the clearest possible way, the whole theory of lightning conductors.
In particular, they give us ocular demonstration that an efficient
lightning rod not only makes the lightning harmless when it comes,
but tends very much to prevent its coming. A remarkable example, on a
large scale, of this important property, is furnished by the town of
Pietermaritzburg, the capital of the colony of Natal, in South Africa.
This town is subject to the frequent visitation of thunderstorms,
at certain seasons of the year, and much damage was formerly done
by lightning, but since the erection of lightning conductors on the
principal buildings, the lightning has never fallen within the town.
Thunderclouds come as before, but they pass silently over the city,
and only begin to emit their lightning flashes when they reach the
open country, and have passed beyond the range of the lightning
conductors.[28]
But it will often happen, even in the case of a pointed conductor,
that the accumulation of electricity goes on so fast that the silent
discharge is insufficient to keep it in check. A disruptive discharge
will then take place, from time to time, and a flash of lightning will
pass. Under these circumstances, the lightning conductor is called upon
to fulfill its second function, and to convey the lightning harmless to
the earth.
=Conditions of a Lightning Conductor.=--From the consideration of
the functions which it has to fulfill, we may now infer what are the
conditions necessary for an efficient lightning conductor. The first
condition is that the end of the conductor, projecting into the air,
should have, at least, one sharp point. Our experiments have shown us
that a pointed conductor tends, in a manner, to suppress the flash
of lightning altogether; whereas a blunt conductor, or one ending in
a ball, tends only to make it harmless when it comes. It is evident,
therefore, that the pointed conductor offers the greater security.
But a fine point is very liable to be melted when the lightning falls
upon it, and thus to be rendered less efficient for future service. To
meet this danger, it has recently been suggested, by the Lightning Rod
Conference, that the extreme end of the conductor should be a blunt
point, destined to receive the full force of the lightning flash, when
it comes; and that, a little lower down, a number of very fine points
should be provided, with a view to favor the silent discharge. This
suggestion, which appears admirably fitted to provide for the twofold
function of a lightning conductor, deserves to be recorded in the exact
terms of the official report.
“It seems best to separate the double functions of the point,
prolonging the upper terminal to the very summit, and merely beveling
it off, so that, if a disruptive discharge does take place, the full
conducting power of the rod may be ready to receive it. At the same
time, having regard to the importance of silent discharge from sharp
points, we suggest that, at one foot below the extreme top of the upper
terminal, there be firmly attached, by screws and solder, a copper
ring bearing three or four copper needles, each six inches long, and
tapering from a quarter of an inch diameter to as fine a point as can
be made; and with the object of rendering the sharpness as permanent
as possible, we advise that they be platinized, gilded, or nickel
plated.”[29]
The second condition of a lightning conductor is, that it should be
made of such material, and of such dimensions, as to offer an easy
passage to the greatest flash of lightning likely to fall on it;
otherwise it might be melted by the discharge, and the lightning,
seeking for itself another path, might force its way through bad
conductors, which it would partly rend asunder, and partly consume
by fire. Copper is now generally regarded as the best material for
lightning conductors, and it is almost universally employed in these
countries. If it is used in the form of a rope, it should not be less
than half an inch in diameter; if a band of copper is preferred--and it
is often found more convenient by builders--it should be about an inch
and a half broad and an eighth of an inch thick. In France it has been
hitherto more usual to employ iron rods for lightning conductors, but
since iron is much inferior to copper in its conducting power, the iron
rod must be of much larger dimensions; it should be at least one inch
in diameter.[30]
The third condition is that the lightning conductor should be
continuous throughout its whole length, and should be placed in good
electrical contact with the earth. This is a condition of the first
importance, and experience has shown that it is the one most likely
of all to be neglected. In a large town the best earth connection is
furnished by the system of water-mains and gas-mains, each of which
constitutes a great network of conductors everywhere in contact with
the earth. Two points, however, must be carefully attended to--first,
that the electrical contact between the lightning conductor and the
metal pipe should be absolutely perfect; and, secondly, that the pipe
selected should be of such large dimensions as to allow the lightning
an easy passage through it to the principal main.
If no such system of water-pipes or gas-pipes is at hand, then the
lightning rod should be connected with moist earth by means of a bed of
charcoal or a metal plate not less than three feet square. This metal
plate should be always of the same material as the conductor, otherwise
a galvanic action would be set up between the two metals, which in
course of time might seriously damage the contact. Dry earth, sand,
rock, and shingle are bad conductors; and, if such materials exist near
the surface of the earth, the lightning rod must pass through them and
be carried down until it reaches water or permanently damp earth.
