a brush is not to all the particles of air that are near the electrified conductor from which the brush issues; only those parts where the ramifications pass are electrified: the air in the central dark parts between them receives no charge, and, in fact, at the time of discharge, has its electric and inductive tension considerably lowered. For consider fig. ”

“ When they are gone, other uncharged particles take their place round the point, and since these cannot shield those next the point itself from the excessive electric tension, a new discharge takes place, after which the newly charged particles move off, and so on as long as the body remains electrified. ”

“ He reasoned out the subject, and concluded that the electric spark between metals, the electric arc between the carbons, and other luminous electric phenomena are secondary effects due to the heating and illumination of electric carriers; that the electric spark of the conductors of ordinary electrical machines is simply a transfer of incandescent particles of metal, which effect a kind of electric convection, known as the disruptive discharge ”

“ Except for a few little storage-battery or Diesel generator systems, that couldn't handle one tenth of one per cent of the barest minimum load, it's been the only source of electric current here since 1962, when the last coal-burning power plant was dismantled. ”

“ The great law which regulates these transactions is this: that the sum of the internal and the external heats is a constant quantity; so that when the platinum wire was ignited we had less heat developed in the battery than before. The zinc in the battery is burned as fuel upon a hearth; the heat, however, being developed either upon the hearth itself or at any distance from it.
“As a primary source of electricity here is the combustion of a metal, the voltaic battery is not an economical source of power for producing electric light. ”

“ One of the most striking consists in their opening a path for a current of electricity. The electrical discharge, feeble in itself, not capable of lifting by means of a motor a pound weight a foot from the floor, is yet competent to open a path for a current which can set all the trolley cars of a great city in motion. To exhibit this mysterious effect we bring the ends of the electrical current which we wish to excite near each other, but not touching, in a glass tube with thin walls, from which the air has been exhausted. ”

“ In fine wires the resistance to the current is large and the energy of the battery is expended in heat rather than in current. If the heat thus produced is very great, serious consequences may arise; for example, the contact of a hot wire with wall paper or dry beams may cause fire. Insurance companies demand that the wires used in wiring a building for electric lights be of a [313] size suitable to the current to be carried, otherwise they will not take the risk of insurance. The greater the current to be carried, the coarser is the wire required for safety. ”

“ Heating, in any form, by electricity, requires a large amount of current proportionally. As a matter of fact, there is less heat to be had in thermal units from a horsepower-hour of electricity than from three ounces of coal. When one is generating current from water-power, or even direct from gasoline or oil, this is not an argument against electric heating devices. But it becomes a very serious consideration when one is installing a storage battery as the source of current, because of the high initial cost, and depreciation of such a battery. ”

“ For this purpose they invariably use a pulsating or alternating current and reduce the resistance by using metal handles and wet sponges for contact with the skin, but even so a very small amount of current passes. The moderate twitching of the muscles seems to be the end sought.
Men who are supposed to be killed by electric shocks often die from other causes. ”

“ Let us say that instead of separating these two mains by air we separated them by the human body—that a man took hold of the bare wires, one in each hand. The resistance of the human body varies from 5,000 to 10,000 ohms. In that case C (amperes) equals 110/5,000, or 110/10,000—about 1/50th, or 1/100th of an ampere. This illustrates why an electric current of 110 volts pressure is not fatal to human beings, under ordinary circumstances. ”

“ As long as each electric system was confined to a single city or town a broken insulator pin could be quickly replaced, and any material interruption of service from such a cause was improbable. Where the light and power supply of a city, however, depends on a long transmission line, as is now the case in many instances, and where the line voltage is so great that contact between a wire and a cross-arm will result in the speedy destruction of the latter by burning, a broken pin may easily lead to a serious interruption of the service. ”

“ This arises in cases where current is consumed during the day for motors, fans, lifts, electric tramways, and other like purposes, and, as the employment of electric energy for these services is rapidly becoming general, no difficulty need be anticipated in the successful working of combined destructor and electric plants where these conditions prevail. ”

“ Although it is impossible to avoid altogether dangers of this nature, the risk can be reduced to the minimum by igniting the explosives by electricity.
Electrical shot firing may be carried out in different ways, according to circumstances. The current is supplied256 either by a dynamo or by a battery, and the firing is controlled from a switchboard placed at a safe distance from the point at which the charge is to be exploded, the connexions being made by long insulated wires. The actual ignition is effected by a hot spark, as in automobile ignition, or by an electric detonator or fuse. ”

“ Were it not for the extremely high pressure of lightning discharges, their high frequency of oscillation would enable ordinary coils to be self-protecting against them. But a discharge of electricity can take place through the air or other insulating medium if its pressure be high enough. A pressure of 70,000 volts can strike across a gap in air of one inch, and lower pressures can strike across smaller distances. ”

“ It should be understood that an electric dynamo or battery does not generate electricity, for if it were only the quantity of electricity that is desired, there would be no use for machines, as the earth may be regarded as a vast reservoir of electricity, of infinite quantity. But electricity in quantity without pressure is useless, as in the case of air or water, we can get no power without pressure, a flow of current. ”

“ Fortunately for the future delivery of energy at great distances from its source, the means to avoid the limit just discussed are not difficult. Other experiments have shown that with a given voltage and distance between conductors the loss of energy from wire to wire decreases rapidly as their diameters increase. The electrical resistance of air, like that of any other substance, increases with the length of the circuit through it. The leakage described is a flow of electrical energy through the air from one wire to another of the same circuit. ”

“ In a transmission with continuous current on the other hand, if the prevailing practice of a constant current and varying pressure on the line is followed, the insulation is subject to the highest voltage only at times of maximum load on the system. Lightning is a very real and pressing danger to machinery connected to long transmission lines, and this danger is much harder to guard against in an alternating system than in a system with continuous constant current. ”

“ It is desirable to exclude the light as perfectly as possible, as it interferes with some experiments. This form of discharger is simple and very effective when properly manipulated. The air being warmed to a certain temperature, has its insulating power impaired; it becomes dielectrically weak, as it were, and the consequence is that the arc can be established at much greater distance. The arc should, of course, be sufficiently insulating to allow the discharge to pass through the gap disruptively. ”