Lippincott's Monthly Magazine/Volume 46/Electric Lighting
ELECTRIC LIGHTING.
TO bring to the understanding of the general reader how the electric light is produced is no easy matter. In order that the untrained mind may realize natural laws and phenomena which are either unknown or unfamiliar to it, the necessity arises to introduce comparisons between them and the phenomena which are within its knowledge. This is not a strictly scientific method of proceeding, and, when employed in this article, any comparisons given must be limited only to the statement made: there must be no further deduction, whether such deduction be right or wrong.
Practical electricity is the application of a certain known force to practical ends, such as telegraphy, telephony, lighting, motive power, welding, chemical operations, medicine, and an infinite number of other purposes. Lighting and motive power only will be treated here. This subject divides itself into three parts: (1) the production of the force,—i.e., electrical energy; (2) the means of conveying it to the point or points where it is to be used; and (3) the apparatus for giving the desired results.
Although there is a variety of ways of producing electricity, from the rubbing of a piece of sealing-wax on the coat-sleeve to the vast engines employed at large central lighting-stations, yet here we will consider only such methods of producing the force as are in practical use on a moderate scale. The current for electric lighting may be obtained either from chemical batteries or from machines termed dynamos. The dynamo is an apparatus a portion of which has to be revolved in order to produce a current, and consequently requires some motive power, which is supplied from a steam-engine or some other source. It must be remembered that a steam-engine derives its power from the fuel in the boiler-furnace, or from gas, petroleum, or some other material, being consumed,—to use a common phrase,—for in reality it is simply the material changing its chemical state, no actual consumption taking place at all. It will thus be observed that the power obtained from all engines is derived, in the first instance, by a chemical process, and is, therefore, comparable with a complex chemical battery. It might be thought that, the current is produced by water-power, chemical action is no longer the source of current-production; but this is not so, for the water which produces the power has been raised from low levels in the form of vapor to high levels, where this vapor is deposited in the form of rain, by the action of the sun’s heat,—which is produced by chemical action. Therefore it may be assumed that electricity can probably be produced only by chemical action, and that whenever any chemical action takes place electricity is produced. However, it is usual to speak of the electrical current being produced either by means of an engine of some kind, or from batteries, without entering into further refinements. The engines employed are usually steam or gas, and both are so familiar to every one that to describe them is unnecessary. The apparatus which is revolved by the engine and termed the dynamo, in which the current is actually or, more strictly speaking, the apparatus in which the power derived from the engine is converted into electrical energy, will claim our attention after a brief reference to batteries.
Batteries are divided into primary and secondary. The former are those wherein the materials, part or the whole of them, become exhausted, and at this stage fail to supply electricity unless some or all of the materials are renewed. The secondary battery is one which becomes exhausted in exactly the same way as the primary, but the chemical contents are of such a nature that it is merely necessary to pass a current of electricity through the battery in order to reinstate it in its original condition. To give a practical instance: every one is familiar with the primary cell, consisting of a pot of weak sulphuric acid, in which are immersed two plates, one of copper, one of zine, not touching each other in the liquid, but connected with each other outside the liquid by means of a wire the ends of which are joined to the plates respectively. Under these conditions the liquid bubbles like soda-water, indicates that some chemical action is proceeding and some of the fluid is being converted into gas; a current also flows through the wire. This is a simple illustration of chemical and elec- trical action proceeding at the same time, which, as already stated, must necessarily take place, although not always self-evident. On disconnecting one end of the wire from one of the plates, the bubbling ceases, indicating that the action has been arrested. In the course of the wire an electrical lamp, a motor, or apparatus for indicating the presence of a current, or any other apparatus, may have been inserted; and all the results, so well known, which can be produced by means of an electrical current, may have made themselves manifest. But of course no appreciable quantity of light or power would be produced from such an experimental cell; a large number of such cells coupled together would be necessary in order to be of practical service; and this combination is termed a battery. In a short time the zinc plate in the cell is dissolved, and the power of the liquid becomes exhausted, when the production of the electric current ceases. But supposing these two plates, instead of being copper and zinc, are one lead and the other double oxide of lead, then, although the current produced ceases after a time, instead of requiring a portion of the contents of the cell to be renewed, it is only necessary to pass through it a current pro- duced from some other source in order to renew its vitality. Such a combination is termed a secondary cell, and a number of these suitably connected form a secondary battery very commonly known under the name of an accumulator. An accumulator, therefore, is nothing more than a primary battery in which the chemicals can be renewed, as often as required, by simply passing a current of electricity through it, such current in practice being produced by means of an engine and a dynamo. It may also be observed that the primary battery at the present moment is not a suitable means for producing electricity on a large scale, for, apart from expense, the necessity to replace new chemicals in the cells from time to time involves considerable trouble and inconvenience; whereas to replenish an accumulator by running an engine is a very simple matter.
