The Story of Electricity Part 3

Figure 36 represents the exterior of an ordinary induction coil of the Ruhmkorff pattern, with its two coils, one over the other C, its commutator R, and its sparkling points D1D2, the whole being mounted on a mahogany base, which holds the condenser.

The intermittent, or rather alternating, currents from the secondary coil are often applied to the body in certain nervous disorders. When sent through gla.s.s tubes filled with rarefied gases, sometimes called "Geissler tubes," they elicit glows of many colours, vieing in beauty with the fleeting tints of the aurora polaris, which, indeed, is probably a similar effect of electrical discharges in the atmosphere.

The action of the induction is reversible. We can not only send a current of low "pressure" from a generator of weak electromotive force through the primary coil, and thus excite a current of high pressure in the secondary coil, but we can send a current of high pressure through the secondary coil and provoke 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 best able to overcome the resistance of the wire convening them, it is customary to transmit high pressure currents from the generator to the distant place where they are wanted by means of small wires, and there transform them into currents of the pressure required to light the lamps or drive the motors.

We come now to another consequence of Oersted's great discovery, which is doubtless the most important of all, namely, the generation of electricity from magnetism, or, as it is usually called, magneto-electric induction. In the year 1831 the ill.u.s.trious Michael Faraday further succeeded in demonstrating that when a magnet M is thrust into a hollow coil of wire C, as shown in figure 37, a current of 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 When the magnet is withdrawn again another current is induced in the reverse direction to the first.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 coil thrust over it similar currents will be induced in the coil. All that is necessary is for the wires to cut the lines of magnetic force around the magnet, or, in other words, the lines of force in a magnetic field We have seen already that a wire conveying a current can move a magnetic pole, and we are therefore prepared to find that a magnetic pole moved near a wire can excite a current in it.

Figure 38 ill.u.s.trates the conditions of this remarkable effect, where N and S are two magnetic poles with lines of force between them, and W is a wire crossing these lines at right angles, which is the best position. If, now, this wire be moved so as 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 oppositely to the arrow. Moreover, if the poles of the magnet N and S exchange places, the directions of the induced currents will also be reversed. This is the fundamental principle of the well known dynamo-electric machine, popularly called a dynamo.

Again, if we send a current from some external source through the wire in the direction of the arrow, the wire will move OF ITSELF across the lines of force away from the reader, 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 direction to the arrow, it will move towards the reader. This is the principle of the equally well-known electric motor. Figure 39 shows a simple method of remembering these directions.

Let the right hand rest on the north pole of a magnet and the forefinger be extended in the direction of the lines of force, then the outstretched thumb will indicate 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 angles to each other, and this relation is the best for inducing the strongest current in a dynamo or the most energetic movement of the conductor in an electric motor.

Of course in a dynamo-electric generator the stronger the magnetic field, the less the resistance of the conductor, and the 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 current produced.

Similarly in an electric motor, the stronger the current and magnetic field the faster will the conductor move.

The most convenient motion to give the conductor in practice is one of rotation, and hence the dynamo usually consists of a coil or series of coils of insulated wire termed the "armature," which is mounted on a spindle and rapidly rotated in 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, these currents can be collected or sifted at will, and led away by wires to an electric lamp, an acc.u.mulator, or an electric motor, as desired. The character of the electricity is precisely the same as that generated in the voltaic battery.

The commutator may only collect the currents as they are generated, and supply what is called an alternating current, that is to say, a current which alternates or changes its direction several hundred times 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 made to supply alternating currents, others continuous currents. Either cla.s.s of current will do for electric lamps, but only continuous currents are used for electo-plating, or, in general, for electric motors.

In the "magneto-electric" machine the FIELD MAGNETS are simply steel bars permanently magnetised, but in the ordinary dynamo the field magnets are electro-magnets excited to a high pitch by means of the current generated in the moving conductor or armature. In the "series-wound" machine the whole of the current generated in the armature also goes through the coils of the field magnets.

