In electricity, as in many other things, simplicity is the key-note of success; and from this little device to employ the alternating current for ridding a house of an insect nuisance sprang the grim apparatus known as the “death chair,” used in the execution of first-degree criminals in the State of New York. Many people think the mechanism for electrocution is a complicated one, but it is quite as simple as the Edison roach-killer. One pole is placed at the head of the criminal and the other at the feet, the latter being bound fast so that perfect contact can be had. Then an alternating current of fifteen hundred to two thousand volts is run through the body, and death is instantaneous and void of pain.
An Electric Mouse-killer
A modification of the simple roach-killer was recently used by the author in his laboratory to get rid of some troublesome mice. A piece of board was cut twelve inches square, the edges being bevelled so that it would be an easy matter for the mice to climb up on it. An inch-wide circle of sheet brass was prepared measuring eleven inches outside diameter and nine inches inside. Another circle was cut measuring eight inches and a half outside and six inches inside diameter. Both circles were attached to the board with copper tacks and polished as bright as possible, the finished board appearing as shown in Fig. 18.
Wires were soldered to each strip, and these in turn were connected to a high-tension current of several thousand volts. Crumbs and small pieces of meat were placed on the board inside the circles, and the trap was set in a convenient place on the floor of the laboratory.
The next morning several mice lay dead on the floor, but at some distance from the board, and this seemed a little mysterious. The following night the author worked late in the laboratory. After finishing what he had on hand, he turned down the lights and sat down and watched the trap. Presently Mr. Mouse appeared from somewhere. He sniffed the air, then approached closer to the board, sniffed again, and, evidently concluding that he was on the right trail, he climbed up the side of the board and stood on the outer strip. He placed one fore-foot on the inner strip, and, bang! up he went in the air, and landed on the floor a foot or more away. His jump into space was due to the electric action on his muscles, for the current literally tore his nervous system into shreds.
Mr. Mouse lost a great many friends and relatives that season in the same manner, and the apparatus is confidently recommended as a certain and humane agent for the destruction of all small vermin.
Chapter XIII
FRICTIONAL ELECTRICITY
Frictional electricity is high potential, current alternating, and of high voltage but very low amperage. Apart from certain uses in laboratory and medical practice, it is valueless. In its greater volume it is akin to the lightning-bolt and is dangerous; but in its smaller volume it is a comparatively harmless toy. From the glass rod, or the amber, rubbed on a catskin to the modern static machines is a long jump, and the period of exploitation covers centuries of interesting experiments, most of which, however, have been practically useless for any commercial purpose.
Static or frictional electricity is generated by friction only, without the aid of magnets, coils of wire, or armatures rotating at high speed. The simple process of the glass and catskin has been variously modified, until at last Wimshurst invented and perfected what is known as the “Wimshurst Influence Machine.” It is self-charging, and does not require “starting.” It will work all the year round in any climate and temperature, and is the greatest improvement ever made in static electric machines.
Apart from its efficiency under all conditions, it is the simplest of all machines to make, and can easily be constructed by a boy who is handy with tools, and who can obtain the glass and brass parts necessary in its construction. The principal parts of an influence machine are the glass disks, wooden bosses, driving pulleys and crank, glass standards, brass arms with the spark-balls at the ends, and the base with the uprights on which these parts are built up and held in position.
A Wimshurst Influence Machine
Obtain a stiff piece of brown paper twenty inches square, and with a compass describe a circle twenty inches in diameter. Inside of this circle make another one fourteen inches in diameter, and near the centre a third circle six inches in diameter. Another circle four inches in diameter should be drawn inside of the six-inch circle, so that when the bosses are made fast to the glass plates they can be properly centred. Also mark sixteen lines radiating from the centre, equal distances apart, as shown in Fig. 1.
From a dealer in glass purchase two clear, white panes of glass eighteen inches square. Be careful not to get the green glass, as this is not nearly so good as the white for static machine construction. If it is possible to get crystal plate so much the better. The panes should be thin, or about one-sixteenth of an inch in thickness, and free from bubbles, wavy places, scratches, or other blemishes.
From these panes cut two disks sixteen inches in diameter with a rotary cutter, as described in the chapter on Miscellaneous Apparatus, page 294, and rub the edges with a water-stone (see chapter on Formulæ, page 330.)