To turn to the dynamo: no attempt can be made in an article of this kind to enter into a full description of this apparatus, which is made in an unlimited number of ways; but sufficient explanation will be given of its general principle to show how a current is produced from it,—the principle being the same in all types, no matter how the machine may be constructed.
Dynamos are of two kinds, one made to produce a continuous current and the other an alternating current. Although these currents are very different in their character, no difference is to be observed when they are employed to give light. A continuous current may be compared to a liquid wherein waves are continually progressing in one direction. The direction towards which these waves are moving is termed the positive direction, and that from whence they come the negative. Consequently, if a wire, with a current flowing in it, is attached to a lamp in such a manner that the waves move towards the point where the wire is attached to the lamp, it would be stated that that point is attached in such a way as to receive a positive current; and the other part of the lamp, where the current leaves it (since the waves will evidently move away from this point), is termed the negative. In this explanation the lamp must be regarded as a part of the wire modified in such a manner as to be capable of converting the current into light. Therefore, when a positive or a negative current is spoken of, it simply indicates direction. Notwithstanding this, positive electricity and negative electricity have a variety of phenomena peculiar to each.
The alternating current may be regarded as a fluid in which a wave proceeds first in one direction and then in the other, seesaw fashion; and in practice these changes of direction take place as frequently as from fifty to two hundred times per second. Instruments employed for continuous currents may contain iron and permanent magnets, but for the alternating currents the latter must be completely absent, and any iron employed in the construction of the instruments must be very soft and much subdivided, or it will become extremely hot.
The dynamo producing the continuous current consists of two parts, one stationary and one to be revolved. In the most usual type the stationary portion is a powerful electro-magnet, horseshoe in shape, the free ends of the horseshoe being termed poles. These poles are so arranged as to permit the armature to revolve between them. The axis of the armature consists of a spindle running in bearings, and is revolved by motive power, either being connected directly with the engine or indirectly by means of a belt, in which latter case the spindle carries a pulley. Upon the spindle is placed a suitable frame-work, and wire is wound upon this in a particular manner so as to form a continuous coil, which may (as it generally does) or may not surround some soft iron. When the armature is removed from a machine, it has somewhat the appearance of a short bolster with a spindle pushed through it. The bolster part, on close examination, will be found to consist of the coil of wire already mentioned. This wire-coil portion revolves between the magnet-poles. The spindle, at one end, carries a number of copper plates, placed radially and close together, so as to form in appearance a solid cylinder, termed the commutator. Each plate is insulated from the next and from the spindle. By insulation is meant that some substance through which the electricity will not pass is placed between plate and plate, and between plates and spindlee. Upon this commutator rest two brushes, to which are attached the wires that lead to any point where electrical energy is required. Every plate in the commutator has a wire joined to it, the other end of the wire being in connection at certain points of the armature-coil. It will thus be seen that any current produced in the armature will flow to the copper plates and enter the brushes which press upon them, and so pass on into the wires (often termed cables, mains, leads, or lines) and travel to the point required. When the armature is revolved, the plates of the commutator successively pass the brushes, consisting of a group of fine copper wires or thin plates, which should be of considerable length, in order to give them elasticity and to allow for wear and tear. A portion of the current produced is employed to excite the electro-magnets. Permanent magnets may be used,—in fact, they are still used in France, for dynamos employed in light-houses,—but for general purposes the electro-magnets are cheaper and have many other advantages. A piece of soft iron, wound round and round with wire which has a current passing through it, becomes a powerful magnet; and the iron is then said to be excited. When the electro-magnet of the dynamo is excited, the armature is in an extremely powerful magnetic field; which means that it is strongly under magnetic influence. If the spindle is revolved under these conditions and the wires leading from the brushes are connected so as to form a closed circuit, it will be found that the power required to turn the spindle increases as the speed is raised. The best comparison to make is that of turning a fan in a barrel of treacle: the more quickly the fan is turned the greater will be the resistance to its motion.
