Part 1

LIGHTNING, THUNDER

AND

LIGHTNING CONDUCTORS.

_WITH AN APPENDIX ON THE RECENT CONTROVERSY ON LIGHTNING CONDUCTORS._

BY

GERALD MOLLOY, D. D., D. Sc.

_ILLUSTRATED._

NEW YORK:

THE HUMBOLDT PUBLISHING CO.,

28 LAFAYETTE PLACE.

LIGHTNING, THUNDER, AND LIGHTNING CONDUCTORS.

CONTENTS.

LECTURE I. Pages 5-26

LIGHTNING AND THUNDER.

Identity of Lightning and Electricity--Franklin’s Experiment--Fatal Experiment of Richman--Immediate Cause of Lightning--Illustration from Electric Spark--What a Flash of Lightning Is--Duration of a Flash of Lightning--Experiments of Professor Rood--Wheatstone’s Experiments--Experiment with Rotating Disc--Brightness of a Flash of Lightning--Various Forms of Lightning--Forked Lightning, Sheet Lightning, Globe Lightning--St. Elmo’s Fire--Experimental Illustration--Origin of Lightning--Length of a Flash of Lightning--Physical Cause of Thunder--Rolling of Thunder--Succession of Peals--Variation of Intensity--Distance of a Flash of Lightning

LECTURE II. Pages 26-53

LIGHTNING CONDUCTORS.

Destructive Effects of Lightning--Destruction of Buildings--Destruction of Ships at Sea--Destruction of Powder Magazines--Experimental Illustrations--Destruction of Life by Lightning--The Return Shock--Franklin’s Lightning Rods--Introduction of Lightning Rods into England--The Battle of Balls and Points--Functions of a Lightning Conductor--Conditions of a Lightning Conductor--Mischief Done by Bad Conductors--Evil Effects of a Bad Earth Contact--Danger from Rival Conductors--Insulation of Lightning Conductors--Personal Safety in a Thunder Storm--Practical Rules--Security Afforded by Lightning Rods

APPENDIX. Pages 55-62

RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.

Theory of Lightning Conductors Challenged--Lectures of Professor Lodge--Short Account of his Views and Arguments--Effect of Self-Induction on a Lightning Rod--Experiment on the Discharge of a Leyden Jar--Outer Shell only of a Lightning Rod Acts as a Conductor--Discussion at the Meeting of the British Association, September, 1888--Statement by Mr. Preece--Lord Rayleigh and Sir William Thomson--Professor Rowland and Professor Forbes--M. de Fonvielle, Sir James Douglass, and Mr. Symons--Reply of Professor Lodge--Concluding Remarks of Professor Fitzgerald, President of the Section--Summary Showing the Present State of the Question

LIST OF ILLUSTRATIONS.

PAGE

THE ELECTRIC SPARK: A TYPE OF A FLASH OF LIGHTNING, 8

CARDBOARD DISC WITH BLACK AND WHITE SECTORS; AS SEEN WHEN AT REST, 12

SAME DISC; AS SEEN WHEN IN RAPID ROTATION, 12

THE BRUSH DISCHARGE, ILLUSTRATING ST. ELMO’S FIRE, 17

ORIGIN OF SUCCESSIVE PEALS OF THUNDER, 22

VARIATIONS OF INTENSITY IN A PEAL OF THUNDER, 24

DISCHARGE OF LEYDEN JAR BATTERY THROUGH THIN WIRES, 27

GLASS VESSEL BROKEN BY DISCHARGE OF LEYDEN JAR BATTERY, 32

GUN COTTON SET ON FIRE BY ELECTRIC SPARK, 33

VOLTA’S PISTOL; EXPLOSION CAUSED BY ELECTRIC SPARK, 34

THE RETURN SHOCK ILLUSTRATED, 35

PROTECTION FROM LIGHTNING BY A CLOSED CONDUCTOR, 48

INDUCTION EFFECT OF LEYDEN JAR DISCHARGE, 56

LECTURE I.

LIGHTNING AND THUNDER.

The electricity produced by an ordinary electric machine exhibits, under certain conditions, phenomena which bear a striking resemblance to the phenomena attendant on lightning. In both cases there is a flash of light; in both there is a report, which, in the case of lightning, we call thunder; and, in both cases, intense heat is developed, which is capable of setting fire to combustible bodies. Further, the spark from an electric machine travels through space with extraordinary rapidity, and so does a flash of lightning; the spark follows a zig-zag course, and so does a flash of lightning; the spark moves silently and harmlessly through metal rods and stout wires, while it forces its way, with destructive effect, through bad conductors, and it is so, too, with a flash of lightning. Lastly, the electricity of a machine is capable of giving a severe shock to the human body; and we know that lightning gives a shock so severe as usually to cause immediate death. For these reasons it was long conjectured by scientific men that lightning is, in its nature, identical with electricity; and that it differs from the electricity of our machines only in this, that it exists in a more powerful and destructive form.

