Read Ebook: The Story of Electricity by Munro John

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The Daniell cell is one of the best, and gives a very constant current. In this battery the copper plate is surrounded by a solution of sulphate of copper , which the hydrogen decomposes, forming sulphuric acid , thus taking itself out of the way, and leaving pure copper to be deposited as a fresh surface on the copper plate. A further improvement is made in the cell by surrounding the zinc plate with a solution of sulphate of zinc , which is a good conductor. Now, when the oxide of zinc is formed by the oxygen uniting with the zinc, the free sulphuric acid combines with it, forming more sulphate of zinc, and maintaining the CONDUCTIVITY of the cell. It is only necessary to keep up the supply of zinc, water, and sulphate of copper to procure a steady current of electricity.

The Daniell cell is constructed in various ways. In the earlier models the two plates with their solutions were separated by a porous jar or partition, which allowed the solutions to meet without mixing, and the current to pass. Sawdust moistened with the solutions is sometimes used for this porous separator, for instance, on board ships for laying submarine cables, where the rolling of the waves would blend the liquids.

In the "gravity" Daniell the solutions are kept apart by their specific gravities, yet mingle by slow diffusion. Figure 15 illustrates this common type of cell, where Z is the zinc plate in a solution of sulphate of zinc, and C is the copper plate in a solution of sulphate of copper, fed by crystals of the "blue vitriol." The wires to connect the plates are shown at WW. It should be noticed that the zinc is cast like a wheel to expose a larger surface to oxidation, and to reduce the resistance of the cell, thus increasing the yield of current. The extent of surface is not so important in the case of the copper plate, which is not acted on, and in this case is merely a spiral of wire, helping to keep the solutions apart and the crystals down. The Daniell cell is much employed in telegraphy. The Bunsen cell consists of a zinc plate in sulphuric acid, and at carbon plate in nitric acid, with a porous separator between the liquids. During the action of the cell, hydrogen, which is liberated at the carbon plate, is removed by combining with the nitric acid. The Grove cell is a modification of the Bunsen, with platinum instead of carbon. The Smee cell is a zinc plate side by side with a "platinised" silver plate in dilute sulphuric acid. The silver is coated with rough platinum to increase the surface and help to dislodge the hydrogen as bubbles and keep it from polarising the cell. The Bunsen, Grove, and Smee batteries are, however, more used in the laboratory than elsewhere.

The Leclanche is a fairly constant cell, which requires little attention. It "polarises" in action but soon regains its normal strength when allowed to rest, and hence it is useful for working electric bells and telephones. As shown in figure 16, it consists of a zinc rod with its connecting wire Z, and a carbon plate C with its binding screw, between two cakes M M of a mixture of black oxide of manganese, sulphur, and carbon, plunged in a solution of sal-ammoniac. The oxide of manganese relieves the carbon plate of its hydrogen. The strength of the solution is maintained by spare crystals of sal-ammoniac lying on the bottom of the cell, which is closed to prevent evaporation, but has a venthole for the escape of gas.

The Bichromate of Potash cell polarises more than the Leclanche, but yields a more powerful current for a short time. It consists, as shown in figure 17, of a zinc plate Z between two carbon plates C C immersed in a solution of bichromate of potash, sulphuric acid , and water. The zinc is always lifted out of the solution when the cell is not in use. The gas which collects in the carbons, and weakens the cell, can be set free by raising the plates out of the liquid when the cell is not wanted. Stirring the solution has a similar effect, and sometimes the constancy of the cell is maintained by a circulation of the liquid. In Fuller's bichromate cell the zinc is amalgamated with mercury, which is kept in a pool beside it by means of a porous pot.

The Latimer Clark "standard" cell is used by electricians in testing, as a constant electromotive force. It consists of a pure zinc plate separated from a pool of mercury by a paste of mercurous proto-sulphate and saturated solution of sulphate of zinc. Platinum wires connect with the zinc and mercury and form the poles of the battery, and the mouth of the glass cell is plugged with solid paraffin. As it is apt to polarise, the cell must not be employed to yield a current, and otherwise much care should be taken of it.