From flat, thin tin-foil cut thirty-two wedge-shaped pieces four inches long. They should be one inch and a half wide at one end and three-quarters of an inch at the other, as shown at Fig. 2 A. Give each plate of glass two thin coats of shellac on both sides; then lay one on the paper pattern (Fig. 1) so that the outside edge of the glass will lie on the largest circle. Place a weight at the middle of the glass to hold it in place; then make sixteen of the tin-foil sectors fast to the plate, using shellac as the sticking medium. But first give one side of each sector a thin coat of shellac, allowing it to dry; then give it another coat when applying it to the glass. The sectors are to be symmetrically arranged on the glass, using a line of the pattern as a centre for each piece (as shown at A in Fig. 1), and the fourteen and six inch circles as the outer and inner boundaries. Each piece, as it is applied, should be pressed down upon the glass, so that it will stick smoothly, without air bubbles or creases. A very good plan is to lay a piece of soft blotting-paper over the sector and drive it down with a small squeegee-roller such as is used in photography, taking care, however, not to shift the sector from its proper position. When all the sectors are on, the plate should appear as shown in Fig. 2. After the shellac, which holds the sectors to the glass, is dry, run a brush full of shellac around the inner and outer extremities of the tin-foil strips for half or three-quarters of an inch in from the ends. The shellac will hold the sectors firmly to the glass, and will slightly insulate them as well, thereby preventing the escape of electricity. Apply the remaining sectors to the other plate of glass in a similar manner; and as a result two disks of glass, with the applied strips, will be ready to mount in the frame.
A hole three-quarters of an inch in diameter should be made in each glass plate, so that a three-eighths spindle may pass through them and into the bosses, so as to keep them in proper line. It is preferable, however, not to bore these holes if bosses and accurately bushed holes can be made in the uprights of the frame which support these disks.
At a wood-working mill have two bosses made that will measure four inches in diameter at the large end, and one inch and a half at the small one. They should be of such length that when the plates and two bosses are arranged in line (to appear as shown in A A at Fig. 9) they will fill the entire space between the uprights B B. Near the small end a groove is turned in each boss, so that a round leather belt will fit in it, as shown in Fig. 3.
The base is made from pine, white-wood, cypress, or any other wood that is soft and easily worked. It is composed of two strips twenty-four inches long, three inches wide, and one inch and a quarter in thickness, and two cross-pieces fourteen inches long, three inches wide, and one inch and a half thick.
These are put together with glue and screws, and at both sides of the base notches are cut to accommodate the feet of the uprights. The uprights are seventeen inches high, three inches wide, and one inch and a half thick. The notch at the foot of each one is cut so that, when fitted in place, the foot of the upright will rest on a table on a line with the bottom of the end cross-pieces under each corner. At the foot of the uprights a piece of sheet rubber should be made fast, with glue or shellac, so that when in operation the machine will not move about or slide.
A groove is cut at one side of each upright six inches above the bottom, as shown at Fig. 4 A. In this groove the driving-wheel axles fit, and near the top holes are made in the uprights through which the spindles pass, which in turn support the bosses and glass disks.
At the middle of each cross-piece forming the ends of the base a one-inch hole, for the glass standard rods, is bored through the wood, as shown at Fig. 4 B B. After attaching the uprights to the base with glue and screws, and giving all the wood-work several successive coats of shellac, the frame will be ready for its mountings.
The driving-wheels are of wood seven-eighths of an inch thick and seven inches in diameter; they should be turned on a lathe and a groove cut in the edge so that a round leather belt will fit in it. These wheels are mounted on a wooden axle that can be made from a round curtain-pole, with a half-inch hole bored through its entire length. The axle is as long as the distance between uprights B B in Fig. 9. The wheels are to be arranged and glued fast to the axle, so that the grooves will line directly under those in the bosses, as shown in Fig. 9. A half-inch axle is driven through the hub, and at one end it is threaded and provided with two washers and nuts; or a square shoulder and one washer and nut may be used, so that a crank and handle may be held fast. Shellac should be put on the shaft to make it hold fast in the hub.
The complete apparatus of wheels, axle, hub, and handle is shown at Fig. 5, and in the frame this is so hung that the iron axle rests in the grooves cut in the uprights. To hold this in place two metal straps, as shown in Fig. 6, are made and screwed fast to the wood. When finally adjusted the driving-wheels should rotate freely whenever the crank is turned. The wooden bosses, Fig. 3, are given two or three coats of shellac; then they are made fast to the glass disks on the same side to which the tin-foil sectors are attached. The disks should be placed over the paper plan, Fig. 1, and so adjusted that the outer line tallies with the large circle. Acetic glue[4] is then applied to the flat surface of the boss, and the latter is placed at the middle of the disk to line with the small circle. Place a weight on the end of the boss to hold it down, and leave it for ten or twelve hours or until thoroughly dry.
