Kitabı oku: «Michael Faraday», sayfa 9

The history of the magneto-electric light affords another remarkable instance of the way in which one of Faraday's most recondite discoveries bore fruit in his own lifetime; and it is the more interesting as it fell to his own lot to assist in bringing the fruit to maturity.

"Brighton, November 29, 1831.

"Dear Phillips,

"For once in my life I am able to sit down and write to you without feeling that my time is so little that my letter must of necessity be a short one; and accordingly I have taken an extra large sheet of paper, intending to fill it with news.

"But how are you getting on? Are you comfortable? And how does Mrs. Phillips do; and the girls? Bad correspondent as I am, I think you owe me a letter; and as in the course of half an hour you will be doubly in my debt, pray write us, and let us know all about you. Mrs. Faraday wishes me not to forget to put her kind remembrances to you and Mrs. Phillips in my letter…

"We are here to refresh. I have been working and writing a paper that always knocks me up in health; but now I feel well again, and able to pursue my subject; and now I will tell you what it is about. The title will be, I think, 'Experimental Researches in Electricity:' – I. On the Induction of Electric Currents; II. On the Evolution of Electricity from Magnetism; III. On a new Electrical Condition of Matter; IV. On Arago's Magnetic Phenomena. There is a bill of fare for you; and, what is more, I hope it will not disappoint you. Now, the pith of all this I must give you very briefly; the demonstrations you shall have in the paper when printed…"

So wrote Faraday to his intimate friend Richard Phillips, on November 29th, 1831, and the letter goes on to describe the great harvest of results which he had gathered since the 29th of August, when he first obtained evidence of an electric current from a magnet. A few days afterwards he was at work again on these curious relations of magnetism and electricity in his laboratory, and at the Round Pond in Kensington Gardens, and with Father Thames at Waterloo Bridge. On the 8th of February he entered in his note-book: "This evening, at Woolwich, experimented with magnet, and for the first time got the magnetic spark myself. Connected ends of a helix into two general ends, and then crossed the wires in such a way that a blow at a b would open them a little. Then bringing a b against the poles of a magnet, the ends were disjoined, and bright sparks resulted."

Next day he repeated this experiment at home with Mr. Daniell's magnet, and then invited some of his best friends to come and see the tiny speck of light.²⁷

But what was the use of this little spark between the shaken wires? "What is the use of an infant?" asked Franklin once, when some such question was proposed to him. Faraday said that the experimentalist's answer was, "Endeavour to make it useful." But he passed to other researches in the same field.

"I have rather been desirous," he says, "of discovering new facts and new relations dependent on magneto-electric induction, than of exalting the force of those already obtained; being assured that the latter would find their full development hereafter." And in this assurance he was not mistaken. Electro-magnetism has been taken advantage of on a large scale by the metallurgist and the telegrapher; and even the photographer and sugar-refiner have attempted to make it their servant; but it is its application as a source of light that is most interesting to us in connection with its discoverer.

Many "electric lights" were invented by "practical men," the power being generally derived from a galvanic battery; and it was discovered that by making the terminals of the wires of charcoal, the brilliancy of the spark could be enormously increased. Some of these inventions were proposed for lighthouses, and so came officially under the notice of Faraday as scientific adviser to the Trinity House. Thus he was engaged in 1853 and 1854 with the beautiful electric light of Dr. Watson, which he examined most carefully, evidently hoping it might be of service, and at length he wrote an elaborate report pointing out its advantages, but at the same time the difficulties in the way of its practical adoption. The Trinity Corporation passed a special vote of thanks for his report, and hesitated to proceed further in the matter.

