CHAPTER V.

METALS AS CONDUCTORS OF ELECTRICITY.

In the history of human inventions and discoveries, the idea of the lightning conductor is almost the sole one which sprang, all but perfect, from one brain, like Minerva, in Greek mythology, from Jupiter’s head. Benjamin Franklin discovered the lightning conductor, and, except some important improvements in its manufacture, due to the progress of the metallurgical arts, the conductor remains the same, in essence, as designed by the world-famous citizen of Philadelphia. The reason of this is plain enough. Though one of the most brilliant discoveries in the annals of mankind, the lightning conductor, by itself, is one of the simplest of things. Franklin found by experiments, that the mysterious so-called ‘electric fluid’ had a tendency to make its way in preference through metals, and so he recommended the laying-down of a metallic line from the clouds to the earth to prevent damage to surrounding objects, such as buildings and the human beings within them. More than this he did not know; and more than this we, to this day, do not know. Of the inner nature, or constitution, of that grand cosmic discharge of electricity to which the name of lightning is given, no scientific explanation can be given. We are utterly ignorant of it, and in all probability ever will be.

But while the general principle laid down by Franklin, that metals will conduct the electric force harmlessly from the clouds to the earth, remains the same, very much has been learnt, in the progress of scientific investigation, as regards the varying conducting capacity of different metals. The first conductors were invariably rods of iron, this metal being preferred by Franklin and his immediate followers as cheap, ready at hand, and answering all purposes in practice. But it was gradually found by experiments that there are other metals through which the electric force will make its way more rapidly than through iron. One of the earliest investigators of this subject was Sir Humphrey Davy, the celebrated inventor of the miner’s safety lamp. It was while studying the decomposition of the fixed alkalies by galvanism, and tracing the metallic nature of their bases, to which he gave the names of sodium and potassium, that the great chemist and natural philosopher was brought to enter upon an examination of what may be called the permeability of the different metals by the electric force. The result of his investigations, as stated by him, was that silver stood highest as a conductor of electricity; next to it coming copper; then gold; next, lead; then platinum; then the new metal called palladium--discovered by Wollaston, 1803, in platinum--and lastly, iron. These were the principal metals experimented upon by Sir Humphrey Davy, and the net result of his inquiries was expressed summarily in the fact of copper being more than six times, and silver more than seven times, as good a conductor as iron. Taking copper at 100, Sir Humphrey Davy drew up the following table of the electrical conductivity of the seven metals:—

Silver 109·10 Copper 100·00 Gold 72·70 Lead 69·10 Platinum 18·20 Palladium 16·40 Iron 14·60

The practical result of these experiments was that it came to be recognised that, among the metals, copper might be employed to greater advantage as a lightning conductor than iron: a much lesser substance of it doing the same service of passing a given quantity of electricity from the clouds harmlessly into the earth.

Sir Humphrey Davy was followed in his researches on the conductivity of the different metals by the electric force, by a number of other scientific men. His immediate successor in entering upon this line of observations was a French naturalist of eminence, Antoine C. Becquerel. Perhaps no man after Benjamin Franklin studied the phenomena of electricity with such thorough insight, free from all misleading theoretical delusions, as Becquerel. He was educated at the Polytechnic School of Paris, and in 1810, at the age of twenty-two, entered the army as an officer of engineers, but quitted it five years afterwards with the rank of colonel, to devote himself entirely to scientific pursuits. Geology and mineralogy first engaged his attention, but he soon quitted these studies to devote himself, heart and soul, to the observation of the phenomena of electricity, which fascinated him as much as they had done Benjamin Franklin. The result was the discovery of a great many facts previously unknown, making Becquerel, amongst others, one of the founders of the science of electro-chemistry. The result of his researches concerning the conducting power of the electric force by different metals may be stated as follows:

Copper 100·00 Gold 93·60 Silver 73·50 Zinc 28·55 Platinum 16·40 Iron 15·80 Tin 15·50 Lead 8·30 Mercury 3·45

It will be seen, in comparing this statement with the result of the investigations of Sir Humphrey Davy, that while the latter places silver before copper in conductivity, Becquerel puts copper at the head of the list. Probably, the explanation of this difference in the result of scientific research, by two men equally learned and equally able, may be found in the fact that the conductivity of copper varies greatly according to the purity of the metal. It has been ascertained that absolutely pure copper of the finest kind--such as that existing in the Isle of Cyprus, youngest of mother Britannia’s colonial children--has a conducting power of upwards of twenty per cent. more than the ordinary copper of commerce. While thus arriving at different estimates, Sir Humphrey Davy and Becquerel are singularly in agreement in one important respect: they both make the relative electrical conductivity of copper and iron about the same, placing it, the one a little under, and the other a little over 100 to 15. In other words, they both say that the value of copper as a lightning conductor to iron is as twenty to three, or between six and seven times as great.