Such a machine is sketched in figure 40, where A is the armature, consisting of an iron core surrounded by coils of wire and rotating in the field 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 pa.s.ses 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 is drawn off from the ring by two copper "brushes" b, be which rub upon the bars of the commutator at opposite ends of a diameter, as shown. One brush is the positive pole of the dynamo, the other is the negative, and the current will flow through any wire or external circuit which may be connected with these, whether electric lamps, motors, acc.u.mulators, electro-plating baths, or other device. The small arrows show the movements of the current throughout the machine, and the terminals are marked (+) positive and (-) negative.

It will be observed that the current excited in the armature also flows through the coils of the electro-magnets, and thus keeps up their strength. When the machine is first started the current is feeble, because the field of 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 strength of the field-magnets, producing a stronger field, which in turn excites a still stronger current in the armature, and this process of give and take goes on until the full strength or "saturation"

of the magnets is attained.

Such is the "series" dynamo, of which the well-known Gramme machine is a type. Figure 41 ill.u.s.trates this machine as it is actually made, A being 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 from a separate engine The brushes b b' of the commutator C collect the current, which in this case is continuous, or constant in its direction.

The current of the series machine varies with the resistance of the external or working circuit, because that is included in the circuit of the field magnets and the armature. Thus, if we vary the number of electric lamps fed by the machine, we shall vary the current it is capable of yielding. With arc lamps in series, by adding to the number in circuit we increase the resistance of the outer circuit, and therefore diminish the strength 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 diminis.h.i.+ng the resistance of the outer circuit in providing more cross-cuts for the current. This, of course, is a drawback to the series machine in places where the number of lamps to be lighted varies from time to time. In the "shunt-wound" machine the field magnets are excited by diverting a small portion of the main current from the armature through them, by means of a "shunt" or loop circuit. Thus in figure 42 where C is the commutator and b b' the brushes, M is a shunt circuit through the magnets, and E is the external or working circuit of the machine.

The small arrows indicate the directions of the currents. With this arrangement the addition of more glow lamps to the external circuit E DIMINISHES the current, because the portion of it which flows through the by-path M, and excites the magnets, is less now that the alternative route for the current through E is of lower resistance 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 ill.u.s.trated in figure 43, where M M' are the field magnets with their poles N S, between which the armature A is revolved by means of the belt B, and a pulley seen behind. The leading wires W W convey the current from the brushes of the commutator to the external circuit. In this machine the conductors of the armature are not coils of wire, but separate bars of copper.

In shunt machines the variation of current due to a varying number of lamps in use occasions a rise and fall in the brightness of the lamps which is undesirable, and hence a third cla.s.s of dynamo has been devised, which combines the principles 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 electromotive force of the current do not vary much, whatever be the number of lamps in circuit. In alternate current machines the electromotive force keeps constant, as the field- magnets are excited by a separate machine, giving a continuous current.

We have already seen that the action of the dynamo is reversible, and that just as a wire moved across a magnetic field supplies an electric current, so a wire at rest, but conducting a current across 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 current be sent through the armature of the Gramme machine, shown in figure 41, the armature will revolve, and the spindle, by means of a belt on the pulley P, can communicate its energy to another machine.

Hence the electric motor can be employed to work lathes, hoists, lifts, drive the screws of boats or the wheels of carriages, and for many other purposes. There are numerous types of electric motor as of the dynamo in use, but they are all modifications of the simple continuous or alternating current dynamo.

Obviously, since mechanical power can be converted into electricity by the dynamo, and reconverted into mechanical power by the motor, it is sufficient to connect a dynamo and motor together by insulated wire in order to transmit mechanical power, whether it be derived from wind, water, or fuel, to any reasonable distance.

CHAPTER V.

ELECTROLYSIS.

Having seen how electricity can be generated and stored in considerable quant.i.ty, let us now turn to its practical uses. Of these by far the most important are based on its property of developing light and heat as in the electric spark, chemical action as m the voltameter, and magnetism as in the electromagnet.