But Faraday's own spark was destined to be more successful. In 1853 some large magneto-electric machines were set up in Paris for producing combustible gas by the decomposition of water. The scheme failed, but a Mr. F. H. Holmes suggested that these expensive toys might be turned to account for the production of light. "My propositions," he told the Royal Commissioners of Lighthouses, "were entirely ridiculed, and the consequence was, that instead of saying that I thought I could do it, I promised to do it by a certain day. On that day, with one of Duboscq's regulators or lamps, I produced the magneto-electric light for the first time; but as the machines were ill-constructed for the purpose, and as I had considerable difficulty to make even a temporary adjustment to produce a fitting current, the light could only be exhibited for a few minutes at a time." He turned his attention to the reconstruction of the machines, and after carrying on his experiments in Belgium, he applied to the Trinity Board in February 1857. Here was the tiny spark, which Faraday had produced just twenty-five years before, exalted into a magnificent star, and for Faraday it was reserved to decide whether this star should shed its brilliance from the cliffs of Albion. A good piece of optical apparatus, intended for the Bishop Rock in the Scillies, happened to be at the experimental station at Blackwall, and with this comparative experiments were made. We can imagine something of the interest with which Faraday watched the light from Woolwich, and asked questions of the inventor about all the details of its working and expense; and we can picture the alternations of hope and caution as he wrote in his report, "The light is so intense, so abundant, so concentrated and focal, so free from under-shadows (caused in the common lamp by the burner), so free from flickering, that one cannot but desire it should succeed. But," he adds, "it would require very careful and progressive introduction – men with peculiar knowledge and skill to attend it; and the means of instantly substituting one lamp for another in case of accident. The common lamp is so simple, both in principle and practice, that its liability to failure is very small. There is no doubt that the magneto-electric lamp involves a great number of circumstances tending to make its application more refined and delicate; but I would fain hope that none of these will prove a barrier to its introduction. Nevertheless, it must pass into practice only through the ordeal of a full, searching, and prolonged trial." This trial was made in the upper of the two light towers at the South Foreland; but it was not till the 8th December, 1858, that the experiment was commenced. Faraday made observations on it for the first two days, but it did not act well, and was discontinued till March 28, 1859, when it again shot forth its powerful rays across the Channel.

It was soon inspected by Faraday inside and outside, by land and by sea. His notes terminate in this way: – "Went to the hills round, about a mile off, or perhaps more, so as to see both upper and lower light at once. The effect was very fine. The lower light does not come near the upper in its power, and, as to colour, looks red whilst the upper is white. The visible rays proceed from both horizontally, but those from the low light are not half so long as those from the electric light. The radiation from the upper light was beautifully horizontal, going out right and left with intenseness like a horizontal flood of light, with blackness above and blackness below, yet the sky was clear and the stars shining brightly. It seemed as if the lanthorn²⁸ only were above the earth, so dark was the path immediately below the lanthorn, yet the whole tower was visible from the place. As to the shadows of the uprights, one could walk into one and across, and see the diminution of the light, and could easily see when the edge of the shadow was passed. They varied in width according to the distance from the lanthorn. With upright bars their effect is considerable at a distance, as seen last night; but inclining these bars would help in the distance, though not so much as with a light having considerable upright dimension, as is the case with an oil-lamp.

"The shadows on a white card are very clear on the edge – a watch very distinct and legible. On lowering the head near certain valleys, the feeble shadow of the distant grass and leaves was evident. The light was beautifully steady and bright, with no signs of variation – the appearance was such as to give confidence to the mind – no doubt about its continuance.

"As a light it is unexceptionable – as a magneto-electric light wonderful – and seems to have all the adjustments of quality and more than can be applied to a voltaic electric light or a Ruhmkorff coil."

The Royal Commissioners and others saw with gratification this beautiful light, and arrangements were made for getting systematic observations of it by the keepers of all the lighthouses within view, the masters of the light-vessels that guard the Goodwin Sands, and the crews of pilot cutters; after which Faraday wrote a very favourable report, saying, among other things: "I beg to state that in my opinion Professor Holmes has practically established the fitness and sufficiency of the magneto-electric light for lighthouse purposes, so far as its nature and management are concerned. The light produced is powerful beyond any other that I have yet seen so applied, and in principle may be accumulated to any degree; its regularity in the lanthorn is great, its management easy, and its care there may be confided to attentive keepers of the ordinary degree of intellect and knowledge."²⁹

The Elder Brethren then wished a further trial of six months, during which time the light was to be entirely under their own control. It was therefore again kindled on August 22, and the experiment happened soon to be exposed to a severe test, as one of the light-keepers, who had been accustomed to the arrangement of the lamps in the lantern, was suddenly removed, and another took his place without any previous instruction. This man thought the light sufficiently strong if he allowed the carbon points to touch, as the lamp then required no attendance whatever, and he could leave it in that way for hours together. On being remonstrated with, he said, "It is quite good enough." Notwithstanding such difficulties as these, the experiment was considered satisfactory, but it was discontinued at the South Foreland, for the cliffs there are marked by a double light, and the electric spark was so much brighter than the oil flames in the other house, that there was no small danger of its being seen alone in thick weather, and thus fatally misleading some unfortunate vessel.

After this Faraday made further observations, estimates of the expense, and experiments on the divergence of the beam, while Mr. Holmes worked away at Northfleet perfecting his apparatus, and the authorities debated whether it was to be exhibited again at the Start, which is a revolving light, or at Dungeness, which is fixed. The scientific adviser was in favour of the Start, but after an interview with Mr. Milner Gibson, then President of the Board of Trade, Dungeness was determined on; a beautiful small combination of lenses and prisms was made expressly for it by Messrs. Chance, and at last, after two years' delay, the light again shone on our southern coast.