Dry cells are more cleanly and portable than wet, they require little or no attention, and are well suited for household or medical purposes. The zinc plate forms the vessel containing the carbon plate and chemical reagents. Figure 19 represents a section of the "E. C. C." variety, where Z is the zinc standing on an insulating sole I, and fitted with a connecting wire or terminal T , which is the negative pole. The carbon C is embedded in black paste M, chiefly composed of manganese dioxide, and has a binding screw or terminal T , which is the positive pole. The black paste is surrounded by a white paste Z, consisting mainly of lime and sal-ammoniac. There is a layer of silicate cotton S C above the paste, and the mouth is sealed with black pitch P, through which a waste-tube W T allows the gas to escape.

The Hellesen dry cell is like the "E. C. C.," but contains a hollow carbon, and is packed with sawdust in a millboard case. The Leclanche-Barbier dry cell is a modification of the Leclanche wet cell, having a paste of sal-ammoniac instead of a solution.

All the foregoing cells are called "primary," because they are generators of electricity. There are, however, batteries known as "secondary," which store the current as the Leyden jar stores up the discharge from an electrical machine.

In the action of a primary cell, as we have seen, water is split into its constituent gases, oxygen and hydrogen. Moreover, it was discovered by Carlisle and Nicholson in the year 1800 that the current of a battery could decompose water in the outer part of the circuit. Their experiment is usually performed by the. apparatus shown in figure 20, which is termed a voltameter, and consists of a glass vessel V, containing water acidulated with a little sulphuric acid to render it a better conductor, and two glass test-tubes OH inverted over two platinum strips or electrodes, which rise up from the bottom of the vessel and are connected underneath it to wires from the positive and negative poles of the battery C Z. It will be understood that the current enters the water by the positive electrode, and leaves it by the negative electrode.

When the power of the battery is sufficient the water in the vessel is decomposed, and oxygen being the negative element, collects at the positive foil or electrode, which is covered by the tube O. The hydrogen, on the other hand, being positive, collects at the negative foil under the tube H. These facts can be proved by dipping a red-hot wick or taper into the gas of the tube O and seeing it blaze in presence of the oxygen which feeds the combustion, then dipping the lighted taper into the gas of the tube H and watching it burn with the blue flame of hydrogen. The volume of gas at the CATHODE or negative electrode is always twice that at the ANODE or positive electrode, as it should be according to the known composition of water.

Now, if we disconnect the battery and join the two platinum electrodes of the voltameter by a wire, we shall find a current flowing out of the voltameter as though it were a battery, but in the reverse direction to the original current which decomposed the water. This "secondary" or reacting current is evidently due to the polarisation of the foils--that is to say, the electro- positive and electro-negative gases collected on them.

Professor Groves constructed a gas battery on this principle, the plates being of platinum and the two gases surrounding them oxygen and hydrogen, but the most useful development of it is the accumulator or storage battery.

The first practicable secondary battery of Gaston Plante was made of sheet lead plates or electrodes, kept apart by linen cloth soaked in dilute sulphuric acid, after the manner of Volta's pile. It was "charged" by connecting the plates to a primary battery, and peroxide of lead was formed on one plate and spongy lead on the other. When the charging current was cut off the peroxide plate became the positive and the spongy plate the negative pole of the secondary cell.

Faure improved the Plante cell by adding a paste of red lead or minium and dilute sulphuric acid , by which a large quantity of peroxide and spongy lead could be formed on the plates. Sellon and Volckmar increased its efficiency by putting the paste into holes cast in the lead. The "E. P. S." accumulator of the Electrical Power Storage Company is illustrated in figure 21, and consists of a glass or teak box containing two sets of leaden grids perforated with holes, which are primed with the paste and steeped in dilute sulphuric acid. Alternate grids are joined to the poles of a charging battery or generator, those connected to the positive pole being converted into peroxide of lead and the others into spongy lead. The terminal of the peroxide plates, being the positive pole of the accumulator, is painted red, and that of the spongy plates or negative pole black. Accumulators of this kind are highly useful as reservoirs of electricity for maintaining the electric light, or working electric motors in tramcars, boats, and other carriages.

THE ELECTRICITY OF HEAT.