[4] See Formulæ, Chapter xiv., for the recipe of acetic glue.
Both bosses should be set at the same time so that they may dry together.
If the bosses are turned on a lathe a hole should be made in each one about half-way through from the small end. This, in turn, should be bushed or lined with a piece of brass tube, which should fit snugly in the hole. A little shellac painted on each piece of tube will make it stick. Pieces of steel rod that will just fit within the tubing are to be cut long enough to enter the full length of the hole, pass through the holes made in the top of the uprights, and extend half an inch beyond, as shown in Fig. 9. The bosses and axles will then appear as shown in Fig. 7.
Flat places should be filed on each rod where it passes through the wood upright; a set-screw will then hold it fast and keep it from revolving. When the hole, or tubing, is oiled so that the boss and disk will revolve freely on the axle, the disks, bosses, and axles are ready to be mounted in the frame.
A red fibre washer, such as is used in faucets, should be made fast to one glass disk at the centre, so as to separate the disks and prevent them from touching when they are revolving in opposite directions. These fibre washers can be had from a plumber or purchased at a hardware store. Shellac or acetic glue will hold the washers in place.
Mount one disk by holding the boss with the small end opposite a hole in one upright, and slip an axle through from the outside of the upright. Hold the other disk in place, and slip the remaining axle through the other upright and into the boss. When both plates are in place and centred, turn the set-screws down on the flattened axles to hold them in place.
To reduce the friction between the bosses and the uprights it would be well to place a fibre washer between them. A few drops of oil on these washers will lubricate them properly, and allow the machine to run easier. An end view of the apparatus, as so far assembled, will appear as shown in Fig. 9, A being the disks, bosses, and axles, B B the uprights supporting them, C the hub, and D D the driving-wheels. The handle and crank (E) extends out far enough from the side to allow a free swinging motion without hitting the upright or base. Having arranged these disks and wheels so as to revolve freely, it will now be necessary to construct and add the other vital parts and the connecting links in the chain of the complete working mechanism.
From a supply-house obtain two solid glass rods an inch in diameter and fifteen inches long. These fit in the holes (B B) bored in the end-pieces of the base, Fig. 4. Procure two brass balls, two or two and a half inches in diameter, such as come on brass beds, and two short pieces of brass tubing, one inch inside diameter, that will fit over the ends of the rods. These tubings are to be soldered fast to the balls so that both tubes and balls will remain at the top of the glass rods.
From brass rod three-sixteenths or a quarter of an inch in diameter make two forks, as shown at Fig. 8, and solder small brass balls at the ends of the rods. The prongs of the fork are six inches long and the shank four inches in length. Along the inside of the forks small holes are bored, and brass wires, or “points,” are soldered fast. These extend out for half an inch from the rods, and are known as the “comb,” or collectors. The forks should be so far apart that when mounted with the glass disks revolving between them the points will not touch or scratch the tin-foil sectors, and yet be as close to them as possible. A hole should be bored in the brass balls, and the shank of the fork passed through and soldered in place, as shown in Fig. 10.
A three-eighth-inch hole is bored directly in the top of each brass ball to hold the quadrant rods, which extend over the top of the disks.
In the illustration of the complete machine (Fig. 12) the arrangement of the glass pillars, balls, combs, and quadrant rods is shown. The rods are three-eighths of an inch in diameter and are loose in the holes at the top of the balls, so that they can be moved or shifted about, according as to whether it is a left or a right handed person who may be turning the crank.
At the upper end of each rod a brass ball is soldered, one being three-quarters of an inch in diameter, the other two inches. The projecting ends of the forks should be provided with metal handles or brass balls, as shown in Fig. 12; these may be slipped over the end or soldered fast. Obtain two small brass balls with shanks, such as screw on iron bed-posts, and have the extending ends of the axles that support the bosses threaded, so that the balls will screw on them. Bore a quarter-inch hole through each ball, and slip a brass rod through it and solder it fast. Each end of these rods should be tipped with a bunch of tinsel or fine copper wires. These are the “neutralizers,” and the ends are curved so that the brushes of fine wires will just touch the disks when the latter are revolved, as shown in Fig. 12. The ball holding the rod is to be screwed fast to the axle; then the axle is pushed back into the boss and made fast in the head of the upright with the set-screw.