It may be well to describe the apparatus. There are 120 permanent magnets, weighing about 50 lbs. each, ranged on the periphery of two large wheels. A steam-engine of about three-horse power causes a series of 180 soft iron cores, surrounded by coils of wire, to rotate past the magnets. This calls the power into action, and the small streams of electricity are all collected together, and by what is called a "commutator" the alternative positive and negative currents are brought into one direction. The whole power is then conveyed by a thick wire from the engine-house to the lighthouse tower, and up into the centre of the glass apparatus. There it passes between two charcoal points, and produces an intensely brilliant continuous spark. At sunset the machine is started, making about 100 revolutions per minute; and the attendant has only to draw two bolts in the lamp, when the power thus spun in the engine-room bursts into light of full intensity. The "lamp" regulates itself, so as to keep the points always at a proper distance apart, and continues to burn, needing little or no attention for three hours and a half, when, the charcoals being consumed, the lamp must be changed, but this is done without extinguishing the light.

Again there were inspections, and reports from pilots and other observers, and Faraday propounded lists of questions to the engineer about bolts and screws and donkey-engines, while he estimated that at the Varne light-ship, about equidistant from Cape Grisnez and Dungeness, the maximum effect of the revolving French light was equalled by the constant gleam from the English tower. But delays again ensued till intelligent keepers could be found and properly instructed; but on the 6th June, 1862, Faraday's own light, the baby grown into a giant, shone permanently on the coast of Britain.

France, too, was alert. Berlioz's machine, which was displayed at the International Exhibition in London, and which was also examined by Faraday, was approved by the French Government, and was soon illuminating the double lighthouse near Havre. These magneto-electric lights on either side of the Channel have stood the test of years; and during the last two years there has shone another still more beautiful one at Souter Point, near Tynemouth; while the narrow strait between England and France is now guarded by these "sentinels of peaceful progress," for the revolving light at Grisnez has been lately illuminated on this principle, and on the 1st of January, 1872, the two lights of the South Foreland flashed forth with the electric flame.³⁰

In describing thus the valuable applications of Faraday's discoveries of benzol, of specific inductive capacity, and of magneto-electricity, it is not intended to exalt these above other discoveries which as yet have paid no tribute to the material wants of man. The good fruit borne by other researches may not be sufficiently mature, but it doubtless contains the seeds of many useful inventions. Yet, after all, we must not measure the worth of Faraday's discoveries by any standard of practical utility in the present or in the future. His chief merit is that he enlarged so much the boundaries of our knowledge of the physical forces, opened up so many new realms of thought, and won so many heights which have become the starting-points for other explorers.

SUPPLEMENTARY PORTRAITS

It has been said that there is no photograph or painting of Faraday which is a satisfactory likeness; not because good portraits have never been published, but because they cannot give the varied and ever-shifting expression of his features. Similarly, I fear that the mental portraiture which I have attempted will fail to satisfy his intimate acquaintance. Yet, as one who never saw him in the flesh may gain a good idea of his personal appearance by comparing several pictures, so the reader may learn more of his intellectual and moral features by combining the several estimates which have been made by different minds. Earlier biographies have been already referred to, but my sketch may well be supplemented by an anonymous poem that appeared immediately after his death, and by the words of two of the most distinguished foreign philosophers – Messrs. De la Rive and Dumas.

 
"Statesmen and soldiers, authors, artists, – still
The topmost leaves fall off our English oak:
Some in green summer's prime, some in the chill
Of autumn-tide, some by late winter's stroke.
 
 
"Another leaf has dropped on that sere heap —
One that hung highest; earliest to invite
The golden kiss of morn, and last to keep
The fire of eve – but still turned to the light.
 
 
"No soldier's, statesman's, poet's, painter's name
Was this, thro' which is drawn Death's last black line;
But one of rarer, if not loftier fame —
A priest of Truth, who lived within her shrine.
 
 
"A priest of Truth: his office to expound
Earth's mysteries to all who willed to hear —
Who in the book of Science sought and found,
With love, that knew all reverence, but no fear.
 
 
"A priest, who prayed as well as ministered:
Who grasped the faith he preached; and held it fast:
Knowing the light he followed never stirred,
Howe'er might drive the clouds thro' which it past.
 
 
"And if Truth's priest, servant of Science too,
Whose work was wrought for love and not for gain:
Not one of those who serve but to ensue
Their private profit: lordship to attain
 
 
"Over their lord, and bind him in green withes,
For grinding at the mill 'neath rod and cord;
Of the large grist that they may take their tithes —
So some serve Science that call Science lord.
 
 
"One rule his life was fashioned to fulfil:
That he who tends Truth's shrine, and does the hest
Of Science, with a humble, faithful will,
The God of Truth and Knowledge serveth best.
 