Among a host of other investigators of the subject there stand forward, besides Sir Humphrey Davy and Antoine Becquerel, two Germans, Professors Lenz and Ohm, and another French savant, Claude Pouillet. In the opinion of many scientific authorities, especially in the United States, the experiments of Professor Lenz regarding the comparative electrical conductivity of different metals were more carefully made than any other, and are therefore deserving of the greatest credit. He had, indeed, ample means and great leisure at his disposal, making his scientific investigations under the patronage of the Grand Duke, afterwards Emperor, Nicholas of Russia, while acting as his private tutor at the university of St. Petersburg. The researches of Professor Lenz as to the comparative power of various metals to conduct the electric force were given in the following results--copper, as before, standing as the centesimal unit:—

Silver 136·25 Copper 100·00 Gold 79·80 Tin 30·84 Brass 29·33 Iron 17·74 Lead 14·62 Platinum 14·16

A comparison of the figures here given with those of Sir Humphrey Davy and of Becquerel shows that the results obtained by Professor Lenz differ from those of both the other investigators. Like Sir Humphrey Davy, Professor Lenz declared silver to be of greater electric conductivity than copper, but, on the other hand, he assigned lead a very low place, putting it under iron, instead of far above it. It is difficult to explain this wide divergence, even on the utmost allowance of purity, or impurity, of metals. As regards the most important question, from a practical point of view--that of the difference between copper and iron--Professor Lenz, it will be noticed, places iron higher in the scale than both Sir Humphrey Davy and Becquerel. Still, in his estimate also, copper was admitted to have about six times the conductive power of iron.

While, as just stated, the experiments of Professor Lenz on the electric conductivity of metals are held in the highest esteem in America, the same is the case in Germany as regards those of Professor Ohm. The latter is held to be there the highest authority on all subjects connected with the measurement of the electric force. The professor, born at Erlangen, 1787, and for many years teacher of natural history at Munich, where he died in 1854, devoted the utmost patience and an immense amount of time to the definite object of ascertaining the electric conductivity of all the metals, registering the result of his experiments in a special work, the most complete existing on the subject. According to Professor Ohm, the principal metals stand to each other in conductivity as follows:—

Copper 100·00 Gold 57·40 Silver 35·60 Zinc 33·30 Brass 28·05 Iron 17·40 Platinum 17·10 Tin 16·80 Lead 9·70

Here again is a striking difference with the statements of other investigators. It seems absolutely inexplicable indeed, how it could happen that scientific men of eminence, and admitted authorities on the subject they are treating, came to vary on the electric conductivity of several of the metals. The difference is most astounding as regards silver, the conductivity of which, compared with the per cent. of copper, Professor Lenz places at 136·25, Sir Humphrey Davy at 109·10, Becquerel at 73·50, and Professor Ohm at only 35·60. The only conclusions that can be come to under the circumstances are, that the record of Professor Ohm’s results as regards silver is incorrect; or, that the relative degrees of purity of the samples of metal experimented upon by him and the other professors differed very widely. What is of more importance than this question, is the comparative rank of copper and iron. Here, it is satisfactory to find, the results ascertained by Ohm agree very nearly with the conclusions of the other investigators, it being laid down that copper has about six times the conductive power of iron.

The place filled in America by Lenz, and in Germany by Ohm, is generally assigned in France to Professor Claude Pouillet, a savant who devoted, perhaps, more time than any other in his own country to the study of the phenomena of electricity. Born in 1791, Professor Pouillet became, at a comparatively early age, the director of the celebrated scientific institution of Paris known as the ‘Conservatoire des arts et métiers,’ which led him to enter upon a course of experiments in electricity, and most particularly, at the request of the government, upon investigations as to the best material for lightning conductors. The result of these was published in a lengthened treatise, in which Professor Pouillet set down the electric conductivity of the principal metals, taking copper at a hundred, as follows:—

Gold 103·05 Copper 100·00 Silver 81·26 Brass from 23·40 to 15·20 Platinum 22·50 Iron from 18·20 to 15·60 Cast Steel 14·75 Mercury 2·60

It will be seen that Professor Pouillet, differing from other investigators, as they among themselves, regarding the relative conductivity of the precious metals, gold, silver, and platinum, agreed in the main with them as regards the relative proportions of copper and iron. Most painstaking and minute in his experiments, he found moreover that iron, as well as brass--the latter a mixed metal, and as such variable in composition--was not always the same in respect to conductivity, the changes being due to difference in temperature, as well as greater or lesser metallic purity. As set down by him, the variations in iron were between a maximum of 18·20 in regard to 100·00 of copper, and a minimum of 15·60, which gives a mean of 16·90. Taking this mean, the comparative list of the positions held by copper and iron in regard to electrical conductivity, according to the five investigators, may be set forth in the following summary:—

Copper Iron Davy 100·00 14·60 Becquerel 100·00 15·80 Lenz 100·00 17·74 Ohm 100·00 17·40 Pouillet 100·00 16·90

Taking the average of these five statements, it will be found that the relative conductivity of copper to iron stands as 100 to 16½--that is, a little over six to one. The approximate correctness of this figure, being the result of all the investigations by the most eminent men who studied the subject, can therefore admit of no reasonable doubt.

