Scientific.

Otago Witness, Issue 1500, 14 August 1880, Page 23

Scientific.

THE ELECTRIC LIGHT. (Specially written fob the Witness.)

If two rods of carbon, connected to the terminals of a battery or other powerful source of eleotricity, are allowed to touch and are then gradually separated, an extremely brilliant, luminous aro appears, and remains so long as the distance between the two carbons is not too great — the light being due to inoandeßcent particles of carbon which pass from point to point between the intervening Bpace whioh is occupied by theeleotric arc.

The electric light was first noticed by Sir H. Davy whilst experimenting (in 1810, at the Royal Institution, London) with a battery composed of 2000 pairs of copper and zinc plates, immersed in a solution of one part sulphuric acid, one parS nitric acid, and sixty parts water. He found on connecting two pieces of charcoal, one inch in length and one- sixth of an inch in diameter, to the terminals of this battery and bringing them together, " a bright spark was , produced, and more than half the volume of the charooal became ignited to whiteness ; and by withdrawing the points from each other a constant discharge took place through the heated air equal to at least four inches, producing a most brilliant ascending arch of light, broad, and conical in form in the middle." Sir Humphrey Davy afterwards used carbon in preference to charcoal, being much harder, the carbon he used being obtained from the inside of gas Tetorta. To the light produced between carbon points the name Voltaic arc has been given, in honour of Volta, the inventor of the battery by means of which it was first produced, and on account of its curved form. Since Sir H. Davy's time, however, more powerful batteries have been invented, and the resulting effects have been on a corresponding scale. The battery most used now for the production of the light is that form known as Grroves's, which consists of an outer vessel, in the midst of which is placed a porous earthenware vessel ; in this is placed a strip of platinum, and in the outer vessel a cylinder of zinc. The battery is charged by pouriDg acidulated water into the outer cell, and strong nitric acid into the porous or inner cell. Bunaen's battery is identical in arrangement with Groves'a, only less expensive, as a plate of carbon takes the place of the Btrip of platinum. The Voltaic battery is, however, scarcely ever used now for the production of the electric light except for the purpose of lecture demonstration, as after working a sbort time it requires recharging — a very expensive, as well as unhealthy and unpleasant operation. In most cases now where the electric light is used, the electricity is generated by a magneto or dynamo electric machine. In order to understand the principle of these machines it will be necessary for me to explain one or two terms used in explaining their construction and action. Permanent magnets are made of hard steel, in which the amount of magnetism is fixed. Electro magnets are made of soft iron, round which is wound insulated oopper wire (wire covered with silk or cotton), through which an electric current can be passed, the iron only remaining magnetic as long as the ourrent passes round it. The magnetism ceases with the current, with the exception of a very small amount whioh is always present in iron, and whioh i$ termed "residual magnetism." Bearing in mind the foregoing explanations, the following descriptions should be easily understood.

In 1831 Faraday showed that a magnet could produce an electric current ; or, more exactly, that the approach or retrogression of a magnet to or from a bar of soft iron on which was coiled thin copper wire, insulated — or, as it is generally termed, an eleotromagnet — would induce an electric current in tJia latter, The simplest form of thisappa-