The words "current," "pressure," and so on point to a certain a.n.a.logy between electricity and water, which helps the imagination to figure what can neither be seen nor handled, though it must not be traced too far. 'Water, for example, runs by the force of gravity from a place of higher to a place of lower level. The pressure of the stream is greater the more the difference of level or "head of water" The strength of the current or quant.i.ty of water flowing per second is greater the higher the pressure, and the less the resistance of its channel. The power of the water or its rate of doing mechanical work is greater the higher the pressure and the stronger the current. So, too, electricity flows by the electromotive force from a place of higher to a place of lower electric level or potential. The electric pressure is greater the more the difference of potential or electromotive force. The strength of the electric current or quant.i.ty of electricity flowing per second is greater the higher the pressure or electromotive force and the less the resistance of the circuit The power of the electricity or its rate of doing work is greater the higher the electromotive force and the stronger the current.

It follows that a small quant.i.ty of water or electricity at a high pressure will give us the same amount of energy as a large quant.i.ty at a low pressure, and our choice of one or the other will depend on the purpose we have in view. As a rule, however, a large current at a comparatively low or moderate pressure is found the more convenient in practice.

The electricity of friction belongs to the former category, and the electricity of chemistry, heat, and magnetism to the latter.

The spark of a factional or influence machine can be compared to a highland cataract of lofty height but small volume, which is more picturesque than useful, and the current from a voltaic battery, a thermopile, or a dynamo to a lowland river which can be dammed to turn a mill. It is the difference between a skittish gelding and a tame carthorse.

Not the spark from an induction coil or Leyden jar, but a strong and steady current at a low pressure, is adapted for electrolysis or electrodeposition, and hence the voltaic battery or a special form of dynamo is usually employed in this work. A flash of lightning is the very symbol of terrific power, and yet, according to the ill.u.s.trious Faraday, it contains a smaller amount of electricity than the feeble current required to decompose a single drop of rain.

In our simile of the mill dam and the battery or dynamo, the dam corresponds to the positive pole and the river or sea below the mill to the negative pole. The mill-race will stand for the wire joining the poles, that is to say, the external circuit, and the mill-wheel for the work to be done in the circuit, whether it be a chemical for decomposition, a telegraph instrument, an electric lamp, or any other appliance. As the current in the race depends on the "head of water," or difference of level between the dam and the sea as well as on the resistance of the channel, so the current in the circuit depends on the "electromotive force," or difference of potential between the positive and negative poles, as well as on the resistance of the circuit. The relation between these is expressed by the well-known law of Ohm, which runs: A current of electricity is directly proportional to the electromotive force and inversely proportional to the resistance of the circuit.

In practice electricity is measured by various units or standards named after celebrated electricians. Thus the unit of quant.i.ty is the coulomb, the unit of current or quant.i.ty flowing per second is the ampere, the unit of electromotive force is the volt, and the unit of resistance is the ohm.

The quant.i.ty of water or any other "electrolyte" decomposed by electricity is proportional to the strength of the current. One ampere decomposes .00009324 gramme of water per second, liberating .000010384 gramme of hydrogen and .00008286 gramme of oxygen.

The quant.i.ty in grammes of any other chemical element or ion which is liberated from an electrolyte or body capable of electrochemical decomposition in a second by a current of one ampere is given by what is called the electrochemical equivalent of the ion. This is found by multiplying its ordinary chemical equivalent or combining weight by .000010384, which is the electrochemical equivalent of hydrogen. Thus the weight of metal deposited from a solution of any of its salts by a current of so many amperes in so many seconds is equal to the number of amperes multiplied by the number of seconds, and by the electrochemical equivalent of the metal.

The deposition of a metal from a solution of its salt is very easily shown in the case of copper. In fact, we have already seen that in the Daniell cell the current decomposes a solution of sulphate of copper and deposits the pure metal on the copper plate. If we simply make a solution of blue vitriol in a gla.s.s beaker and dip the wires from a voltaic cell into it, we shall find the wire from the negative pole become freshly coated with particles of new copper. The sulphate has been broken up, and the liberated metal, being positive, gathers on the negative electrode. Moreover, if we examine the positive electrode we shall find it slightly eaten away, because the sulphuric acid set free from the sulphate has combined with the particles of that wire to make new sulphate. Thus the copper is deposited on one electrode, namely, the cathode, by which the current leaves the bath, and at the expense of the other electrode, that is to say, the anode, by which the current enters the bath.