The important researches as to the greatly varying degree in which given quantities of metals will act as conductors of the electric force, were made possible only by the discovery of the singular phenomena of electro-magnetism, due chiefly to the Danish philosopher and naturalist, Hans Christian Oersted. His career, in some respects, was not unlike that of Benjamin Franklin. The son of an apothecary, born in 1777, he set up in the same business, not despising trade, but devoting himself actively to it, as means to an honourable end, that of gaining independence. Fascinated by the study of the phenomena of electricity, Oersted devoted himself to it heart and soul, as Franklin had done; and the result achieved, if not fully as important as the invention of the lightning conductor, was one filling a prominent place in modern scientific discovery. It had been observed, long before Oersted, that there was a close connection between what was known as magnetism and lightning, or rather, to state it more directly, it was known that lightning exercised a strong influence upon the magnetic needle. One of the most notable reports, and one of the first on the subject, came from the captains of two English vessels, sailing in company from London to the West Indies in the year 1675. When near the Bermudas, a stroke of lightning fell upon the mast of one of the vessels, doing considerable damage, and, as the captain believed, swinging his ship round, the men at the helm seeing the compass violently disturbed. He continued steering in what he believed the old direction, but noticed, a few minutes afterwards, that the other vessel, his former companion on the route, and which had not been struck by lightning, was following an opposite course. He had the good sense to approach it, and explanations ensued, the result being the discovery that the lightning had completely reversed the polarity of the magnetic needle, it pointing now south instead of north. The story of this met with much doubt at the outset, but it was amply verified before long by the report of many similar occurrences. It became known, not only that the polarity of the magnetic needle might be reversed by a stroke of lightning, but that the effect of the latter frequently was to magnetise iron and steel. An instance of this kind, on a large scale, occurred at Wakefield, Yorkshire, in the month of June 1731, during a violent thunderstorm. The lightning here entered the warehouse of a merchant who had just packed a case of knives, forks, and other articles of steel and cutlery ware, for despatch to the colonies. The case was placed immediately under the chimney, which the lightning entered, breaking open the box, and scattering over the floor of the room its contents, which, when afterwards examined, were all found to be strongly magnetic. These, and many similar facts, were all clearly established; yet a considerable time elapsed before important conclusions were drawn therefrom. As in the case of Franklin, so in that of Oersted, it required not merely scientific acumen, but a thoroughly practical mind, to trace, in the one instance, the actual connection between electricity and lightning, and in the other that between magnetism and electricity.

It was in the year 1819 that Hans Oersted, now settled as a lecturer at Copenhagen, announced the result of a series of investigations which laid the foundation for the new science of electro-magnetism. He stated that he had found that if a magnetic needle, free to move like that of a compass, was brought parallel to a wire charged with electricity, it would leave its natural place and take up a new one, dependent on the position of the wire and the needle relative to each other. If the needle, he said, was placed horizontally under the wire, the pole of the needle nearest the negative end of the electric battery would move westward, but, on the other hand, if the needle was placed above the wire, the same pole would move eastward. Again, if the needle was placed on the same horizontal plane as the wire, no motion would be on that plane, but the inclination would be to a vertical movement. Finally, if the wire was laid to the west of the needle, the pole nearest the negative side of the battery would be depressed, but it would be raised if the wire was placed to the east of the needle. From these observations, verified in numerous experiments, Oersted concluded that the magnetic action of the electric force moved in a circular manner around the conducting object, which he expressed in the formula that ‘the pole _above_ which the negative enters is turned to the west,’ and that ‘the pole _under_ which it enters is turned to the east.’ The discoveries of Oersted resulted in the creation of that wonderful production of modern science--the electric telegraph. A minor result, highly important as regards the erection, and still more the maintenance, of lightning conductors, was the construction of galvanometers.