The very fact of revolving the armature, which is nothing more than a specially-wound coil of wire close to the poles of a magnet, produces a current of electricity in the coil; and the faster this coil is turned the greater will be the pressure of the current produced. The quantity produced is dependent upon the resistance of the circuit. The resistance is made up, in the simple case imagined, of the wire in the armature, the length of which is invariable, and the length of the wire joined to the brushes, which is variable. Evidently the longer this wire is, the more resistance will be offered to the passage of the current; and if the pressure were to remain unaltered for two different lengths of this wire, the quantities of current which would flow through it myst necessarily vary with its length,—i.e., if the armature-coil resistance is neglected. The resistance of the armature-coil is always very small, and may practically be neglected, so that the current flowing in the system may be considered proportional to the resistance of the circuit outside the machine. In an electric-light installation it would be practically proportional to the number of lamps in use at any time.
In a well-constructed dynamo, running at a given speed and intended to produce current for a certain number of lamps, the pressure should remain constant, whether one lamp or more is in use upon the circuit; and this is of great importance, for otherwise the light from the lamps will vary. When such self-regulation cannot be obtained, electrical governors are employed to secure constant pressure.
One form of alternating-current dynamo consists of a number of coils placed upon the periphery of a wheel, which are revolved before electro-magnets excited from a small continuous-current machine such as that which has just been described. The arrangement is such that an alternating current is produced in the moving coils. This current is collected by means of brushes, as in the last case; but the commutator consists of two rings of metal insulated from each other and from the machine, in connection, however, with the moving coils. The con- ditions for regular pressure and quantity of current produced are the same as in the last case. These descriptions of the two most usual types of dynamo are only of the most superficial kind, the aim being to give merely a general idea of their mode of action.
Having dealt with the production of the electric current, it becomes necessary to consider how it is conveyed to those points where it is required. After this, the apparatus used for obtaining practical results may be examined. In order that the current may travel, the circuit must be complete. The circuit may be compared to a system of hot-water pipes such as are used for warming hot-houses. In a hot-water system there are a boiler, a flow and a return circulating pipe, and pipe-coils at various points for giving off heat at places where warmth is required. In an electrical installation the dynamo replaces the boiler, flow- and return-pipes are represented by the two conducting mains, and the pipe-coils by lamps, motors, and other apparatus. In a hot-water system, it is perfectly evident that the quantity of water passing through every part of the main must be the same. It is so with the electric current: whatever may be the quantity of the current starting from the dynamo, the same quantity comes back to it. But it is not so with the pressure: this diminishes in proportion to the work the current does; consequently the pressure diminishes as the current advances on its path. The passage of a current through a conductor cannot be effected without loss,—i.e., diminished pressure. Loss means work done. If this work has a useful purpose, it is not a loss in the common sense of the word; but in all other cases it is waste. Any pressure of the current lost in a lamp produces a desired result. Since this is not so for the mains, the system must be constructed in such a manner that, except where practical results are needed, as little loss as possible shall occur in conducting the current from point to point,—which is effected by making the mains as large as possible. The size of the mains is limited only by the consideration of their cost. To sum this up, the whole pressure of the current may be regarded as being lost in passing through the lamps, motors, and other apparatus in a well-designed installation.