In the year 1821 Professor Seebeck, of Berlin, discovered a third source of electricity. Volta had found that two dissimilar metals in contact will produce a current by chemical action, and Seebeck showed that heat might take the place of chemical action. Thus, if a bar of antimony A and a bar of bismuth S are in contact at one end, and the junction is heated by a spirit lamp to a higher temperature than the rest of the bars, a difference in their electric state or potential will be set up, and if the other ends are joined by a wire W, a current will flow through the wire. The direction of the current, indicated by the arrow, is from the bismuth to the antimony across the joint, and from the antimony to the bismuth through the external wire. This combination, which is called a "thermo-electric couple," is clearly analogous to the voltaic couple, with heat in place of chemical affinity. The direction of the current within and without the couple shows that the bismuth is positive to the antimony. This property of generating a current of electricity by contact under the influence of heat is not confined to bismuth and antimony, or even to the metals, but is common to all dissimilar substances in their degree. In the following list of bodies each is positive to those beneath it, negative to those above it, and the further apart any two are in the scale the greater the effect. Thus bismuth and antimony give a much stronger current with the same heating than copper and iron. Bismuth and selenium produce the best result, but selenium is expensive and not easy to manipulate. Copper and German silver will make a cheap experimental couple:--

POSITIVE Bismuth Cobalt Potassium Nickel Sodium Lead Tin Copper Platinum Silver Zinc Cadmium Arsenic Iron Red phosphorus Antimony Tellurium Selenium NEGATIVE

Other things being equal, the hotter the joint in comparison with the free ends of the bars the stronger the current of electricity. Within certain limits the current is, in fact, proportional to this difference of temperature. It always flows in the same direction if the joint is not overheated, or, in other words, raised above a certain temperature.

The electromotive force and current of a thermo-electric couple is very much smaller than that given by an ordinary voltaic cell. We can, however, multiply the effect by connecting a number of pairs together, and so forming a pile or battery. Thus figure 23 shows three couples joined "in series," the positive pole of one being connected to the negative pole of the next. Now, if all the junctions on the left are hot and those on the right are cool, we will get the united effect of the whole, and the total current will flow through the wire W, joining the extreme bars or positive and negative poles of the battery. It must be borne in mind that although the bismuth and antimony of this thermo-electric battery, like the zinc and copper of the voltaic or chemico-electric battery, are respectively positive and negative to each other, the poles or wires attached to these metals are, on the contrary, negative and positive. This peculiarity arises from the current starting between the bismuth and antimony at the heated junction.

The internal resistance of a "thermo-electric pile" is, of course, very slight, the metals being good conductors, and this fact gives it a certain advantage over the voltaic battery. Moreover, it is cleaner and less troublesome than the chemical battery, for it is only necessary to keep at the required difference of temperature between the hot and cold junctions in order to get a steady current. No solutions or salts are required, and there appears to be little or no waste of the metals. It is important, however, to avoid sudden heating and cooling of the joints, as this tends to destroy them.

Clammond, Gulcher, and others have constructed useful thermo-piles for practical purposes. Figure 24 illustrates a Clammond thermo- pile of 75 couples or elements. The metals forming these pairs are an alloy of bismuth and antimony for one and iron for the other. Prisms of the alloy are cast on strips of iron to form the junctions. They are bent in rings, the junctions in a series making a zig-zag round the circle. The rings are built one over the other in a cylinder of couples, and the inner junctions are heated by a Bunsen gas-burner in the hollow core of the battery. A gas- pipe seen in front leads to the burner, and the wires WW connected to the extreme bars or poles are the electrodes of the pile.

Thermo-piles are interesting from a scientific point of view as a direct means of transforming heat into electricity. A sensitive pile is also a delicate detector of heat by virtue of the current set up, which can be measured with a galvanometer or current meter. Piles of antimony and bismuth are made which can indicate the heat of a lighted match at a distance of several yards, and even the radiation from certain of the stars.

Thermo-batteries have been used in France for working telegraphs, and they are capable of supplying small installations of the electric light or electric motors for domestic purposes.

A very feeble thermo-electric effect can be produced by heating the junction of two different pieces of the same substance, or even by making one part of the same conductor hotter than another. Thus a sensitive galvanometer will show a weak current if a copper wire connected in circuit with it be warmed at one point. Moreover, it has been found by Lord Kelvin that if an iron wire is heated at any point, and an electric current be passed through it, the hot point will shift along the wire in a direction contrary to that of the current.

THE ELECTRICITY OF MAGNETISM.

We have already seen how electricity was first produced by the simple method of rubbing one body on another, then by the less obvious means of chemical union, 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 subtle process of generation, not requiring actual contact, which, as might be expected, was discovered later, that, mainly through the medium of magnetism.

The curious mineral which has the property of attracting iron was known to the Chinese several thousand years ago, and certainly to the Greeks in the times of Thales, who, as in the case of the rubbed amber, ascribed the property to its possession of a soul.