What the microscope is to the student of the inner secrets of animal and vegetable life, the galvanometer is to the investigator of the phenomena of electricity, in their practical applications. Until its invention, there existed no means of practically testing the strength of the electric force, or the ‘current,’ as it is usually called, and it was not possible, therefore, to ascertain, in any given case, whether lightning conductors, among others, were really efficient or not. Perhaps, had it been only for this purpose, the galvanometer would have waited long in being constructed, but what brought it into existence, and led it to its present perfection, was that greatest of practical uses of electricity, the telegraph. As it arose from small beginnings to gradually more extended employment, embracing ultimately some of the highest interests of civilised mankind, there came the necessity of having instruments for gauging accurately the effects of the mysterious force thus put in harness at the bidding of science. The galvanometer having been devised, the next step, indispensable for its use, was to frame a standard by which electrical energy might be measured, and to invent terms by which the amount of such energy could be expressed. It is well known that in order to be able to measure the dimensions of any material object, standard units are required. In this country the units adopted are: for length, the foot; for weight, the pound; for time, the second; and so on. To express mechanical force or power, the foot pound is the unit employed--that is, the mechanical energy necessary to raise a weight of one pound to a height of one foot. On the Continent, where the units of length and weight are the metre and the gramme, the unit of mechanical energy is the metre gramme. Apart from the fact that the latter units are very generally adopted by all the Continental States, the simplicity of the decimal method of multiplying and sub-multiplying them renders the system of particular usefulness for scientific purposes; and they are therefore very extensively employed even in England in scientific research. Thus experimental results obtained in one country are at once understood, and are directly comparable with results obtained in any other country, without the necessity of reducing the figures to terms of units of other kinds than those in which they are expressed.

Now electrical energy being merely a form of mechanical energy--the one being capable of conversion into the other--it follows that the units of the functions of either of the two powers can be expressed in units of the other; and this being the case, it is manifestly both convenient and desirable that in forming the dimensions of the standard electrical units, they should be constructed in terms of the metre gramme, second units.

The proposition to do this originated with Dr. Weber, and acting upon this proposition a committee of the British Association, comprising nearly all the leading electricians of Great Britain, was formed some years ago, which committee, with almost perfect experimental skill, determined an absolute measure for the values of the several units required for electrical measurement. Taking as the unit quantity of electricity that amount which would be generated by a gramme weight falling through a distance of one metre in one second, the value given to the unit of resistance was such as would allow this unit quantity to flow through it in one second. The means by which the values were experimentally arrived at cannot be described here. It suffices to say, that the unit of resistance being once determined, copies of it, formed of lengths of wire of a platinum-iridium alloy, were issued, from which copies the sets of resistances now so largely employed by electricians were adjusted. Out of compliment to the great German physicist who first proposed the fundamental law which governed the flow of the electrical current, the unit of resistance was called the ‘Ohm.’ It was a marked progress on the practical application of the electric force to be enabled to measure it, and, as it were, bring it under control.

Without its help the electric telegraph could not have become what it is; nor has it been without notable use in the art of protection against lightning. One of the greatest steps in advance in the application of the lightning conductor, from its discovery to the present day, has been the invention of the galvanometer. Franklin could not, but we can, test our lightning conductors.

Some of the simplest and most practical galvanometers, specially designed for ascertaining the actual efficiency of conductors, have been made in recent years in Germany. The author of this work had constructed for him by Mr. H. Yeates, of Covent Garden, the one, with some improvements, as shown in the subjoined engravings: the first, fig. 1, exhibiting the arrangement of the battery and resistance coils, and the second, fig. 2, giving a diagram of the battery current. The battery consists of three cells, and is a modification of the old manganese cell, in which the carbon and oxide of manganese occupy the outer, and the zinc plate the inner, or porous, cell. By this arrangement, the surface of the negative element is greatly increased, and hence a more constant current is obtained, on account of the battery not polarising so rapidly as in the old form. Another advantage of this arrangement is, that the cells can be almost entirely sealed up, the air-openings being made within the porous cell. In the centre of the lid of the box is placed the galvanometer with a ‘tangent’ scale; and on the left are two terminals, by the connection of which the conductor can be examined. On the right hand end of the lid are placed five keys, marked respectively, L, B, 1, 2, 3. Under B is one pole of the battery, so that by depressing this key, as will be seen by the connections in the diagram (fig. 2), the battery current is sent through the galvanometer direct. If, however, key No. 1 is depressed, the battery is connected with the galvanometer through a known resistance--key No. 2 has a larger resistance, and No. 3. still larger. The fifth key, L, closes the circuit within the limit of the instrument, but on being depressed opens it, and includes the line or conductor placed between the two terminals at the other end, the battery key at the same time being pressed down. By this arrangement it will be seen that the resistance of the line or conductor may be compared with the known resistance connected with any of the keys Nos. 1, 2, 3, or any of these resistances may be included with that of the line, so as to get a convenient deflection of the galvanometer needle. In the case, with the battery, is a bobbin of insulated wire for connecting the instrument with the conductor and earth which is to be tested. The whole arrangement here described and illustrated is exceedingly portable, being in the form of a small carpet bag, and therefore particularly fitted for persons inspecting lightning conductors and making periodical tests, without which it cannot be too widely known there is really no trustworthy security of protection in lightning conductors.