ratus, or magneto-electric machine, is that used for the purpoae of giving " shocks." If a moderately powerful machine of this description be taken, and two thin pieces of carbon are attaohed, one to each of the handles by meanß of a piece of wire, on turning the handle of the maohine rapidly, and slightly separating the oarbons from eaoh other (having first allowed them to touch), a very small bright light will be produced— small certainly, but still a veritable electric light. In the large machines such as that lately used in Dunedin it requires a motive power of three or four horses to rotate the magnet oube at the necessary speed of 900 revolutions' per minute. The large maeneto-eleotrio machines used for the production ot the electric light differ only ia form and construction from these small machines ; in all of them the prinoiple remains the same. In the Voltaic battery the ourrent flows in one continuous direction — that is, the current leaves by the wire connected to the copper plate, and enters by the wire connected to the zinc plate— the former being termed the poßitive pole, the latter the negative. In the magneto- electric machines the presence or absence of a " commutator,' or "current changer," determines whether the machine produces a " continuous " or an "alternating" current. Where it is present the. current flows, as in a battery, in one continuous direction ; bnt where absent, the current is alternating, and changes its direction in each wire many hundred times a second, The uses of these different forms of machine will be apparent when we come to consider the lamps or regulators used in conjunction with them for the production of the light. The most recent form of maohine is that known as the dyuamo electric maohine. I append a very good description taken from Shoolbred's work on " Electric Lighting" :— "lf an induction coil (which ia merely a maBS of iron surrounded by insulated copper wire, similar to an electromagnet) be made- to revolve in front of a soft iron electro-magnet, instead of before a permanent magnet, as in the earlier machineß, the small amount of residual magnetism always latent in the iron, especially if it has once been magnetised, causes feeble currents to be induced in the ooil ; and if theße ourrents, or a portion of them, are Bent round the Iron bar — that is, into the wire surrounding it— the magnetisation of the iron 1b increased. This again produces a proportionate inoreaae in the induced ourrent in the coll, and thus by a Berieß of successive mutual reaotions intense magnetisation and very powerful currents are produced. Machines constructed upon this principle of action— that is, in whioh eleotromagnets only are used — have been termed 'dynamo electric' machines, as indicating that dynamloal f oroe is converted into electricity, and which is gradually augmented ■in intensity ; while for the older form, when permanent steel magnets are employed, the term * magneto electric ' machine haa been retained, their power being limited to the strength of the magnetisation of the permanent magnets made use of. This discovery, by adding greatly to the power of electric maohines> led to a corresponding diminution in their bulk and also in their costliness. Almost all the most recent maohines are 1 dynamo electric ' in construction, and are based upon the principle of mutual reaction." The electric light machines in this city—owned by the Harbour Board, and D. Proudfoot, Esq.— are the best known forms of this kind of machine, the ," Gramme" and " Siemens," so called after their respective inventors. Count dv Moncel, the celebrated French electrician, speaking of Edison's " new " electrical generator, says that it is a bad Imitation of the last named machine. The carbon on the end of the conductor, by which the electricity pasßes from the machine or battery, is called the "positive," and that on the conductor, by which it returns, the " negative." With direct currents, such as are produced by the battery, the positive carbon burns away at double the rate of the negative. The positive carbon wastes away in a cavernous form, a cup-like hollow being formed in its centre, whilst the negative beoomeß pointed. The positive carbon has also a much higher temperature than the negative, as it is heated to whiteness for a considerable lengtb, whilst the negative carbon is only heated to a dull redness. The wasting away of the carbons can be almost entirely prevented by burning the Voltaic arc in a vacuum. We have seen that the carbons in the Voltaic^ arc burn away unequally— that is, the positive carbon wastes away at about twice the rate of the negative. This is why the clockwork lamps or regulators are used, their duty being to briiag the carbons together at different speeds, and, should the light by any means go out, to bring them together, and then separate them to the proper distance for the production of the full power of the light ; for should they by any accident cease to burn, the light cannot be re- established until the oarbon points are brought into contaot. For the above purpoae a host of regulators have been devised, probably the two beat being those of Serrier and Siemen, both of whioh have been successfully operated in this city in connection with the Gramme and Siemens machines before mentioned. The Voltaic arc is very suitable for the production of one light on a single circuit. Where a number of lights are required, recourse is generally had to one of the systems of lighting by incandescence. Lighting bp incandescence is, as its name implies, the light emitted by a subatanoe rendered intensely hot. It is generally arrived at by making a break in the continuity of the electric circuit where it is desirable to have a light, and inserting some material capable of offering sufficient resistance to the passage of the current bo as to become intensely hot. Platinum, carbon, and several alloys, have been used for this purpose. Carbon is, however, most used. This system does not require for its successful operation the complicated regulators made use of in the produotion of the Voltaic arc, but the light resulting from it is not near so powerful aB that emitted by carbons in the Voltaic arc. Mr Edison's new lamp, that we have heard so much of lately, ia on this prinoiple, his resisting medium being a piece of carbonised Bristol board, in the shape of a horseshoe, enclosed in a hermetically sealed glass globe containing nitrogen. In its present stage it is not a success. I have spoken of the unequal consumption of

the oarbons with a direct current using the Voltaic arc; with an alternating current machine, however, the carbons are consumed at exactly the same rate. The famous Jablochkoff candle was invented to be used witb one r 'of these machines. It consists of two slender rods of carbon, separated from each other by a thin partition of plaster of pana or gypsum j across the top was laid a very thin piece of carbon connecting the two rods together, by means of which the arc was first established. The great drawback to these '* candles " (as they are termed) is that, should they go out, the attendant has to go and connect them together on purpose to re-establish the light. Messrs Wilde, Rapieff, Siemens, and others have, however, constructed candles on this principle doing away with the insulating partition, and adapting a small electromagnetic apparatus to the base of the stand, which allows the ' carbons to touch eaoh other should they by i any means go out, and alss regulates them the proper distance from each other to produce the maximum light. With frictional or Btatical electricity contaot ia nob necessary to ensure the travelling of a current of electricity, as with an ordinary electrical machine it will strike acrosß several inches from one conductor to the other ; but it has been proved by very delicate experiments that, with the current even from a very powerful battery, the eleotricity will not 1 pass over a space of air equal to the 400 bh part of an inch, bub upon connecting them merely by a touch it will manifest itself, in full power. A word as to the carbons whioh are used in both Bystems. Formerly the carbons were cut out of the carbon found lining the inside of gas retorts that had I been in use some time ', but since the eleo- j trie light haß assumed such a practioal form j — since it has removed from the bounds of experiment, and beoome an established fact — it has baen adopted by a great many factories, ironworks, &c, and to meet the growing demand for carbons factories for their produotion on a large soale have been established In France, Germany, England, and America. Aa to the future of this great light I have no doubt in my own mind, and do not think I could do better than by finishing this with quoting "Mrs Gingham's Dithramblo" to Punch :—: — But 'fcain'fc no good— the coming light will come, You'll see ; But all I know is, I have no patience with the indiwiddles, Greedy monopolists or timorous fogies, As looks on new inventions as on bogies. Sich is the parties as would sneer and scoff At Edison and that there Jabloehkoff ; But them as knows says there's no call for fightThat this electric light Is bright And white, Don't give no heat, nor yet no smoke, Nor nasty sooty fumes as soil and choke (Which going nowadays to a theayter Is bad as dwelling in Etny's crater). No j gas may have its uses still, no doubt — But them as makes it ain't been so perlite That we should fret ourselves for their look out — Not quite ! ! If Jablochkoff, or Edison, or others, Can give us better light and fewer bothers, More wital aira and not ho noxious wapors— In spite of all the croakers in the papers I beg to say, emphatical, So be it ! And may I live to see it. Electron.