The rod-and-ball at the opposite side of the disks is arranged in a similar manner, but the rod points in an opposite direction to that on the first side. Cord or leather belts connect the driving-pulleys and bosses, the belt on one side running up straight over the boss and down again around the driving-pulley. The belt at the opposite side is crossed, so that the direction of the boss is reversed; and in this manner the disks are made to revolve in opposite directions, although the driving-pulleys are both going in the same direction.
A portion of the sectors are omitted in the illustration (Fig. 12) so that a better view of the working parts may be had. When the disks are revolving the accumulated electricity discharges from one ball to the other, above the plates, in the form of bright blue sparks sufficiently powerful to puncture cardboard if it is held midway between the balls.
A Large Leyden-jar
When experimenting with this machine it would be well to have one or more Leyden-jars to accumulate static charges. A large one of considerable capacity is easily made from a battery jar, tin-foil, brass rods and chain, and some other small parts.
Obtain a bluestone battery jar, and after heating it to drive all moisture from the surface, give it a coat of shellac inside and out. With tin-foil, set with shellac, cover the bottom and inside of the jar for two-thirds of its height from the bottom, as shown in Fig. 11. Cover the outside and bottom in a similar manner, and the same height from the bottom, and provide a cork, or wooden cap, for the top. If a large, flat cork cannot be had, then make a stopper by cutting two circular pieces of wood, each half an inch thick, the inner one to fit snugly within the jar, the other to lap over the edges a quarter of an inch all around. Fasten these pieces together with glue, as shown at Fig. 13, and give them several good coats of shellac. Make a small hole at the middle of this cap and pass a quarter-inch rod through it, leaving six inches above and below the cap. To the top of the rod solder a brass ball. At the foot a piece of brass chain is to be made fast, so that several links of it rest on the tin-foil at the bottom of the jar.
To charge a jar from the Wimshurst machine, stand the jar on a glass-legged stool, and connect a copper wire between one of the overhead balls on the machine and the ball at the top of the rod in the stopper of the jar. Make another wire fast to the other ball at the top of the machine, and place it under the jar so that the tin-foil on the bottom touches it. By operating the machine the jar is charged.
To discharge the jar make a T-yoke, as shown at Fig. 14, by nailing a brass rod fast to a wooden handle and soldering brass knobs, or hammering a lead bullet, on the ends of the rod. Hold one knob against the top knob of the jar and bring the other near the foil coating at the outside, when a spark will jump from the foil to the knob with a loud snap.
A Glass-legged Stool
One of the most useful accessories in playing with frictional electricity will be a glass-legged stool. A stool with glass feet is perhaps too expensive for a boy to purchase, but one may be made at little or no cost from a piece of stout plank, four glass telegraph line-insulators, and the wooden screw-pins on which they rest when attached to a pole.
The general plan of the stool is shown at Fig. 15, and the top measures twelve by fifteen by two inches. Under each corner a screw-pin is made fast by boring a hole in the wood and setting the pin in glue. The pins are cut at the top, as shown in Fig. 16, and when they are set in place the glass insulators may be screwed on. The wood-work should be given a few coats of shellac to lend it a good appearance and help to insulate it.
There are a great many interesting experiments that may be performed with static or frictional electricity, and these may be looked up in the text-books on electricity used in school. A word of caution will not be misplaced. Remember that the current, in large volume, is dangerous. For example, a series of charged Leyden-jars may contain enough electricity to give a very severe shock to the nervous system of the person who chances to discharge it. Its medical use should be under the advice and supervision of a physician.
Chapter XIV
FORMULÆ
In the construction of electrical apparatus there are many things, such as paint, cement, non-conducting compounds, and acid-proof substances, that are necessary in assembling the parts which make up complete working outfits. Accurate formulas and directions for these things will save the amateur trouble and expense, since they indicate the materials which have been put to the test of time and wear by others who have had abundant experience along these lines.
The amateur will not need a large number of compounds, but such as are necessary should be of the best. Those which are described in this chapter can be relied upon to give working results.
