The Story of Electricity

nion, and next by the finer agency of heat. In all these, it will be observed, a substantial contact is necessary. We have now to consider a still more s

several thousand years ago, and certainly to the Greeks in the times of Thales, who,

chy, and perchance its property of pointing out the north and south direction was discovered by dropping a light piece of the stone, if not a sewing needle made of it, on the surface of still water. At all events, we read in Pere Du Halde's Description de la Chine, that sometim

dress who pointed to the south, and was doubtless moved by a magnet floating in water or turning on a pivot. This rude appliance was afterward

inor. It was called the "Heraclean Stone" by the people, but came at length to bear the name of "Magnet" after the ci

n experiment in which iron filings are made to rise up and "rave" in a brass basin by a magnet held underneath. We are told by other writers that images of the gods and goddesses were suspended in the air by lodestone in the

natural magnet of this kind is shown in figure 25, where L is the stone shod with two iron "pole-pieces

minstrel at the court of Barbarossa, which was written in or about the year 890, contains the first mention of the magnet in the West. Guiot relates how mariners have an "art which cannot deceive" of finding the position of the polestar, that does not move. After touching a needle with the magnet, "an ugly brown stone which draws iron to itself," he says they put the needle on a straw and float it on water so that its point turns to the hidden star, and enables th

certainly known in China during the twelfth, and in Europe during the thirteenth century. Columbus also found that the variation changed its value as he sailed towards A

trated beyond a doubt that the whole earth was a great magnet. A magnet, as is well known, has, like an electric battery, always two poles or centres of attraction, which are situated near its extremities. Sometimes, indeed, when the magnet is imperfect,

of his knife is only efficacious when the blade itself becomes magnetic after being touched with the magnet. A piece of steel is readily magnetis

s a south pole, while, on the other hand, a south pole always repels a south pole and attracts a north pole. This can be proved by suspending a magnetic needle like a pithball, and approaching another towards it, as illustrated in fi

unlike the north and like the south magnetic pole of the earth. Instead of calling it t

hey do not, and, moreover, they are slowly moving round the geographical poles, hence the declination of the needle, that is to say its angle of divergence from the true meridian or north and south line, is gradually changing. The north magnetic pole of the earth was actually discovered by Sir John Ross north of British A

hat masses of lodestone or magnetic iron exist in the crust. Coulomb found that not only iron, but all substances are more or less magnetic, and Fara

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ind as the south and north-magnetic poles of the earth. If we break each of these again, we get four smaller magnets, and we can repeat the process as often as we like. It is supposed, therefore, that every atom of the bar is a little magnet in itself having its two opposite poles, and that in magne

a piece of soft iron we can destroy its magnetism by striking it so as to agitate its atoms and throw them out of line. In steel, which is iron with a small admixture of carbon, the atoms are not so free as in soft iron, and hence, while ir

n the ranks to take up their new positions. Even if this molecular theory is true, however, it does not help us to explain why a molecule of matter is a tiny magnet. We have only pushed the mystery back to the atom. Something more is wanted,

e arrangement of the poles. In the horse- shoe magnet, as seen, they chiefly issue from one pole and sweep round to the other. They are never broken, and apparently they are lines of stress in the circumambient ether. A pivoted magnet tends to range itself along these lines, and thus the compass guides the sailor on the ocean by keeping itself in the line between the north and south magnetic poles of the earth. Faraday called them lines of magnetic force, and said that the stron

clue, and a happy chance is said to have rewarded him. His experiment is shown in figure 29, where a wire conveying a current of electricity flowing in the direction of the arrow is held over a pivoted magnetic needle so that the current flows from south to north. The needle will tend to set itself at right angles to the wire, its north or nor

a galvanoscope, as shown in figure 30, where N is the needle and B the bobbin. When a proper scale is added to the needle by which its deflections can be accurately read,

f you face the needle you will see its north pole go to the left and its south pole to the right. I find it simpler to recollect that if the current flows from your head to your

tion of a wire carrying a current, with the iron filings arranged in circles round it. Since a magnetic pole tends to move in the direction of the lines of force, we now see why a north or south pole tends to move ROUND a current, and why a compass needle tries to set itself at right angles to a current, as in

nd repelled each other when they flowed in opposite directions. Thus, in figure 32, if A and B are the two parallel wires, and A is mounted on pivots and free to move in liquid "contacts" of mercury, it will be attracted or repelled by B according as the two currents flow in the same or in opposite directions. If the wires cross each other at right angles there is no attraction or repulsion. If they cross at an acute angle, they will tend to become parallel like two compass

