_PROTECTION FROM LIGHTNING_
A flash of lightning is one of the most feared of nature’s
manifestations of power; and yet by the use of proper precautions its
ability to injure persons and property can be lessened greatly.
Speculations as to the nature of lightning were vague until Benjamin
Franklin boldly sent a kite into the teeth of a storm and tapped the
accumulated electricity in the cloud to charge one of his storage jars.
He connected the cloud with his jar by a wire made of a material which
he knew would conduct the electrical charge, and at the same time he
took the precaution not to hold the end of this wire himself. He
introduced between the end of the wire and his hand a piece of silk
cord, which is a non-conductor of electricity. Had he taken hold of the
end of the wire, the charge would have passed through him with probably
fatal results.
What is lightning? One naturally inquires for the reason of this storage
of electrical energy in the clouds. The explanation is not
forthcoming--at least there is none which is entirely satisfactory--but
the facts are well known. The mass of water-vapor which forms the clouds
becomes electrically charged just as a rubber comb does when rubbed on
the hair on a dry day, or as an ebonite ruler does when rubbed on a
cat-skin. Perhaps by contact with the air, which is in motion, the
particles of water become charged, and by the union of multitudes of
these the clouds are charged to a tremendous pressure. Lightning can be
produced artificially on a small scale by means of electric machines,
and the results of study of these artificial discharges have been to
show the following facts: The air is not a conductor of electricity, but
when the electrical pressure between two points becomes sufficiently
great the electric charge jumps suddenly between the two points at which
the pressure exists. It punctures a hole for itself through the air.
Lightning is the result. This discharge is very violent, and it is
accompanied by a strong smell of ozone, which is only very strong
oxygen. If one were to examine the points of the electric machine
between which the discharge took place, they might be found either hot
or cold, depending upon their size and the material of which they were
made. Some materials offer more resistance to the passage of the
electric charge than others, and when a considerable resistance is
offered, heat is produced in appreciable amounts at the places at which
the resistance is met. The application of this principle will be seen
when the effects of real lightning are considered.
In Figs. 131, 132, and 133 are shown lightning flashes taken by Mr. W.
N. Jennings.[9] These flashes are so soon over that without the aid of
the sensitive photographic plate it would be impossible to study them.
It will be noticed that the path of the charge is not straight, but
quite irregular; this path being that in which there is the least
resistance to the passage of the electricity. One strange phenomenon
which is brought out clearly in the pictures is that the discharge very
frequently divides into several branches. This is because it finds easy
paths in several directions and divides into smaller discharges, thus
finally disappearing.
[9] These three pictures are drawn, by permission, from photographic
illustrations by Mr. Jennings in Journal of the Franklin Institute,
vol. 133 (1892).
[Illustration: Fig. 131. Horizontal discharge of lightning.]
[Illustration: Fig. 132.
Meandering discharge.]
[Illustration: Fig. 133. Tree-form discharge.]
_Protection from lightning._--Having noticed briefly something of the
nature of lightning, the next point to be considered is its control, so
that the dangerous effects of a sudden discharge may be avoided. It has
long been known that by repeating Franklin’s experiment and connecting
the clouds with the earth, dangerous flashes of lightning can be avoided
to some extent; and this fact has given rise to much swindling on the
part of the “lightning-rod man,” who has frequently imposed on the
people through their fear of the results of lightning bolts. Any person
of average intelligence, with the knowledge of a few simple principles,
can put up a rod himself for the protection of his barn or dwelling at a
very reasonable expense.
METAL ROOFS
It has been noticed that metal roofs protect buildings even when no
lightning rods are used, especially if there are tin or iron water
pipes running to the ground. Even steam and gas pipes are good if
connected with the roof. Tin and copper roofs are not so common in the
country as in the city, and this is one of the many reasons why city
houses are less frequently struck by lightning than country ones. Copper
roofs are not used now as they once were on account of the great
expense; but from the electrical standpoint they are an excellent
protection to a house in a thunder storm. The writer has noticed in a
room in a city house, in which steam heat is used, that the lightning
will come in and down on the steam pipes without doing any harm. If one
will go into a telegraph station during a storm he will frequently
notice the discharges of lightning which take place through devices
provided for the purpose, and this without the least fire risk. This is
an illustration of the fact that, if properly provided for, the
dangerous element can be largely eliminated from a lightning discharge.