=Identity of Lightning and Electricity.=--But it was reserved for the celebrated Benjamin Franklin to demonstrate the truth of this conjecture by direct experiment. He first conceived the idea of drawing electricity from a thundercloud in the same way as it is drawn from the conductor of an electric machine. For this purpose he proposed to place a kind of sentry-box on the summit of a lofty tower, and to erect, on the sentry-box, a metal rod, projecting twenty or thirty feet upward into the air, pointed at the end, and having no electrical communication with the earth. He predicted that when a thundercloud would pass over the tower, the metal rod would become charged with electricity, and that an observer, stationed in the sentry-box, might draw from it, at pleasure, a succession of electric sparks.

With the magnanimity of a really great man, Franklin published this project to the world; being more solicitous to extend the domain of science by new discoveries, than to secure for himself the glory of having made them. The project was set forth in a letter to Mr. Collinson, of London, which bears date July 29, 1750, and which, in the course of a year or two, was translated into the principal languages of Europe. Two years later the experiment suggested by Franklin was made by Monsieur Dalibard, a wealthy man of science, at his villa near Marly-la-Ville, a few miles from Paris. In the middle of an elevated plain Monsieur Dalibard erected an iron rod, forty feet in length, one inch in diameter, and ending above in a sharp steel point. The iron rod rested on an insulating support, and was kept in position by means of silk cords.

In the absence of Monsieur Dalibard, who was called by business to Paris, this apparatus was watched by an old dragoon, named Coiffier; and on the afternoon of the tenth of May, 1752, he drew sparks from the lower end of the rod at the time that a thundercloud was passing over the neighborhood. Conscious of the importance that would be attached to this phenomenon, the old dragoon summoned, in all haste, the prior of Marly to come and witness it. The prior came without delay, and he was followed by some of the principal inhabitants of the village. In the presence of the little group, thus gathered together, the experiment was repeated--electric sparks were again drawn, in rapid succession, from the iron rod; the prediction of Franklin was fulfilled to the letter; and the identity of lightning and electricity was, for the first time, demonstrated to the world.

=Franklin’s Experiment.=--Meanwhile Franklin had been waiting, with impatience, for the completion of the tower of Christchurch, in Philadelphia, on which he intended to make the experiment himself. He even collected money, it is said, to hasten on the building. But, notwithstanding his exertions, the progress of the tower was slow; and his active mind, which could ill brook delay, hit upon another expedient, remarkable alike for its simplicity and for its complete success. He constructed a boy’s kite, using, however, a silk pockethandkerchief, instead of paper, that it might not be damaged by rain. To the top of the kite he attached a pointed iron wire about a foot long, and he provided a roll of hempen twine, which he knew to be a conductor of electricity, for flying it. This was the apparatus with which he proposed to explore the nature of a thundercloud.

The thundercloud came late in the afternoon of the fourth of July, 1752, and Franklin sallied out with his kite, accompanied by his son, and taking with him a common door-key and a Leyden jar. The kite was soon high in air, and the philosopher awaited the result of his experiment, standing, with his son, under the lee of a cowshed, partly to protect himself from the rain that was coming, and partly, it is said, to shield himself from the ridicule of passers-by, who, having no sympathy with his philosophical speculations, might be inclined to regard him as a lunatic. To guard against the danger of receiving a flash of lightning through his body, he held the kite by means of a silk ribbon, which was tied to the door-key, the door-key being itself attached to the lower end of the hempen string.

A flash of lightning soon came from the cloud, and a second, and a third; but no sign of electricity could be observed in the kite, or the hempen cord, or the key. Franklin was almost beginning to despair of success, when suddenly he noticed that the little fibres of the cord began to bristle up, just as they would if it were placed near an electric machine in action. He presented the door-key to the knob of the Leyden jar, and a spark passed between them. Presently a shower began to fall; the cord, wetted by the rain, became a better conductor than it had been before, and sparks came more freely. With these sparks he now charged the Leyden jar, and found, to his intense delight, that he could exhibit all the phenomena of electricity by means of the lightning he had drawn from the clouds.

In the following year a similar experiment, with even more striking results, was carried out, in France, by de Romas. Though it is said he had no knowledge of what Franklin had done in America, he, too, used a kite; and, with a view of making the string a better conductor, he interlaced with it a thin copper wire. Then, flying his kite in the ordinary way, when it had risen to a height of about 550 feet, he drew sparks from it which, we are told, were upwards of nine feet long, and emitted a sound like the report of a pistol.