Acid-proof Cements
One of the best acid-proof cements is made by adding shellac, dissolved in grain alcohol, to red-lead until it is at the right consistency. It can be used in liquid form or in a putty-like paste. The consistency is governed by the amount of shellac added to the red-lead. The lead should be pulverized and free from lumps. This cement can be mixed in a small tin cup or on a piece of glass, with a knife having a thin blade.
It should be used as soon as it is mixed, since it “sets” as quickly as shellac, and then dries from the outside towards the middle. In a week or two it will become dry and hard like stone.
Another cement, which will also dry as hard as a stone and will hold soapstone slabs together as if they were of one solid piece, is made of litharge (yellow lead) and glycerine. The glycerine is added to the pulverized litharge to make a paste, or it can be mixed and kneaded like thin putty. It should be used very soon after mixing, as it sets rapidly.
Hard Cement
A medium hard cement is made from plaster of Paris, six parts; silex, or fine sand, two parts; dextrine, two parts (by measure). Mix with water until soft; then work with a trowel or knife.
Soft Cement
A good soft cement is made of plaster of Paris, five parts; pulverized asbestos, five parts (by weight). Add water enough to make a soft paste, and use with a trowel or knife. This is a heat-proof compound and is commonly known as asbestos cement.
Very Hard Cement
One of the hardest cements that can be made is composed of hydraulic cement (Portland or Edison), five parts; silex, or white sand, five parts (by measure). Mix with water and use like plaster with a trowel or spatula.
Care must be taken when the parts are combined to see that the cement is free from lumps. These must be broken before the silex, or sand, and water are added. Then the two parts should be mixed together at first and moistened afterwards. The proper way is to place some water at the bottom of a pan; then add the dry mixture by the handfuls, sprinkling it around so that the water can enter into it without forming lumps. Keep adding and mixing until the mass is at the right consistency to work.
All these cements are acid-proof.
Clark’s Compound
For exterior insulation, where the parts are exposed to the weather, a superior compound is made up of mineral pitch, ten parts; silica, six parts; tar, one part (all parts by weight). This is called Clark’s compound, after the man who invented it and used it successfully.
It is heated, thoroughly mixed, and used with a brush or spatula.
Battery Fluid
A depolarizing solution for use in zinc-carbon batteries like the Grenet is composed as follows:
Dissolve one pound of bichromate potash or soda in ten pounds of water (by weight). When it is thoroughly dissolved add two and one-half pounds of sulphuric acid, slowly pouring it into the bichromate solution and stirring it with a glass rod. The addition of the acid will heat the solution. Do not use it until it has entirely cooled.
Glass Rubbing
To rub the edges of glass, such as the disks for Wimshurst machines, obtain a piece of hard sandstone, such as is used for sharpening knives or scythes. The glass is placed on a table so that the edge extends beyond. Oil of turpentine is rubbed or dropped on the surface of the stone, and the edge of the glass is moistened with a rag soaked in the turpentine. Hold the glass down securely with one hand, and with the other grasp the stone and give it a forward and backward motion, grinding the glass along its edge and not crosswise. With care and patience a rough edge can soon be brought to a smooth one, and a soft, rounded corner substituted for the hard, angular, cutting edge that makes the glass a difficult thing to handle. Use plenty of lubricant in the form of oil of turpentine to make the work easy.
Acetic Glue
One of the best glues for glass and wood or glass and fibre is made by placing some high-grade glue (either flake or granulated) in a cup or tin and covering it with cold water. Allow it to stand several hours until the glue absorbs all the water it will and becomes soft; then pour the water off, and add glacial acetic acid to cover the glue. The proportion should be eighteen parts of glue to two of acid. Heat the mass until it is reduced to liquid, stirring it until it is thoroughly mixed. When ready for use it should be poured into a bottle and well corked to keep the air away from it.
Insulators
Apart from glass and porcelain, insulators can be made from non-conducting compounds, the best of which is molded mica. Ground mica or mica dust is mixed with thick shellac until it is in a putty-like state. It may then be forced into oiled molds of any desired shape. Hydraulic pressure is employed for almost every form of molded mica that is made for commercial purposes; but as a boy cannot employ that means to get the best results, he must use all the pressure that his hands and a flat board will give.
Another compound is made from pulverized asbestos and shellac, with a small portion of ground or pulverized mica added, in the proportion of asbestos, six parts; mica, four parts. Shellac is added to make a pasty mass, which is kneaded into a thick putty and forced into oiled molds until it sets. It is then removed and allowed to dry in the open air, and the mold used for more insulators.