rents, such as the spiral S in figure 33 gives, when connected to a battery C Z, is in fact a skeleton ELECTRO-MAGNET having its north and south poles at the extremities. If a rod or core of soft iron I be suspended by fibres

er magnetised in the same way, and supporting nails at both ends. The poker has become the core of the electromagnet. On reversing the direction of the current through the spiral we reverse the pole

a current is STARTED in the wire B (fig. 32) in direction of the arrow, it will induce or give rise to a momentary current in the wire A, flowing in a contrary direction to itself. Again, if the current in B be STOPEED, a momentary current is set up in the wire A in a direction the same as that of the excitin

ed automatically by the magnetism of the iron core I C, for the hammer H has a soft iron head which is attracted by the core when the latter is magnetised, and being thus drawn away from the contact screw C S the circuit of the primary is broken, and the current is stopped. The iron core then ceases to be a magnet, the hammer H springs back to the contact screw, and the current again flows in the primary circuit only to be interrupted again as before. In this way the current in the primary coil is rapidly started and stopped many times a second, and this, as we know, induces corresponding currents in the secondary which appear as sparks at the discharging points. The effect of the apparatus is enhanced by interpolating a "condenser" C C in the primary circuit. A condenser is a form of Leyden jar, suitable for current electricity, and consists of layers

ttern, with its two coils, one over the other C, its commutator R, and its sparklin

n sent through glass tubes filled with rarefied gases, sometimes called "Geissler tubes," they elicit glows of many colours, vieing in be

ovoke a current of low pressure in the primary coil The transformer or converter, a modified induction coil used in distributing electricity to electric lamps and motors, can not only transform a low pressure current into a high, but a high pressure current into a low. As the high pressure currents are b

f electricity is set up in the coil whilst the motion lasts. When the magnet is withdrawn again another current is induced in the reverse direction to the first. If the coil be closed through a small galvanometer G the movements of the needle to one side or the other will indicate these temporary currents. It follows from the principle of action and reaction that if the magnet is kept still and the

to sink bodily through the paper away from the reader, an electric current flowing in the direction of the arrow will be induced in it. If, on the contrary, the wire be moved across the lines of force towards the reader, the induced current will flow

, that is to say, in the direction it would need to be moved in order to excite such a current; and if, on the other hand, the current be sent through it in the reverse directio

the direction in which the wire or conductor moves and the bent middle finger the direction of the current. These three digits, as will be noticed, are all at right

faster it is moved across the lines of force, that is to say, the more lines it cuts in a second the stronger is the curre

n a strong magnetic field between the poles of powerful magnets. Currents are generated in the coils, now in one direction then in another, as they revolve or cross different parts of the field; and, by means of a device termed a commutator

a second, or it may sift the currents as they are produced and supply what is termed a continuous current, that is, a current always in the same direction, like that of a voltaic battery. Some machines are ma

ield of a powerful electro-magnet NS in the direction of the arrows. For the sake of simplicity only twelve coils are represented. They are all in circuit one with another, and a wire connects the ends of each coil to corresponding metal bars on the commutator C. These bars are insulated from each other on the spindle X of the armature. Now, as each coil passes through the magnetic field in turn, a current is excited in it. Each coil therefore resembles an individual cell of a voltaic battery, connected in series. The current

the magnets in which the armature revolves is merely that due to the dregs or "residual magnetism" left in the soft iron cores of the magnet since the last time the machine was used. But this feeble current exalts the streng

g the armature revolving between the poles NS of the field-magnets M, M, M' M', on a spindle which is driven by means of a belt on the pulley P

h of the current yielded by the machine, because the current, weakened by the increase of resistance, fails to excite the field magnets as strongly as before. On the other hand, with glow lamps arranged in parallel, the reverse is the case, and putting more lamps in circuit increases the power of the machine, by diminishing the resistance of the outer circuit in providing more cross-cuts for the current. This, of course, is a drawback

ce than before. When fewer glow lamps are in the external circuit E, and its resistance therefore higher, the current in the shunt circuit M is greater than before, the magnets become stronger, and the electromotive force of the armature is increased. The Edison machine is of this type, and is illustrated in figure 43, where M M' a

es of both the series and shunt machines. This is the "compound-wound" machine, in which the magnets are wound partly in shunt and partly in series with the armature, in such a manner that the strength of the field-magnets and the electromot

ss a magnetic field, will move. The electric motor is therefore essentially a dynamo, which on being traversed by an electric current from an external source puts itself in motion. Thus, if a curren

e wheels of carriages, and for many other purposes. There are numerous types of electric motor as of th

anical power by the motor, it is sufficient to connect a dynamo and motor together by insulated wire in orde