PROTECTING WOODEN ROOFS
If a metal roof is out of the question, the protection of the wooden
roof must be provided for. Very little attention has been paid in this
country to the proper erection and maintenance of lightning rods. It is
not sufficient to put up a point in an out-of-the-way place, and with a
careless ground connection, and then expect immunity from lightning. The
lightning rod will protect a wooden-roofed building if it is properly
installed; and in order that this simple but important piece of
apparatus be thoroughly understood it will now be considered in detail.
In the first place, it should be noted that there are two forms of
electric discharge or lightning which are provided for in equipping a
building with lightning protection: the brush discharge and the
disruptive discharge. The brush-form is so named because the fine
streamers of sparks which are emitted have somewhat the appearance of a
brush. This discharge is harmless, and one of the important functions of
the bunch of points on the upper end of the lightning rod is to quietly
take from the surrounding atmosphere the electricity there generated,
and thus prevent its accumulation to a dangerous extent. Very high
towers, such as steel windmills, high trees, and steeples do the
community a good service in this respect. But sometimes the discharges
cannot be dissipated through the brush form, but reach a high pressure,
and exhibit themselves with great violence, producing the booming and
crackling noise of thunder. This is the second form; and although the
points may be useful in this case too, yet if they are too far apart the
discharge may not seek them, but may take a shorter path through the
moist hay from which the hot, damp air is rising to the roof and forming
another lightning conductor. Protection from this can be partly provided
by the use of several points, not over forty feet apart; but in cases in
which lightning is very violent and frequent, the conductor should be
run all around the edges of the roof, and in several places to the
ground.
An experiment made by a noted electrician some years ago will illustrate
this point: A frame was made of iron wire in the shape of a barn, the
wire representing the edges of the walls and roof. The frame was
connected to the ground, or “grounded,” as the electricians say, and
then artificial lightning was allowed to play upon it from a distance of
a foot or more above. This gave a model about in proportion to the real
barn and actual lightning. All the discharge followed the wire frame,
and did not ignite a dummy of gun-cotton which was placed inside. The
instant that the metal barn frame was removed the dummy was struck and
burned violently. One can draw his own conclusions from an experiment of
this sort.
[Illustration: Fig. 134. Proper adjustment of lightning rods on a barn.]
A barn properly fitted with lightning rods is shown in Fig. 134. The
location of the points is such that there is not more than forty feet
between two adjacent ones. The rod projects about six feet above the
roof, and these projections are all connected by means of rod of the
same form as the vertical conductors. Sharp turns are avoided in
erecting the conductor, for an electric discharge would prefer to go
straight through the air rather than turn a corner.
It will now be necessary to go into some practical details of the
construction of lightning rods, and the suggestions that will be made
have been included here because good points or rods may not always be
readily obtainable. Their manufacture is easy and can be performed with
the limited facilities of a small village. If the raw materials have to
be bought at a distance, this can be easily done by correspondence.
Parts of the system: The equipment will consist of three parts--the
conductor and its support, the points, and the ground connection.
[Illustration: Fig. 135.
Supporting a rod.]
The conductor, or so-called “rod,” first demands attention. All metals
conduct electricity to some extent, but certain ones are very much
better than others. For example, lead, platinum, brass, and iron are
poor conductors, which is equivalent to saying that they heat up readily
on the passage of an electric current. On the other hand, silver,
copper, and aluminum are good conductors. In making a lightning rod, the
best all-round conductor should be used, when cost and conductivity are
the basis for the selection. As an example, take the metals iron,
copper, and aluminum for comparison. Iron is cheapest in price per
pound, but its electrical conductivity is small, while copper, though
more expensive, has so much more conductivity that to get rid of a
certain charge of electricity requires much less of it. So with
aluminum, which has slightly less conductivity and which costs more than
copper, but which is so light that a rod having the same conducting
ability when made of this metal actually costs less than one made of
copper, and the price of aluminum is constantly lessening, while that of
copper cannot fall much on account of the limited supply. To compare
actual figures, call the conductivity of copper 100, then that of steel
or iron will be about 18, and that of aluminum about 60. As to relative
weights, copper weighs about 550 pounds per cubic foot, iron or steel
480, and aluminum 160. As the prices of these materials are constantly
varying, it would be impossible to say at this time what the relative
costs would be at any other time; but it can be said that on the score
of cost there is little choice among them. For a number of reasons aside
from cost, copper is at present the best material, and these reasons
are: That it is smaller than the others for a given conducting ability,
and thus is more sightly; that it is easier to support on account of
this small size, and that it can be readily soldered to the ground
plate, which will be considered later.