Non-conductors
When working in different materials that seem adapted to electrical apparatus, it is necessary to know whether they can be used safely or not. Often a material seems to be just the thing, but if it should be a partial conductor, when a non-conductor is desired, it would be hazardous to use it. A list of non-conductors is therefore valuable to the amateur. Some of the principal non-conductors, among the many, are as follows: glass, porcelain, slate, marble, hard stone, soapstone, concrete (dry), hard rubber, soft rubber, composition fibre, mica, asbestos, pitch, tar, shellac, cotton, silk; cotton, silk and woollen fabrics, transite (dry), electrobestus (dry), duranoid; celluloid, dry wood (well seasoned), paper, pith, leather, and oil.
While this account of formulæ and material might be extended, this is not necessary inasmuch as the formulæ and practical directions which have been given will answer all usual practical requirements.
Insulating Varnish
There are several good insulating varnishes that can be used in electrical work, the most valuable being shellac dissolved in alcohol and applied with a brush. To make good shellac, purchase the orange-colored flake shellac by the pound from a paint-store, place some of it in a wide-necked bottle, and cover it with alcohol; then cork the bottle and let it stand for a few hours. Shake the bottle occasionally until the shellac is thoroughly dissolved. It can be thinned by adding alcohol. Always keep the bottle corked, taking out only what is necessary from time to time.
Another varnish can be made by dissolving red sealing-wax in alcohol and adding a small portion of shellac. This can be applied with a soft brush, and is a good varnish. When colors are to be applied to distinguish the poles, red is used for the positive current-poles and blue or black for the negative, if they are colored at all.
A very good black varnish is made by adding lampblack to shellac; another consists of thick asphaltum or asphaltum varnish. This is waterproof, and dries hard, yet with an elastic finish.
Battery Wax
For the upper edges of glass cells, such as the Leclanché or other open-circuit batteries, there is nothing superior to hot paraffine brushed about the upper edge to prevent the sal-ammoniac or other fluids from creeping up over the top. The paraffine can be colored with red-lead, green dust, or powders of various colors if desired, but generally the paraffine is used without color, so that it has a frosted-glass appearance when it is cool and dry.
A black wax for use in stopping the tops of dry cells and coating the tops of carbons is composed of paraffine, eight parts; pitch, one part; lampblack, one part. Heat the mixture and stir it until thoroughly mixed; then apply with a brush, or dip the parts into the warm fluid.
Another good black wax is composed of tar and pitch in equal parts. They are made into a pasty mass with turpentine heated over a stove, but not over an open flame, because the ingredients are inflammable. The compound should be like very thick molasses, and can be worked with an old table-knife.
Chapter XV
ELECTRIC LIGHT, HEAT, AND POWER
For the use of the cuts in this chapter, the Publishers desire to acknowledge the courtesy of the General Electric Company, the Thomson Electric Welding Company, and the Cooper Hewitt Electric Company.
With the discovery of the reversibility of the dynamo, the invention of the telephone, and the improvements in the electric light began the great modern development of electricity which proved that marvellous agent to be a master-workman.
Many of the things electrical that we ordinarily think of as modern inventions are merely modern applications of phenomena that were discovered many years ago. The pioneers in the science of dynamic electricity performed their experiments with the electric light, electro-magnets, etc., by using galvanic batteries. But for practical purposes the consuming of zinc and chemicals in such batteries was too expensive a way to generate electricity, and prevented any commercial use of the results of their experiments until cheaper electricity could be had.
The Work of the Dynamo
The invention of the dynamo, with which we obtain electricity from mechanical power, changed all that. Instead of consuming zinc in primary batteries, men could obtain it by burning coal, which is much cheaper, under the boiler of a steam-engine used to drive the dynamo. Thus it is that modern electricity comes from mechanical power. It is really the energy of a steam-engine or a water-wheel, or some other “prime mover,” working through the medium of electricity, that is transmitted to a distance and distributed over wires. The electricity may then be transmuted into light, heat, or chemical energy as the case may be, to light our electric lamps, develop the intense heat of the electric furnace, and charge storage-batteries.
Moreover, some time after the invention of the dynamo it was found that the mechanical power put into one of these machines could be transmitted electrically and reproduced as mechanical power. In other words, a dynamo could be made to revolve and give out power, as a motor, by supplying it with current from another dynamo. This showed the way to transmute electricity back again into mechanical power, to run our electric cars and trains, and all kinds of machinery in our factories and elsewhere. Nowadays the dynamo is used to generate nearly all the electricity that we need. Even in such comparatively old electrical applications as electro-plating and the telegraph and telephone, primary batteries are being supplanted by motor dynamos, which we shall learn about later.
