Every axle of a railway train carries a wheel at each end, rigidly attached to it. When rounding a corner the outside wheel has further to travel than the other, and consequently one or both wheels must slip.
The curves are made so gentle, however, that the amount of slip is very small. But with a traction-engine, motor car, or tricycle the case is different, for all have to describe circles of very small diameter in proportion to the length of the vehicle. Therefore in every case a _compensating gear_ is fitted, to allow the wheels to turn at different speeds, while permitting them both to drive. Fig. 49 is an exaggerated sketch of the gear. The axles of the moving wheels turn inside tubes attached to the springs and a central casing (not shown), and terminate in large bevel-wheels, C and D. Between these are small bevels mounted on a shaft supported by the driving drum. If the latter be rotated, the bevels would turn C and D at equal speeds, a.s.suming that both axles revolve without friction in their bearings. We will suppose that the drum is turned 50 times a minute. Now, if one wheel be held, the other will revolve 100 times a minute; or, if one be slowed, the other will increase its speed by a corresponding amount. The _average_ speed remains 50. It should be mentioned that drum A has incorporated with it on the outside a bevel-wheel (not shown) rotated by a smaller bevel on the end of the propeller shaft.
THE SILENCER.
The petrol-engine, as now used, emits the products of combustion at a high pressure. If unchecked, they expand violently, and cause a partial vacuum in the exhaust pipe, into which the air rushes back with such violence as to cause a loud noise. Devices called _silencers_ are therefore fitted, to render the escape more gradual, and split it up among a number of small apertures. The simplest form of silencer is a cylindrical box, with a number of finely perforated tubes pa.s.sing from end to end of it. The exhaust gases pouring into the box maintain a constant pressure somewhat higher than that of the atmosphere, but as the gases are escaping from it in a fairly steady stream the noise becomes a gentle hiss rather than a "pop." There are numerous types of silencers, but all employ this principle in one form or another.
THE BRAKES.
Every car carries at least two brakes of band pattern--one, usually worked by a side hand-lever, acting on the axle or hubs of the driving-wheel; the other, operated by the foot, acting on the transmission gear (see Fig. 48). The latter brake is generally arranged to withdraw the clutch simultaneously. Tests have proved that even heavy cars can be pulled up in astonishingly short distances, considering their rate of travel. Trials made in the United States with a touring car and a four-in-hand coach gave 25-1/3 and 70 feet respectively for the distance in which the speed could be reduced from sixteen miles per hour to zero.
SPEED OF CARS.
As regards speed, motor cars can rival the fastest express trains, even on long journeys. In fact, feats performed during the Gordon-Bennett and other races have equalled railway performances over equal distances.
When we come to record speeds, we find a car, specially built for the purpose, covering a mile in less than half a minute. A speed of over 120 miles an hour has actually been reached. Engines of 150 h.p. can now be packed into a vehicle scaling less than 1-1/2 tons. Even on touring cars are often found engines developing 40 to 60 h.p., which force the car up steep hills at a pace nothing less than astonishing. In the future the motor car will revolutionize our modes of life to an extent comparable to the changes effected by the advent of the steam-engine. Even since 1896, when the "man-with-the-flag" law was abolished in the British Isles, the motor has reduced distances, opened up country districts, and generally quickened the pulses of the community in a manner which makes it hazardous to prophesy how the next generation will live.
_Note._--The author is much indebted to Mr. Wilfrid J. Lineham, M. Inst.
C.E., for several of the ill.u.s.trations which appear in the above chapter.
[8] Steam-driven cars are not considered in this chapter, as their principle is much the same as that of the ordinary locomotive.
[9] On some cars natural circulation is used, the hot water flowing from the top of the cylinder to the tank, from which it returns, after being cooled, to the bottom of the cylinder.
[10] For explanation of the induction coil, see p. 122
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 pa.s.sed through wires to produce mechanical motion, heat, or light. We must remain content, therefore, with a.s.suming 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, pa.s.ses 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 pa.s.sing 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 ill.u.s.tration. 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.
[Ill.u.s.tration: 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 pa.s.s 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.
[Ill.u.s.tration: FIG. 51.--Electro-magnet: A, armature; B, battery.]
Having now considered electricity in three of its forms--static, current, and rotatory--we will pa.s.s 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 b.u.t.ton 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.
[Ill.u.s.tration: 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.