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.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
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.