It is from the invention of the dynamo and the discovery that it was reversible that we date the beginning of what are known as heavy electrical engineering applications, including electric light, heat, and power. In this closing chapter it is purposed to learn a little about these applications, and in so doing to summarize briefly the things that we have already studied.
The Electric Light
In the chapter on Electrical Resistance we learned that an electric current always encounters a resistance in passing through a conductor, and that when the current is strong enough the conductor is heated up. The electric light is produced by the heating of a conductor of one kind or another to incandescence by the electrical friction of the current passing through it.
The first electric light was made by Sir Humphry Davy over a hundred years ago. He discovered that when a current from a great many cells of battery was interrupted the spark did not simply appear for an instant and then go out, as it does when only a few cells are used, but remained playing between the terminals of the circuit. He found by experiment that if pieces of carbon are used as the terminals—or “electrodes,” as they are called—the electricity passes between them in an intensely hot flame, or “arc.” The latter, which is due to the electrical resistance of the vapor of carbon, heats up the carbon-points so that they give a brilliant white light.
Before the dynamo came into use, the electric light was rarely seen, except as a philosophical experiment; but as soon as cheap electricity became available, commercial electric arc-lamps were made by many inventors and have been continually improved. Fig. 1 shows one form of modern arc-lamp, with its case removed to show the interior mechanism. In most arc-lamps the lamp itself consists of a pair of carbon or other electrodes in the form of long rods arranged vertically, with their tips normally in contact. When the current is turned on, the mechanism lifts the upper electrode away from the lower one. The interruption of the circuit thus caused “strikes the arc” between the tips, and the mechanism keeps the arc-distance unchanged as the carbons burn away. Some arc-lamps are made to burn on continuous-current, and others on alternating-current circuits. When continuous current is used, the upper (or positive) carbon burns away about twice as fast as the lower one, forming a cup, or “crater,” from which most of the light comes.
Uses of the Arc-Light
The first commercial use of the arc-light on a large scale was for street-lighting, to replace the old-fashioned gas-lamps. But another important use is in search-lights, in which the arc-lamp is fitted with a powerful reflector for throwing a very bright light to a distance. Fig. 2 is a view of a search-light arranged to go on top of a ship’s pilot-house. In war-time the ships carry search-lights to help them find the enemy’s ships and repel attack; and they are used in the army also, by having a portable dynamo and engine drawn by horses. The arc is also employed in projectors for lecture-rooms, and sometimes for the headlights of steam and electric locomotives and interurban electric cars.
Incandescent and Other Lamps
The arc-lamp came into wide use for lighting large spaces like streets, stores, and public halls, but was found to be too intense for lighting smaller places like private houses. After many experiments, Edison succeeded in subdividing the electric light into the small pear-shaped “incandescent” lamps that we now see everywhere. In this kind of electric lamp the light comes from a thin “filament” of carbon, contained in a glass globe from which all air has been removed. Since there is no oxygen to support combustion, the filament may be heated white-hot by the current without being consumed.
Fig. 3
In certain other forms of incandescent lamps that are just coming into use, the filaments are made of rare metals—osmium, tantalum, etc.—that will stand a high temperature without melting. The Nernst lamp has a filament consisting of a mixture of certain materials that has to be heated before it will conduct electricity.
Then there are the so-called “vapor” lamps, consisting of a glass tube full of conducting metallic vapor which gives out light when a current is passed through it. The best-known form is the Cooper Hewitt mercury vapor-lamp shown in Fig. 3, which gives a peculiar greenish light.
From the point of view of efficiency, the electric light, wonderful as it is, leaves much to be desired. The light always comes from a hot resistance; and whether this resistance is a mass of conducting vapor, as in the arc and vapor lamps, or a solid conducting filament, as in the so-called “incandescent” lamps, much more heat than light is produced. A needed improvement, therefore, is in the direction of obtaining a greater percentage of light for a given amount of electrical energy.
Electric Heat
The generation of heat in electrical devices usually means wasted energy—sometimes a very serious waste, as we have just seen. There are certain kinds of electrical apparatus, however, that are designed to transform all of the electrical energy delivered to them into heat, for various industrial and household purposes.