The larger the section of a wire, the less resistance does it offer to the passage of the current. Therefore one wire double the diameter of another will offer four times less resistance. On the other hand, if the current passing through any wire is doubled, the loss of pressure in travelling a given distance will be four times greater than before; three times the current, nine times: i.e., the waste increases as the square of the current. In electric lighting, the waste invariably consists in producing heat at places in the circuit where heat is not required; and if this is generated in too great a degree, by reason of the mains offering too much resistance,—that is, being too small in section,—a fire may result. To avoid the possibility of such an accident, the current is made to pass in its course, at suitable points, through short pieces of metal far more fusible in nature than the material of which the conductors are made. The cables and wires are usually of copper. The fuses, as a rule, consist of tin wire, and are generally called "safety- junctions." Then, if the current rises beyond a certain limit from any cause, the safety-junctions melt and cut the circuit before any damage is done.
The only apparatus in connection with an electric-light installation that need be considered here are the arc lamp, the incandescent lamp, the motor, the switches, and the instruments for indicating the quantity and pressure of the current.
In the arc lamp the current passes through two carbon rods, which are separated from each other by a very short distance. In order that the current shall leap this interval, the rods are made to touch each other, and then they are separated: a flame, consisting of heated gases, passes between these carhon rods, which flame must not be mistaken for visible electricity. The powerful light is produced by the intense heat to which the ends of the rods are raised. Suitable apparatus is connected with these carbons, in order that they may be fed as they burn away. Otherwise the distance between them will increase, and eventually the current will cease to flow. This form of light is termed "arc" light, because the flame resembles in shape an arc or a crescent.
The incandescent or glow lamp consists of a very fine filament of carbon, hermetically sealed in a glass globe from which the air has been exhausted. The ends of the filament reach the outside of this globe by being attached within it to two platinum wires which pass through the glass to the outside, where they are dealt with in some convenient way whereby they may be attached to the circuit. The current consequently enters the filament at one end and leaves it by the other. The filament becomes white hot during the time that the current passes through it, and is not consumed, since it is not in the presence of air. The high resistance of the carbon filament necessitates a great loss of pressure in the current during its passage, and is oonverted into light-giving heat. If the pressure of the current is greater than that for which the lamp was constructed, too much current will pass through the filament, and it will be destroyed. On the other hand, if the pressure is insufficient, the temperature to which the filament ought to be raised will not be reached, and the light will be far less than it should be under normal conditions. The light given by any lamp diminishes in far greater proportion than the fall in the pressure of the current; and the inverse is true. For instance, a lamp intended to give a certain light with a given pressure of current would give less than half its light with a fall of ten per cent. in pressure. On the other hand, a four-per-cent. increase of pressure above the normal would produce at least double the light intended.
A motor is identical with the dynamo. In the latter case, the armature is revolved and a current produced, but when a current is sent into a dynamo it will be found that the armature revolves: in other words, it becomes a motor. Here is perhaps the simplest way of conveying power that has thus far been discovered. If it is required to turn a lathe or other machine by power at any given place, it will simply be necessary to convey to that place two wires to conduct the current, and then attach a motor. Although heavy, a motor is compact, and a man can, without difficulty, move from place to place, on a properly-designed truck, such a machine up to the size of say ten horse-power. Apart from the rapidity and ease with which motive power may be installed wherever it may be desired, there are also eliminated the dangers, disadvantages, and complications which exist more or less in connection with the forms of motive power hitherto employed.
In order to light and put out a lamp, or to start and stop a motor, the current must be cut: to effect this, simple apparatus are used that break the metallic continuity of the circuit, this being all that is necessary. Such devices are termed switches.