 
"And from his humbleness what heights he won!
By slow march of induction, pace on pace,
Scaling the peaks that seemed to strike the sun,
Whence few can look, unblinded, in his face.
 
 
"Until he reached the stand which they that win
A bird's-eye glance o'er Nature's realm may throw;
Whence the mind's ken by larger sweeps takes in
What seems confusion, looked at from below.
 
 
"Till out of seeming chaos order grows,
In ever-widening orbs of Law restrained,
And the Creation's mighty music flows
In perfect harmony, serene, sustained;
 
 
"And from varieties of force and power,
A larger unity, and larger still,
Broadens to view, till in some breathless hour
All force is known, grasped in a central Will,
 
 
"Thunder and light revealed as one same strength —
Modes of the force that works at Nature's heart —
And through the Universe's veinèd length
Bids, wave on wave, mysterious pulses dart.
 
 
"That cosmic heart-beat it was his to list,
To trace those pulses in their ebb and flow
Towards the fountain-head, where they subsist
In form as yet not given e'en him to know.
 
 
"Yet, living face to face with these great laws,
Great truths, great myst'ries, all who saw him near
Knew him for child-like, simple, free from flaws
Of temper, full of love that casts out fear:
 
 
"Untired in charity, of cheer serene;
Not caring world's wealth or good word to earn;
Childhood's or manhood's ear content to win;
And still as glad to teach as meek to learn.
 
 
"Such lives are precious: not so much for all
Of wider insight won where they have striven,
As for the still small voice with which they call
Along the beamy way from earth to heaven."
 

Punch, September 7, 1867.

The estimate of M. A. de la Rive is from a letter he addressed to Faraday himself: —

"I am grieved to hear that your brain is weary; this has sometimes happened on former occasions, in consequence of your numerous and persevering labours, and you will bear in mind that a little rest is necessary to restore you. You possess that which best contributes to peace of mind and serenity of spirit – a full and perfect faith, a pure and tranquil conscience, filling your heart with the glorious hopes which the Gospel imparts. You have also the advantage of having always led a smooth and well-regulated life, free from ambition, and therefore exempt from all the anxieties and drawbacks which are inseparable from it. Honour has sought you in spite of yourself; you have known, without despising it, how to value it at its true worth. You have known how to gain the high esteem, and at the same time the affection, of all those acquainted with you.

"Moreover, thanks to the goodness of God, you have not suffered any of those family misfortunes which crush one's life. You should, therefore, watch the approach of old age without fear and without bitterness, having the comforting feeling that the wonders which you have been able to decipher in the book of nature must contribute to the greater reverence and adoration of their Supreme Author.

"Such, my dear friend, is the impression that your beautiful life always leaves upon me; and when I compare it with our troubled and ill-fulfilled life-course, with all that accumulation of drawbacks and griefs by which mine in particular has been attended, I put you down as very happy, especially as you are worthy of your good fortune. This leads me to reflect on the miserable state of those who are without that religious faith which you possess in so great a degree."

In M. Dumas' Eloge at the Académie des Sciences, occur the following sentences: —

"I do not know whether there is a savant who would not feel happy in leaving behind him such works as those with which Faraday has gladdened his contemporaries, and which he has left as a legacy to posterity: but I am certain that all those who have known him would wish to approach that moral perfection which he attained to without effort. In him it appeared to be a natural grace, which made him a professor full of ardour for the diffusion of truth, an indefatigable worker, full of enthusiasm and sprightliness in his laboratory, the best and most amiable of men in the bosom of his family, and the most enlightened preacher amongst the humble flock whose faith he followed.

"The simplicity of his heart, his candour, his ardent love of the truth, his fellow-interest in all the successes, and ingenuous admiration of all the discoveries of others, his natural modesty in regard to what he himself discovered, his noble soul – independent and bold, – all these combined gave an incomparable charm to the features of the illustrious physicist.

"I have never known a man more worthy of being loved, of being admired, of being mourned.

"Fidelity to his religious faith, and the constant observance of the moral law, constitute the ruling characteristics of his life. Doubtless his firm belief in that justice on high which weighs all our merits, in that sovereign goodness which weighs all our sufferings, did not inspire Faraday with his great discoveries, but it gave him the straightforwardness, the self-respect, the self-control, and the spirit of justice, which enabled him to combat evil fortune with boldness, and to accept prosperity without being puffed up…

"There was nothing dramatic in the life of Faraday. It should be presented under that simplicity of aspect which is the grandeur of it. There is, however, more than one useful lesson to be learnt from the proper study of this illustrious man, whose youth endured poverty with dignity, whose mature age bore honours with moderation, and whose last years have just passed gently away surrounded by marks of respect and tender affection."