=Fatal Experiment of Richman.=--There can be no doubt that experiments of this kind, made with the electricity of a thundercloud, were extremely dangerous; and this was soon proved by a fatal accident. Professor Richman, of St. Petersburgh, had erected on the roof of his house a pointed iron rod, the lower end of which passed into a glass vessel, intended, as we are informed, to measure the strength of the charge which he expected to receive from the clouds. On the sixth of August, 1753, observing the approach of a thunderstorm, he hastened to his apparatus; and as he stood near it, with his head bent down, to watch the effect, a flash of lightning passed through his body and killed him on the spot. This catastrophe served to fix public attention on the danger of such experiments, and gave occasion to the saying of Voltaire: “There are some great lords whom we should always approach with extreme precaution, and lightning is one of them.”[1] From this time the practice of making experiments directly with the lightning of the clouds seems to have been, by common consent, abandoned.

=Immediate Cause of Lightning.=--And now, having set before you some of the most memorable experiments by which the identity of lightning and electricity has been demonstrated, I will try to give you a clear conception regarding the immediate cause of lightning, so far as the subject is understood at the present day by scientific men. You know that there are two kinds of electricity, which are called _positive_ and _negative_; and that each of them repels electricity of the same kind as itself, while it attracts electricity of the opposite kind. Now, every thundercloud is charged with electricity of one kind or the other, positive or negative; and, as it hovers over the earth, it develops, by what is called _induction_, or influence, electricity of the opposite kind in that part of the earth which is immediately under it. Thus we have two bodies--the cloud and the earth--charged with opposite kinds of electricity, and separated by a stratum of the atmosphere. The two opposite electricities powerfully attract each other; but for a time they are prevented from rushing together by the intervening stratum of air, which is a non-conductor of electricity, and acts as a barrier between them. As the electricity, however, continues to accumulate, the attraction becomes stronger and stronger, until at length it is able to overcome the resistance of this barrier; a violent disruptive discharge then takes place between the cloud and the earth, and the flash of lightning is the consequence of the discharge.

The whole phenomenon may be illustrated, on a small scale, by means of this electric machine of Carré’s which you see before you. When my assistant turns the handle of the machine negative electricity is developed in that large brass cylinder, which in our experiment will represent the thundercloud. At a distance of five or six inches from the cylinder I hold a brass ball, which is in electrical communication with the earth through my body. The electrified brass cylinder acts by induction, or influence on the brass ball, and develops in it, as well as in my body, a charge of positive electricity. Now, the positive electricity of the ball and the negative electricity of the cylinder are mutually attracting each other, but the intervening stratum of air offers a resistance which prevents a discharge from taking place. My assistant, however, continues to work the machine; the two opposite electricities rapidly accumulate on the cylinder and the ball; at length their mutual attraction is strong enough to overcome the resistance interposed between them; a disruptive discharge follows, and at the same moment a spark is seen to pass, accompanied by a sharp snapping report.

This spark is a miniature flash of lightning; and the snapping report is a diminutive peal of thunder. Furthermore, at the moment the spark passes you may observe a slight convulsive movement in my hand and wrist. This convulsive movement represents, on a small scale, the violent shock, generally fatal to life, which is produced by a flash of lightning when it passes through the body.

I can continue to take sparks from the conductor as long as the machine is worked; and it is interesting to observe that these sparks follow an irregular zig-zag course, just as lightning does. The reason is the same in both cases: a discharge between two electrified bodies takes place along the line of least resistance; and, owing to the varying condition of the atmosphere, as well as of the minute particles of matter floating in it, the line of least resistance is almost always a zig-zag line.

=What a Flash of Lightning is.=--Lightning, then, may be conceived as an electrical discharge, sudden and violent in its character, which takes place, through the atmosphere, between two bodies highly charged with opposite kinds of electricity. Sometimes this electrical discharge passes, as I have said, between a cloud and the earth; sometimes it passes between one cloud and another; sometimes, on a smaller scale, it takes place, between the great mass of a cloud and its outlying fragments.

But, if you ask me in what the discharge itself consists, I am utterly unable to tell you. It is usual to speak and write on this subject as if electricity were a material substance, a very subtle fluid, and as if, at the moment the discharge takes place, this fluid passes like a rapid stream, from the body that is positively electrified to the body that is negatively electrified. But we must always remember that this is only a conventional mode of expression, intended chiefly to assist our conceptions, and to help us to talk about the phenomena. It does not even profess to represent the objective truth. All that we know for certain is this: that immediately before the discharge the two bodies are highly electrified with opposite kinds of electricity; and, that immediately after the discharge, they are found to have returned to their ordinary condition, or, at least, to have become less highly electrified than they were before.