Two instruments are employed to observe what is taking place upon the circuit. The one is termed an ammeter, equivalent to a water-meter, and indicates the quantity of current,—the unit being termed an ampère. The other instrument is called a voltmeter, and registers pressure,—the unit being termed a volt. There are also meters in use, equivalent to a gas-meter, whereby the electrical energy used during any given time is recorded. When current is supplied from a public installation by the unit (which in England is 1000 Watt-hours), the price per unit multiplied by ten gives the equivalent value for gas per thousand cubic feet, light for light, when glow-lamps are used. Thus, electrical energy sold at sevenpence per unit in London is equivalent to gas at six shillings and tenpence per thousand cubic feet.
There are two systems of distribution, one the series and the other the parallel. Of these systems there are many subdivisions of each, but only the simple methods need be referred to.
In the series system the conductor consists of one continuous circle. The lamps and other apparatus which are inserted form a part of this conducting ring. In this method high-pressure currents have to be used, because there is a successive fall of pressure as the current passes through each succeeding lamp or other piece of apparatus. The most usual pressure adopted is that of one hundred volts. (The volt is the unit-measure of pressure.) Suppose, therefore, a circuit contained one hundred such lamps, ten thousand volts would be required in order that they might give their normal light. Such a pressure is fatal to life. A pressure exceeding five hundred volts with continuous current, and two hundred volts with alternating current, may prove fatal under certain conditions.
In the parallel system, which is free from danger, since high pressure is rarely employed with it, the flow and return mains may be supposed to be of equal length and laid parallel with each other. At various points along these mains, branch wires start, a lamp or motor being placed in the course of each branch, so that the current, in passing from one main to the other, traverses the lamp or motor. If this system is drawn on paper, it will have the appearance of a ladder, the lower ends of the sides, which may be taken to represent the mains, being connected to the dynamo and the top ends free. The rounds of the ladder will then represent the connecting cross-wires, each having in its course a lamp or any other piece of apparatus. Evidently, if the large mains, represented by the sides of the ladder, are of very low resistance, the current traversing the branches will simply be pro- portional to the resistance of each branch; and for lamps made to give equal light, placed in these branches, the resistances of the latter must be made approximately equal.
The resistance of the one-hundred-volt sixteen-candle-power glow- lamp most commonly in use at the present time is about one hundred and seventy times greater than that of the mains and branches leading to it. Consequently, if the flow- and the return-wire of such a lamp were to come in contact before reaching it, what is termed a short circuit would result, since the electric current, like steam and water, flows in the direction of least resistance. In this case the wires leading to the lamp will pass one hundred and seventy times more current than was intended, which would raise these wires to a white heat, or even fuse them, if no safety-junction is inserted in the circuit. If it exists, the fusible wire melts, and no mischief will be done.
In practice, one horse-power will produce one thousand candle-power in an arc-light; but an increase in the horse-power gives a far larger corresponding increase of light in the case of the arc lamp. For instance, seven horse-power will produce as great a light as fifteen thousand candles, or more. But, on the other hand, the light given by the incandescent lamp is directly proportional to the power of production; and, in practice, one horse-power will incandesce about eight sixteen-candle lamps of this type.
Although electricity was known to the ancient Greeks, the uses to which it might be applied have remained unknown for thousands of years. It was reserved to Ampere, Volta, Faraday, and a few others, to discover the laws which govern this force. The development of the science in its application to the practical needs of human life may be considered to date from the time of Faraday, whose career ended about the middle of the present century. In the past forty years electrical science has advanced in a degree probably unequalled by the progress of any other science since the commencement of the world’s history. The electric telegraph was discovered about fifty years ago. The transmission of messages across the ocean dates so recently as to be within the memory of those who can look back some thirty years, The electric light can only be considered to have entered the practical stage since 1880; and, though the advances during the last few years have been great, we know that greater achievements are in store for the present and future generations. It would be possible to speculate to an unlimited degree as to the future of science, but experience shows that the knowledge of to-day is as nothing compared with that of to-morrow. A few new facts often upset pet theories. One cannot but be reminded of the words spoken by one of the greatest of philosophers who ever lived, Sir Isaac Newton, who is reported to have said, when dying, "I seem only like a child playing on the sea-shore, and diverting myself in now and themn finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."
David Salomons.