=Mischief Done by Bad Conductors.=--If the earth contact is bad, a
lightning conductor does more harm than good. It invites the lightning
down upon the building without providing for it, at the same time, a
free passage to earth. The consequence is that the lightning forces
a way for itself, violently bursting asunder whatever opposes its
progress, and setting fire to whatever is combustible.
I will give you some recent and striking examples. In the month of May,
1879, the church of Laughton-en-le-Morthen, in England, though provided
with a conductor, was struck by lightning and sustained considerable
damage. On examination it was found that the lightning followed the
conductor down along the spire as far as the roof; then, changing its
course, it forced its way through a buttress of massive masonwork,
dislodging about two cartloads of stones, and leaped over to the leads
of the roof, about six feet distant. It now followed the leads until it
came to the cast-iron down-pipes intended to discharge the rain-water,
and through these it descended to the earth. When the earth contact
of the lightning conductor was examined, it was found exceedingly
deficient. The rod was simply bent underground, and buried in dry
loose rubbish at a depth not exceeding eighteen inches. This is a very
instructive example. The lightning had a choice of two paths--one by
the conductor prepared for it, the other by the leads of the roof
and the down-pipes--and, by a kind of instinct which, however we may
explain, we must always contemplate with wonder, it chose the path of
least resistance, though in doing so it had to burst its way at the
outset through a massive wall of solid masonry.[31]
On the 5th of June, in the same year, a flash of lightning struck the
house of Mr. Osbaldiston, near Sheffield, and, notwithstanding the
supposed protection of a lightning conductor, it did damage to the
amount of about five hundred pounds. The lightning here followed the
conductor to a point about nine feet from the ground, then passed
through a thick wall to a gas-pipe at the back of the drawing-room
mirror. It melted the gas-pipe, set fire to the gas, smashed the
mirror to atoms, broke the Sevres vases on the chimney-piece, and
dashed the furniture about. In this case, as in the former, it was
found that the earth contact was bad; and, in addition, the conductor
itself was of too small dimensions. Hence, the electric discharge
found an easier path to earth through the gas-pipes, though to reach
them it had to force for itself a passage through a resisting mass of
non-conductors.[32]
Again in the same year, on the 28th of May, the house of Mr. Tomes, of
Caterham, was struck by lightning, and some slight damage was done.
After a careful examination it was found that the greater part of
the discharge left the lightning conductor with which the house was
provided, and passed over the slope of the roof to an attic room, into
which it forced its way through a brick wall, and reached a small iron
cistern. This cistern was connected by an iron pipe of considerable
dimensions with two pumps in the basement story; and through them the
lightning found an easy passage to the earth, and did but little harm
on its way. When the earth contact of the lightning conductor was
examined, it was discovered that the end of the rod was simply stuck
into a dry chalky soil to a depth of about twelve inches. Thus in this
case, as in the two former, it was made quite clear that the lightning
conductor failed to fulfill its functions because the earth contact was
bad.[33]
Cases are not uncommon in which builders provide underground a
carefully constructed reservoir of water, into which the lower end
of the lightning rod is introduced. The idea seems to prevail that a
reservoir of water constitutes a good earth contact; and this is quite
true of a natural reservoir, such as a lake, where the water is in
contact with moist earth over a considerable area. But an artificial
reservoir may have quite an opposite character, and practically
insulate the lightning conductor from the earth. One which came under
my notice lately, in the neighborhood of this city, consists of a large
earthenware pipe set on end in a bed of cement, and kept half full of
water. Now, the earthenware pipe is a good insulator, and so is the bed
of cement in which it rests; and the whole arrangement is identical,
in all essential features, with the apparatus of Professor Richman, in
which he introduced his lightning rod into a glass bottle, and by which
he lost his life a hundred and thirty years ago.
A conductor mounted in this manner will, probably enough, draw down
lightning from the clouds; but it is more likely to discharge it, with
destructive effect, into the building it is intended to guard, than
to transmit it harmlessly to the earth. An example is at hand in the
case of Christ Church, in the town of Clevedon, in Somersetshire.
This church was provided with a very efficient system of lightning
conductors, five in number, corresponding to the four pinnacles and the
flagstaff, on the summit of the principal tower. The five conductors
consisted of good copper-wire rope; all were united together inside the
tower, through which they were carried down to earth, and there ended
in an earthenware drain. This kind of earth contact might be pretty
good as long as water was flowing in the drain; but whenever the drain
was dry the conductor was practically insulated from the earth. On the
fifteenth of March, 1876, the church was struck by lightning, which
for some distance followed the line of the conductor; then finding its
passage barred by the earthenware drain, which was dry at the time, it
burst through the walls of the church, displacing several hundredweight
of stone, and making its way to earth through the gas-pipe.[34]
Another very instructive example is furnished by the lightning
conductor attached to the lighthouse of Berehaven, on the south-west
coast of Ireland. It consists of a half-inch copper-wire rope, which
is carried down the face of the tower “until it reaches the rock
at its base, where it terminates in _a small hole, three inches by
three inches, jumped out of the rock, about six inches under the
surface_.” Here, again, we have a good imitation of Professor Richman’s
experiment, with only this difference, that a small hole in the rock
is substituted for a glass bottle. A lightning conductor of this kind
fulfills two functions: it increases the chance of the lightning coming
down on the building, and it makes it positively certain that, having
come, it cannot get to earth without doing mischief.