The flash of light that accompanies an electric discharge is often supposed to be the electricity itself, passing from one body to the other. But it is not; it is simply an effect produced by the discharge. Heat is generated by the expenditure of electrical energy, in overcoming the resistance offered by the atmosphere; and this heat is so intense, that it produces a brilliant incandescence along the path of the discharge. When a spark appears, for example, between the conductor of the machine and this brass ball, it can be shown, by very satisfactory evidence, that minute particles of these solid bodies are first converted into vapor, and then made to glow with intense heat. The gases, too, of which the air is composed, and the solid particles floating in the air, are likewise raised to incandescence. So, too, with lightning; the flash of light is due to the intense heat generated by the electrical discharge, and owes its character to the composition and the density of the atmosphere through which the discharge passes.

=Duration of a Flash of Lightning.=--How long does a flash of lightning last? You are aware, I dare say, that when an impression of light is made on the eye, the impression remains for a sensible interval of time, not less than the tenth of a second, after the source of light has been extinguished or removed. Hence we continue, in fact, to see the light, for at least the tenth of a second, after the light has ceased. Now, if you reflect how brief is the moment for which a flash of lightning is visible, and if you deduct the tenth of a second from that brief moment, you will see, at once, that the period of its actual duration must be very short indeed.

The exact duration of a flash of lightning is a question on which no settled opinion has yet been accepted generally by scientific men. Indeed, the most widely different statements have been made on the subject, quite recently, by the highest authorities, each speaking apparently with unhesitating confidence. Thus, for example, Professor Mascart describes an experiment, which he says was made by Wheatstone, and which showed that a flash of lightning lasts for less than _one_-thousandth of a second;[2] Professor Everett describes the same experiment, without saying by whom it was made, and gives, as the result, that “the duration of the illumination produced by lightning is certainly less than the _ten_-thousandth of a second;”[3] Professor Tyndall, in his own picturesque way, tells us that “a flash of lightning cleaves a cloud, appearing and disappearing in less than the _hundred_-thousandth of a second;”[4] and according to Professor Tait, of Edinburgh, “Wheatstone has shown that lightning certainly lasts less than the _millionth_ of a second.”[5]

=Experiments of Professor Rood.=--I cannot say which of these statements is best supported by actual observation; for none of the writers I have quoted gives any reference to the original memoir from which his statement is derived. As far as my own reading goes, I have only come across one original record of experiments, made directly on the flash of lightning itself, with a view to determine the period of its duration. These experiments were carried out by Professor Ogden Rood, of Columbia College, New York, between the years 1870 and 1873, and are recorded in the _American Journal of Science and Arts_.[6]

For the description of his apparatus, and for the details of his observations, I must refer you to the memoir itself; but I may tell you briefly that the results at which he arrived, if they be accepted, must lead to a considerable modification of the views previously entertained on the subject. In the first place, he satisfied himself that what appears to the eye a single flash of lightning is usually, if not always, multiple in its character; consisting, in fact, of a succession of distinct flashes, which follow one another with such rapidity as to make a continuous impression on the retina. Next, he proceeded to measure approximately the duration of these several component flashes; and he found that it varied over a wide range, amounting sometimes to fully the twentieth of a second, and being sometimes less than the sixteen-hundredth of a second.

=Wheatstone’s Experiments.=--These results are extremely interesting; but we can hardly regard them as finally established, until they have been confirmed by other observers. I may remark, however, that they fit in very well with the experiments made by Professor Wheatstone, many years ago, on the duration of the electric spark, which, as I told you, is a miniature flash of lightning. In these classical experiments, which leave nothing to be desired in point of accuracy, Professor Wheatstone showed that a spark taken directly from a Leyden jar, or a spark taken from the conductor of a powerful electric machine, that is, just such a spark as you have seen here to-day, lasts for less than the millionth of a second.

But he also showed that the duration of the spark is greatly increased, when a resisting wire is introduced into the path of the discharge. Thus, for example, when the discharge from a Leyden jar was made to pass through half a mile of copper wire, with breaks at intervals, the sparks that appeared at these breaks were found to last for ¹⁄₂₄₀₀₀ of a second.[7] Hence we should naturally expect that the period of illumination would be still further increased, in the case of a flash of lightning, where the resistance interposed is enormously greater than in either of the experiments made by Wheatstone.[8]

=Experiment of the Rotating Disk.=--It would be tedious, on an occasion like the present, to enter into an account of Wheatstone’s beautiful and ingenious method of investigation, by which the above facts have been established; but I will show you a much more simple experiment which brings home very forcibly to the mind how exceedingly short must be the duration of the electric spark. Here is a circular disk of cardboard, the outer part of which, as you see, is divided into sectors, black and white alternately, while the space about the centre is entirely white. The disk is mounted on a stand, by means of which I can make it rotate with great velocity. When it is put in rotation, the effect on the eye is very striking--the central space remains white as before, but in the outer rim the distinction of black and white absolutely disappears and gives place to a uniform gray. This color is due to the blending together of black and white in equal proportions; the blending being effected, not on the cardboard disk, but on the retina of the eye.