The fact that the weight of metal deposited in this way from its salts is proportional to the current, has been utilised for measuring the strength of currents with a fine degree of accuracy.

If, for example, the tubes of the voltameter described on page 38 were graduated, the volume of gas evolved would be a measure of the current. Usually, however, it is the weight of silver or copper deposited from their salts in a certain time which gives the current in amperes.

Electro-plating is the princ.i.p.al application of this chemical process. In 1805 Brugnatelli took a silver medal and coated it with gold by making it the cathode in a solution of a salt of gold, and using a plate of gold for the anode. The shops of our jewellers are now bright with teapots, salt cellars, spoons, and other articles of the table made of inferior metals, but beautified and preserved from rust in this way.

Figure 44 ill.u.s.trates an electro-plating bath in which a number of spoons are being plated. A portion of the vat V is cut away to show the interior, which contains a solution S of the double cyanide of gold and pota.s.sium when gold is to be laid, and the double cyanide of silver and pota.s.sium when silver is to be deposited. The electrodes are hung from metal rods, the anode A being a plate of gold or silver G, as the case may be, and the cathode C the spoons in question. When the current of the battery or dynamo pa.s.ses through the bath from the anode to the cathode, gold or silver is deposited on the spoons, and the bath recuperates its strength by consuming the gold or silver plate.

Enormous quant.i.ties of copper are now deposited in a similar way, sulphate of copper being the solution and a copper plate the anode. Large articles of iron, such as the parts of ordnance, are sometimes copper-plated to preserve them from the action of the atmosphere. Seamless copper pipes for conveying steam, and wires of pure copper for conducting electricity, are also deposited, and it is not unlikely that the kettle of the future will be made by electrolysis.

Nickel-plating is another extensive branch of the industry, the white nickel forming a cloak for metals more subject to corrosion.

Nickel is found to deposit best from a solution of the double sulphate of nickel and ammonia. Aluminium, however, has not yet been successfully deposited by electricity.

In 1836 De la Rue observed that copper laid in this manner on another surface took on its under side an accurate impression of that surface, even to the scratches on it, and three years later Jacobi, of St. Petersburg, and Jordan, of London, applied the method to making copies or replicas of medals and woodcuts. Even non-metallic surfaces could be reproduced in copper by taking a cast of them in wax and lining the mould with fine plumbago, which, being a conductor, served as a cathode to receive the layer of metal. It is by the process of electrotyping or galvano- plastics that the copper faces for printing woodcuts are prepared, and copies made of seals or medals.

Natural objects, such as flowers, ferns, leaves, feathers, insects, and lizards, can be prettily coated with bronze or copper, not to speak of gold and silver, by a similar process.

They are too delicate to be coated with black lead in order to receive the skin of metal, but they can be dipped in solutions, leaving a film which can be reduced to gold or silver. For instance, they may be soaked in an alcoholic solution of nitrate of silver, made by shaking 2 parts of the crystals in 100 parts of alcohol in a stoppered bottle. When dry, the object should be suspended under a gla.s.s shade and exposed to a stream of sulphuretted hydrogen gas; or it may be immersed in a solution of 1 part of phosphorus in 15 parts of bisulphide of carbon, 1 part of bees-wax, 1 part of spirits of turpentine, 1 part of asphaltum, and 1/8 part of caoutchouc dissolved in bisulphide of carbon. This leaves a superficial film which is metallised by dipping in a solution of 20 grains of nitrate of silver to a pint of water. On this metallic film a thicker layer of gold and silver in different shades can be deposited by the current, and the silver surface may also be "oxidised" by was.h.i.+ng it in a weak solution of platinum chloride.

Electrolysis is also used to some extent in reducing metals from their ores, in bleaching fibre, in manufacturing hydrogen and oxygen from water, and in the chemical treatment of sewage.

CHAPTER VI.

THE TELEGRAPH AND TELEPHONE.