The lightning did come down on the Berehaven Lighthouse, about five
years ago. As might have been expected, it made no use of the lightning
conductor in finding a path to earth, but forced its way through the
building, dealing destruction around as it descended from stage to
stage. The Board of Irish Lights furnished a detailed report of this
accident to the Lightning Rod Conference, in March, 1880, from which
the above particulars have been derived.[35]
=Precaution Against Rival Conductors.=--But it is not enough to
provide a good lightning conductor, which is itself able to convey
the electric discharge harmless to the earth; we must take care that
there are no rival conductors near at hand in the building, to draw
off the lightning from the path prepared for it, and conduct it by
another route in which its course might be marked with destruction.
This precaution is of especial importance at the present day, owing to
the great extent to which metal, of various kinds, is employed in the
construction and fittings of modern buildings. I will take a typical
case which will bring home this point clearly to your minds.
A great part of the roof of many large buildings is covered with lead.
The lead, at one or more points may come near the gutters intended
to collect the rain water; the gutters are in connection with the
cast-iron down-pipes into which the water flows, and these down-pipes
often pass into the earth, which, under the circumstances, is generally
moist, and, therefore, in good electrical contact with the metal pipes.
Here, then, is an irregular line of conductors, which, though it has
gaps here and there, may, under certain conditions, offer to the
lightning discharge a path not less free than the lightning conductor
itself. What is the consequence? The flash of lightning, or a part of
it, will quit the lightning rod, and make its way to earth through the
broken series of conductors, doing, perhaps, serious mischief, as it
leaps across, or bursts asunder, the non-conducting links in the chain.
Another illustration may be taken from the gas and water-pipes, with
which almost all buildings in great cities are now provided, and which
constitute a network of conductors, spreading out over the walls and
ceilings, and stretching down into the earth, with which they have
the best possible electrical contact. Now, it often happens that a
lightning conductor, at some point in its course, comes within a short
distance of this network of pipes. In such a case, a portion of the
electrical discharge is apt to leave the lightning conductor, force
its way destructively through masses of masonry, enter the network of
pipes, melt the leaden gas-pipe, ignite the gas, and set the building
on fire.
These are not merely the speculations of philosophers. All the various
incidents I have just described have occurred, over and over again,
during the last few years. You will remember, in some of the examples
I have already set before you, when the electric discharge failed to
find a sufficient path to earth through the lightning rod, it followed
some such broken series of chance conductors as we are now considering.
But this broken series of conductors seems to bring with it a special
danger of its own, even when the lightning conductor is otherwise in
efficient working order. I will give you just one case in point.
On the fifth of June, 1879, the Church of Saint Marie, Rugby, was
struck by lightning and set on fire, and narrowly escaped being
burned to the ground. A number of workmen were engaged on that day in
repairing the spire of the church. About three o’clock they saw a dense
black cloud approaching, and they came down to take shelter within the
building. In a few minutes they heard a terrific crash just overhead;
at the same moment the gas was lighted under the organ loft and the
woodwork was set in a blaze. The men soon succeeded in putting out the
fire, and the church escaped with very little damage.
Now, in this case there was no reason to suppose that the lightning
conductor was in any way defective. But about half-way up the spire
there was a peal of eight bells. Attached to these bells were iron
wires, about the eighth of an inch in diameter, leading from the
clappers down to the organ-loft, where they came within a short
distance of a gas-pipe fixed in the wall. It would seem that a great
part of the discharge was carried safely to earth by the lightning
conductor. But a part branched off at the bells in the spire, descended
by the iron wires, and forced its way into the organ loft, to reach
the network of gas-pipes, through which it passed down to the earth,
melting the soft leaden gas-pipe in its course and lighting the gas.
The remedy for this danger is obvious. All large masses of metal used
in the structure of a building--the leads and gutters of the roof,
the cast-iron down-pipes, the iron gas and water mains--should be put
in good metallic connection with the lightning conductor, and, as
far as may be, with one another. Connected in this way they furnish
a continuous and effective line of conductors leading safely down to
earth; and, instead of being a dangerous rival, they become a useful
auxiliary to the lightning rod.
