Chapter V.

ELECTRICAL APPARATUS. What is electricity?--Forms of electricity--Magnetism--The permanent magnet--Lines of force--Electro-magnets--The electric bell--The induction coil--The condenser--Transformation of current--Uses of the induction coil. WHAT IS ELECTRICITY? Of the ultimate nature of electricity, as of that of heat and light, we are at present ignorant. But it has been clearly established that all three phenomena are but manifestations of the energy pervading the universe. By means of suitable apparatus one form can be converted into another form. The heat of fuel burnt in a boiler furnace develops mechanical energy in the engine which the boiler feeds with steam. The engine revolves a dynamo, and the electric current thereby generated can be passed through wires to produce mechanical motion, heat, or light. We must remain content, therefore, with assuming that electricity is energy or motion transmitted through the ether from molecule to molecule, or from atom to atom, of matter. Scientific investigation has taught us how to produce it at will, how to harness it to our uses, and how to measure it; but not _what_ it is. That question may, perhaps, remain unanswered till the end of human history. A great difficulty attending the explanation of electrical action is this--that, except in one or two cases, no comparison can be established between it and the operation of gases and fluids. When dealing with the steam-engine, any ordinary intelligence soon grasps the principles which govern the use of steam in cylinders or turbines. The diagrams show, it is hoped, quite plainly "how it works." But electricity is elusive, invisible; and the greatest authorities cannot say what goes on at the poles of a magnet or on the surface of an electrified body. Even the existence of "negative" and "positive" electricity is problematical. However, we see the effects, and we know that if one thing is done another thing happens; so that we are at least able to use terms which, while convenient, are not at present controverted by scientific progress. FORMS OF ELECTRICITY. Rub a vulcanite rod and hold one end near some tiny pieces of paper. They fly to it, stick to it for a time, and then fall off. The rod was electrified--that is, its surface was affected in such a way as to be in a state of molecular strain which the contact of the paper fragments alleviated. By rubbing large surfaces and collecting the electricity in suitable receivers the strain can be made to relieve itself in the form of a violent discharge accompanied by a bright flash. This form of electricity is known as _static_. Next, place a copper plate and a zinc plate into a jar full of diluted sulphuric acid. If a wire be attached to them a current of electricity is said to _flow_ along the wire. We must not, however, imagine that anything actually moves along inside the wire, as water, steam, or air, passes through a pipe. Professor Trowbridge says,[11] "No other agency for transmitting power can be stopped by such slight obstacles as electricity. A thin sheet of paper placed across a tube conveying compressed air would be instantly ruptured. It would take a wall of steel at least an inch thick to stand the pressure of steam which is driving a 10,000 horse-power engine. A thin layer of dirt beneath the wheels of an electric car can prevent the current which propels the car from passing to the rail, and then back to the power-house." There would, indeed, be a puncture of the paper if the current had a sufficient voltage, or pressure; yet the fact remains that _current_ electricity can be very easily confined to its conductor by means of some insulating or nonconducting envelope. MAGNETISM. The most familiar form of electricity is that known as magnetism. When a bar of steel or iron is magnetized, it is supposed that the molecules in it turn and arrange themselves with all their north-seeking poles towards the one end of the bar, and their south-seeking poles towards the other. If the bar is balanced freely on a pivot, it comes to rest pointing north and south; for, the earth being a huge magnet, its north pole attracts all the north-seeking poles of the molecules, and its south poles the south-seeking poles. (The north-_seeking_ pole of a magnet is marked N., though it is in reality the _south_ pole; for unlike poles are mutually attractive, and like poles repellent.) There are two forms of magnet--_permanent_ and _temporary_. If steel is magnetized, it remains so; but soft iron loses practically all its magnetism as soon as the cause of magnetization is withdrawn. This is what we should expect; for steel is more closely compacted than iron, and the molecules therefore would be able to turn about more easily.