Lodestone, a magnetic oxide of iron , is found in various parts of China, especially at T'szchou in Southern Chihli, which was formerly known as the "City of the Magnet." It was called by the Chinese the love-stone or thsu-chy, and the stone that snatches iron or ny-thy-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 sometime in or about the year 2635 B.C. the great Emperor Hoang-ti, having lost his way in a fog whilst pursuing the rebellious Prince Tchiyeou on the plains of Tchou-lou, constructed a chariot which showed the cardinal points, thus enabling him to overtake and put the prince to death.

A magnetic car preceded the Emperors of China in ceremonies of state during the fourth century of our era. It contained a genius in a feather 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 afterwards refined into the needle compass for guiding mariners on the sea, and assisting the professors of feng- shui or geomancy in their magic rites.

Magnetite was also found at Heraclea in Lydia, and at Magnesium on the Meander or Magnesium at Sipylos, all in Asia Minor. It was called the "Heraclean Stone" by the people, but came at length to bear the name of "Magnet" after the city of Magnesia or the mythical shepherd Magnes, who was said to have discovered it by the attraction of his iron crook.

The ancients knew that it had the power of communicating its attractive property to iron, for we read in Plato's "Ion" that a number of iron rings can be supported in a chain by the Heraclean Stone. Lucretius also describes an 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 ceilings of the temples of Diana of Ephesus, of Serapis at Alexandria, and others. It is surprising, however, that neither the Greeks nor Romans, with all their philosophy, would seem to have discovered its directive property.

During the dark ages pieces of Lodestone mounted as magnets were employed in the "black arts." A small natural magnet of this kind is shown in figure 25, where L is the stone shod with two iron "pole-pieces," which are joined by a "keeper" A or separable bridge of iron carrying a hook for supporting weights.

Apparently it was not until the twelfth century that the compass found its way into Europe from the East. In the Landnammabok of Ari Frode, the Norse historian, we read that Flocke Vildergersen, a renowned viking, sailed from Norway to discover Iceland in the year 868, and took with him two ravens as guides, for in those days the "seamen had no lodestone in the northern countries." The Bible, a poem of Guiot de Provins, 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 them to keep their course. Arab traders had probably borrowed the floating needle from the Chinese, for Bailak Kibdjaki, author of the Merchant's Treasure, written in the thirteenth century, speaks of its use in the Syrian sea. The first Crusaders were probably instrumental in bringing it to France, at all events Jacobus de Vitry and Vincent de Beauvais mention its use, De Beauvais calling the poles of the needle by the Arab words aphron and zohran.

Ere long the needle was mounted on a pivot and provided with a moving card showing the principal directions. The variation of the needle from the true north and south was 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 America on his memorable voyage of 1492. Moreover, in 1576, Norman, a compass maker in London, showed that the north- seeking end of the needle dipped below the horizontal.

In these early days it was supposed that lodestone in the pole- star, that is to say, the "lodestar" of the poets or in mountains of the far north, attracted the trembling needle; but in the year 1600, Dr. Gilbert, the founder of electric science, demonstrated 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, there are "consequent poles" of weaker force between them. One of the poles is called the "north," and the other the "south," because if the magnet were freely pivotted like a compass needle, the former would turn to the north and the latter to the south.

Either pole will attract iron, but soft or annealed iron does not retain the magnetism nearly so well as steel. Hence a boy's test for the steel of his knife is only efficacious when the blade itself becomes magnetic after being touched with the magnet. A piece of steel is readily magnetised by stroking it from end to end in one direction with the pole of a magnet, and in this way compass needles and powerful bar magnets can be made.

The poles attract iron at a distance by "induction," just as a charge of electricity, be it positive or negative, will attract a neutral pith ball; and Dr. Gilbert showed that a north pole always repels another north pole and attracts 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 figure 26, where the north pole N attracts the south S. Obviously there are two opposite kinds of magnetic poles, as of electricity, which always appear together, and like magnetic poles repel, unlike magnetic poles attract each other.

It follows that the magnetic pole of the compass needle which turns to the north must be unlike the north and like the south magnetic pole of the earth. Instead of calling it the "north," it would be less confusing to call it the "north-seeking" pole of the needle.

Gilbert made a "terella," or miniature of the earth, as a magnet, and not only demonstrated how the compass needle sets along the lines joining the north and south magnetic poles, but explained the variation and the dip. He imagined that the magnetic poles coincided with the geographical poles, but, as a matter of fact, they 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 America, on the coast of Boothia , where, as foreseen, the needle entirely lost its directive property and stood upright, or, so to speak, on its head. The south magnetic pole lies in the Prince Albert range of Victona Land, and was almost reached by Sir James Clark Ross.