I would observe, however, that the lightning conductor ought not to
be connected directly with the soft leaden pipes which are commonly
employed to convey gas and water to the several parts of a building.
Such pipes, as we have seen, are liable to be melted when any
considerable part of the lightning discharge passes through them; and
thus much harm might be done, and the building might even be set on
fire by the lighting of the gas. Every good end will be attained if the
conductor is put in metallic connection with the iron gas and water
_mains_ either inside or outside the building.
=Insulation of Lightning Conductors.=--It is a question often asked
whether a lightning rod should be insulated from the building it
is intended to protect. I believe that this practice was formerly
recommended by some writers, and I have observed that glass insulators
are still employed not infrequently by builders in the erection of
lightning conductors; but, from the principles I have set before
you to-day, it seems clear that any insulation of this kind is, to
say the least, altogether useless. The building to be protected is
itself in electrical communication with the earth, and the lightning
conductor, if efficient, is also in electrical communication with the
earth--therefore, the lightning conductor and the building are in
electrical communication with each other through the earth, and any
attempt at insulating them from one another above the earth is only
labor thrown away.
Further, I have just shown you that the masses of metal employed in
the structure or decoration of a building ought to be electrically
connected with each other and with the lightning conductor. Now, if
this be done, the lightning conductor is, by the fact, in direct
communication with the building, and the glass insulators are utterly
futile. Again, the building itself, during a thunderstorm, becomes
highly electrified by the inductive action of the cloud, and needs to
be discharged through the conductor just as the surrounding earth needs
to be discharged; therefore, the more thoroughly it is connected with
the conductor, the more effectively will the conductor fulfill its
functions.
=Personal Safety in a Thunderstorm.=--I suppose there is hardly any one
to whom the question has not occurred, at some time or another, what
he had best do to secure his personal safety during a thunderstorm.
This question is of so much practical interest that I think I shall
be excused if I say a few words about it, though perhaps, strictly
speaking, it is somewhat beside the subject of lightning conductors.
At the outset, perhaps, I shall surprise you when I say that you would
enjoy the most perfect security if you were in a chamber entirely
composed of metal plates, or in a cage constructed of metal bars, or
if you were incased, like the knights of old, in a complete suit of
metal armor. This kind of defense is looked upon as so perfect, among
scientific men, that Professor Tait does not hesitate to recommend
his adventurous young friends devoted to the cause of science to
provide themselves with a light suit of copper, and, thus protected,
take the first opportunity of plunging into a thundercloud, there
to investigate, at its source, the process by which lightning is
manufactured.[36]
The reason why a metal covering affords complete protection is that,
when a conductor is electrified, the whole charge of electricity
exists on the outside surface of the conductor; and therefore, when a
discharge takes place, it is only the outside surface that is affected.
Thus, if you were completely incased in a metal covering, and then
charged with electricity by the inductive action of a thundercloud, it
is only the metal covering that would undergo any change of electrical
condition; and when the lightning flash would pass, it is only the
metal covering that would be discharged.
Let me show you a very pretty and interesting experiment to illustrate
this principle: Here is a hollow brass cylinder, open at the ends,
mounted on an insulating stand. On the outside is erected a light brass
rod with two pith balls suspended from it by linen threads. Two pith
balls are also suspended by linen threads from the inner surface of
the cylinder. You know that these pith balls will indicate to us the
electrical condition of the surfaces to which they are attached. If the
surface be electrified, the pith balls attached to it will share in
its electrical condition, and will repel each other; if the surface be
neutral, the pith balls attached to it will be neutral, and will remain
at rest.
I now put this apparatus under the influence of our thundercloud, that
is, the large brass conductor of our machine. The moment my assistant
turns the handle, the electricity begins to be developed on the
conductor, and you see, at once, the effect on the brass cylinder. The
pith balls attached to the outer surface fly asunder; those attached
to the inner surface remain at rest. And now a spark passes; our
thundercloud is discharged; the inductive action ceases; the pith balls
on the outside suddenly collapse, while those on the inside are in no
way affected.
[Illustration: PROTECTION FROM LIGHTNING FURNISHED BY A CLOSED
CONDUCTOR.]
It is not necessary that the brass cylinder should be insulated. To
vary the experiment, I will now connect it with the earth by a chain;
you will observe that the effect is precisely the same as before.
Flash after flash passes while the machine continues in action; the
outside pith balls fly about violently, being charged and discharged
alternately; the inside pith balls remain all the time at rest. Thus
you see clearly that, if you were sitting inside such a metal chamber
as this, or covered with a complete suit of metal armor, you would
be perfectly secure during a thunderstorm, whether the chamber were
electrically connected with the earth or insulated from it.