[12] It is fortunate for us that this is so, since on the rapid magnetization and demagnetization of soft iron depends the action of many of our electrical mechanisms. THE PERMANENT MAGNET. Magnets are either (1) straight, in which case they are called bar magnets; or (2) of horseshoe form, as in Figs. 50 and 51. By bending the magnet the two poles are brought close together, and the attraction of both may be exercised simultaneously on a bar of steel or iron. LINES OF FORCE. In Fig. 50 are seen a number of dotted lines. These are called _lines of magnetic force_. If you lay a sheet of paper on a horseshoe magnet and sprinkle it with iron dust, you will at once notice how the particles arrange themselves in curves similar in shape to those shown in the illustration. It is supposed (it cannot be _proved_) that magnetic force streams away from the N. pole and describes a circular course through the air back to the S. pole. The same remark applies to the bar magnet. ELECTRICAL MAGNETS. [Illustration: FIG. 50.--Permanent magnet, and the "lines of force" emanating from it.] If an insulated wire is wound round and round a steel or iron bar from end to end, and has its ends connected to the terminals of an electric battery, current rotates round the bar, and the bar is magnetized. By increasing the strength and volume of the current, and multiplying the number of turns of wire, the attractive force of the magnet is increased. Now disconnect the wires from the battery. If of iron, the magnet at once loses its attractive force; but if of steel, it retains it in part. Instead of a simple horseshoe-shaped bar, two shorter bars riveted into a plate are generally used for electromagnets of this type. Coils of wire are wound round each bar, and connected so as to form one continuous whole; but the wire of one coil is wound in the direction opposite to that of the other. The free end of each goes to a battery terminal. In Fig. 51 you will notice that some of the "lines of force" are deflected through the iron bar A. They pass more easily through iron than through air; and will choose iron by preference. The attraction exercised by a magnet on iron may be due to the effort of the lines of force to shorten their paths. It is evident that the closer A comes to the poles of the magnet the less will be the distance to be travelled from one pole to the bar, along it, and back to the other pole. [Illustration: FIG. 51.--Electro-magnet: A, armature; B, battery.] Having now considered electricity in three of its forms--static, current, and rotatory--we will pass to some of its applications. THE ELECTRIC BELL. A fit device to begin with is the Electric Bell, which has so largely replaced wire-pulled bells. These last cause a great deal of trouble sometimes, since if a wire snaps it may be necessary to take up carpets and floor-boards to put things right. Their installation is not simple, for at every corner must be put a crank to alter the direction of the pull, and the cranks mean increased friction. But when electric wires have once been properly installed, there should be no need for touching them for an indefinite period. They can be taken round as many corners as you wish without losing any of their conductivity, and be placed wherever is most convenient for examination. One bell may serve a large number of rooms if an _indicator_ be used to show where the call was made from, by a card appearing in one of a number of small windows. Before answering a call, the attendant presses in a button to return the card to its normal position. In Fig. 52 we have a diagrammatic view of an electric bell and current. When the bell-push is pressed in, current flows from the battery to terminal T^1, round the electro-magnet M, through the pillar P and flat steel springs S and B, through the platinum-pointed screw, and back to the battery through the push. The circulation of current magnetizes M, which attracts the iron armature A attached to the spring S, and draws the hammer H towards the gong. Just before the stroke occurs, the spring B leaves the tip of the screw, and the circuit is broken, so that the magnet no longer attracts. H is carried by its momentum against the gong, and is withdrawn by the spring, until B once more makes contact, and the magnet is re-excited. The hammer vibrations recur many times a second as long as the push is pressed in. [Illustration: FIG. 52.--Sketch of an electric-bell circuit.] The electric bell is used for so many purposes that they cannot all be noted. It plays an especially important part in telephonic installations to draw the attention of the subscribers, forms an item in automatic fire and burglar alarms, and is a necessary adjunct of railway signalling cabins. THE INDUCTION OR RUHMKORFF COIL. Reference was made in connection with the electrical ignition of internal-combustion engines (p. 101) to the _induction coil_. This is a device for increasing the _voltage_, or pressure, of a current. The two-cell accumulator carried in a motor car gives a voltage (otherwise called electro-motive force = E.M.F.) of 4·4 volts. If you attach a wire to one terminal of the accumulator and brush the loose end rapidly across the other terminal, you will notice that a bright spark passes between the wire and the terminal. In reality there are two sparks, one when they touch, and another when they separate, but they occur so closely together that the eye cannot separate the two impressions. A spark of this kind would not be sufficiently hot to ignite a charge in a motor cylinder, and a spark from the induction coil is therefore used. [Illustration: FIG. 53.