The magnetism of the earth is such as might be produced by a powerful magnet inside, but its origin is unknown, although there is reason to believe that 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 Faraday showed in 1845 that while some are attracted by a magnet others are repelled. The former he called paramagnetic and the latter diamagnetic bodies.

The following is a list of these.--

Paramagnetic Diamagnetic Iron Bismuth Nickel Phosphorus Cobalt Antimony Aluminium Zinc Manganese Mercury Chromium Lead Cerium Silver Titanium Copper Platinum Water Many ores and Alcohol salts of the Tellurium above metals Selenium Oxygen Sulphur Thallium Hydrogen Air

We have theories of magnetism that reduce it to a phenomenon of electricity, though we are ignorant of the real nature of both. If we take a thin bar magnet and break it in two, we find that we have now two shorter magnets, each with its "north" and "south" poles, that is to say, poles of the same kind 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 magnetising the bar we have merely partially turned all these atoms in one direction, that is to say, with their north poles pointing one way and their south poles the other way, as shown in figure 27. The polarity of the bar only shows itself at the ends, where the molecular poles are, so to speak, free.

There are many experiments which support this view. For example, if we heat a magnet red hot it loses its magnetism, perhaps because the heat has disarranged the particles and set the molecular poles in all directions. Again, if we magnetise 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 iron easily loses its magnetism, steel retains it, even under a shock, but not under a cherry red- heat. Nevertheless, if we put the atoms of soft iron under a strain by bending it, we shall find it retain its magnetism more like a bit of steel.

It has been found, too, that the atoms show an indisposition to be moved by the magnetising force which is known as HYSTERESIS. They have a certain inertia, which can be overcome by a slight shock, as though they had a difficulty of turning in 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, and electricians look for it in the constitution of the atom, and in the luminiferous ether which is believed to surround the atoms of matter, and to propagate not merely the waves of light, but induction from one electrified body to another.

We know in proof of this ethereal action that the space around a magnet is magnetic. Thus, if we lay a horse-shoe magnet on a table and sprinkle iron filings round it, they will arrange themselves in curving lines between the poles, as shown in figure 28. Each filing has become a little magnet, and these set themselves end to end as the molecules in the metal are supposed to do. The "field" about the magnet is replete with these lines, which follow certain curves depending on the 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 stronger the magnet the more of these lines pass through a given space. Along them "magnetic induction" is supposed to be propagated, and a magnet is thus enabled to attract iron or any other magnetic substance. The pole induces an opposite pole to itself in the nearest part of the induced body and a like pole in the remote part. Consequently, as unlike poles attract and like repel, the soft iron is attracted by the inducing pole much as a pithball is attracted by an electric charge.

The resemblances of electricity and magnetism did not escape attention, and the derangement of the compass needle by the lightning flash, formerly so disastrous at sea, pointed to an intimate connection between them, which was ultimately disclosed by Professor Oersted, of Copenhagen, in the year 1820. Oersted was on the outlook for the required 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 north-seeking pole moving towards the west. If the direction of the current is reversed, the needle is deflected in the opposite direction, its north pole moving towards the east. Further, if the wire is held below the needle, in the first place, the north pole will turn towards the east, and if the current be reversed it will move towards the west.

The direction of a current can thus be told with the aid of a compass needle. When the wire is wound many times round the needle on a bobbin, the whole forms what is called 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, the instrument becomes a current measurer or galvanometer, for within certain limits the deflection of the needle is proportional to the strength of the current in the wire.

A rule commonly given for remembering the movement of the needle is as follows:--Imagine yourself laid along the wire so that the current flows from your feet to your head; then if 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 feet a north pole will move round you from left to right in front. Or, again, if a current flows from north to south, a north pole will move round it like the sun round the earth.

The influence of the current on the needle implies a magnetic action, and if we dust iron filings around the wire we shall find they cling to it in concentric layers, showing that circular lines of magnetic force enclose it like the water waves caused by a stone dropped into a pond. Figure 31 represents the section 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 the original experiment of Oersted. The needle, having two opposite poles, is pulled in opposite directions by the lines, and being pivoted, sets itself tangentically to them. Were it free and flexible, it would curve itself along one of the lines. Did it consist of a single pole, it would revolve round the wire.

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