--Sketch of an induction coil.] We give a sketch of the induction coil in Fig. 53. It consists of a core of soft iron wires round which is wound a layer of coarse insulated wire, denoted by the thick line. One end of the winding of this _primary_ coil is attached to the battery, the other to the base of a hammer, H, vibrating between the end of the core and a screw, S, passing through an upright, T, connected with the other terminal of the battery. The action of the hammer is precisely the same as that of the armature of an electric bell. Outside the primary coil are wound many turns of a much finer wire completely insulated from the primary coil. The ends of this _secondary_ coil are attached to the objects (in the case of a motor car, the insulated wire of the sparking-plug and a wire projecting from its outer iron casing) between which a spark has to pass. As soon as H touches S the circuit is completed. The core becomes a powerful magnet with external lines of force passing from one pole to the other over and among the turns of the secondary coil. H is almost instantaneously attracted by the core, and the break occurs. The lines of force now (at least so it is supposed) sink into the core, cutting through the turns of the "secondary," and causing a powerful current to flow through them. The greater the number of turns, the greater the number of times the lines of force are cut, and the stronger is the current. If sufficiently intense, it jumps any gap in the secondary circuit, heating the intermediate air to a state of incandescence. THE CONDENSER. The sudden parting of H and S would produce strong sparking across the gap between them if it were not for the condenser, which consists of a number of tinfoil sheets separated by layers of paraffined paper. All the "odd" sheets are connected with T, all the "even" with T^1. Now, the more rapid the extinction of magnetism in the core after "break" of the primary circuit, the more rapidly will the lines of force collapse, and the more intense will be the induced current in the secondary coil. The condenser diminishes the period of extinction very greatly, while lengthening the period of magnetization after the "make" of the primary current, and so decreasing the strength of the reverse current. TRANSFORMATION OF CURRENT. The difference in the voltage of the primary and secondary currents depends on the length of the windings. If there are 100 turns of wire in the primary, and 100,000 turns in the secondary, the voltage will be increased 1,000 times; so that a 4-volt current is "stepped up" to 4,000 volts. In the largest induction coils the secondary winding absorbs 200-300 miles of wire, and the spark given may be anything up to four feet in length. Such a spark would pierce a glass plate two inches thick. It must not be supposed that an induction coil increases the _amount_ of current given off by a battery. It merely increases its pressure at the expense of its volume--stores up its energy, as it were, until there is enough to do what a low-tension flow could not effect. A fair comparison would be to picture the energy of the low-tension current as the momentum of a number of small pebbles thrown in succession at a door, say 100 a minute. If you went on pelting the door for hours you might make no impression on it, but if you could knead every 100 pebbles into a single stone, and throw these stones one per minute, you would soon break the door in. Any intermittent current can be transformed as regards its intensity. You may either increase its pressure while decreasing its rate of flow, or _amperage_; or decrease its pressure and increase its flow. In the case that we have considered, a continuous battery current is rendered intermittent by a mechanical contrivance. But if the current comes from an "alternating" dynamo--that is, is already intermittent--the contact-breaker is not needed. There will be more to say about transformation of current in later paragraphs. USES OF THE INDUCTION COIL. The induction coil is used--(1.) For passing currents through glass tubes almost exhausted of air or containing highly rarefied gases. The luminous effects of these "Geissler" tubes are very beautiful. (2.) For producing the now famous X or Röntgen rays. These rays accompany the light rays given off at the negative terminal (cathode) of a vacuum tube, and are invisible to the eye unless caught on a fluorescent screen, which reduces their rate of vibration sufficiently for the eye to be sensitive to them. The Röntgen rays have the peculiar property of penetrating many substances quite opaque to light, such as metals, stone, wood, etc., and as a consequence have proved of great use to the surgeon in localizing or determining the nature of an internal injury. They also have a deterrent effect upon cancerous growths. (3.) In wireless telegraphy, to cause powerful electric oscillations in the ether. (4.) On motor cars, for igniting the cylinder charges. (5.) For electrical massage of the body. [11] "What is Electricity?" p. 46. [12] If a magnetized bar be heated to white heat and tapped with a hammer it loses its magnetism, because the distance between the molecules has increased, and the molecules can easily return to their original positions.