In addition to the material of the lightning rod, its form is a matter
of considerable importance. The cable forms have been used extensively
and successfully, but the ribbon or flat form is better on account of
the smaller cost, and because there is a greater area exposed for the
dissipation of the heat generated by the lightning in passing from the
points to the ground. A rectangular section of three-quarters by
one-eighth of an inch is recommended.
In supporting the conductor from the wall or roof, it should be
separated or “insulated” from these surfaces. There is a slight chance
that the lightning might leave the conductor if the building were wet. A
more important reason for the use of the insulator is that the heat
which is generated on the surface of the rod when a heavy discharge
occurs will not be able, if supported away from the wall, to heat up any
inflammable material near it. Fig. 135 shows a method of support in
which one of the standard insulators used in running electric light and
other wires is employed. These insulators, which are made of porcelain
and iron, can be screwed into the wood or into a plug driven into the
joints between the stones very readily. The insulator shown is
manufactured by the General Electric Company, of Schenectady, New York,
and similar ones are made by other manufacturers of electrical
materials.
[Illustration: Fig. 136. Efficient points for a lightning rod.]
In order to attract the discharge, the rods must project some distance
above the roof, about 6 feet being the proper height. This projection
must be supported, and there are two ways to do this. The first is to
screw or nail a piece of timber to the side of the building, projecting
about 5 feet above the roof. Two insulators on this will provide the
necessary support for the rod. As this might be considered unsightly in
some places, a neater but more expensive method is to use a piece of
³⁄₄-inch copper, brass or iron rod for the upper 9 or 10 feet of the
rod. This can be easily joined and soldered to the copper ribbon and is
strong enough to support itself in any wind. A brace from the vertical
to the horizontal rod will provide additional support if desired, and
will give a more substantial appearance. At the point at which the
horizontal rod passes through a timber support, in case such plan is
used, a hole 1¹⁄₂ inches should be bored in the timber to avoid any risk
of its being burned. In joining the horizontal to the vertical rod, the
former should be bent up at right angles for an inch, and the surfaces
should then be well cleaned and soldered.
The points for attracting the discharge should be made very carefully,
and with a view to accommodating the brush discharge particularly. As a
rule, the more points in the bunch at the head of the rod the better
will the brush discharge be attracted; and for the same reason these
points should be sharp and bright. These facts have been determined by
experiment, from which it has been learned that the discharge is quieter
and at a lower pressure from sharp, bright terminals than from others.
Aluminum wire fulfils the requirements for the points better than any
other metal of reasonable cost. Unfortunately this metal is difficult to
solder, but if the directions here given are carefully followed there
will be no difficulty in producing a good bunch.
The sketch (Fig. 136) shows the general construction. In the end of a
block of copper of the dimensions shown, drill a hole ⁵⁄₈ of an inch in
diameter and 1 inch deep. Cut off a number of pieces of aluminum wire,
of about ¹⁄₁₆ of an inch in diameter, about 4 inches long. This wire
can be obtained from the Pittsburg Reduction Company, of Pittsburg,
Pennsylvania. These wires must then be filed to sharp points on one end,
the opposite ends being roughened with coarse sandpaper. Push as many of
the wires into the hole in the block as it will hold and bend the points
back so as to form a brush. Now heat some solder in a ladle and pour in
around the lower ends of the aluminum wires, having first taken the
precaution to heat the copper block so that the solder will flow well.
The conductor rod is then soldered into a slot filed in the lower end of
the block, and the bunch of points is complete.
The ground connection is the most important part of the whole equipment.
With poor ground connections, the rods become a menace to a building
rather than a protection. Examples could be cited where buildings were
actually struck and destroyed, even though “apparently properly rodded.”
In one case the wire entered but two inches into dry soil, while in
another the lower end was buried in concrete. It is absolutely essential
that the lower end of the rod be connected with moist earth in some way,
as this is the only method which will insure safety. If there are water
pipes in the building, they should be attached to the rod in the
basement in addition to the main ground connection.
As the charge is to be dissipated in the earth, it will be necessary to
expose a considerable area of metal under ground. If a spring is near,
the rod should be run to the vicinity of the spring and there soldered
to the ground plate, which should be below the level of the surface of
the spring. Moist soil is the only kind which will conduct electricity,
hence the insistence on a moist place for the terminal of the rod. In
case the plate must be planted some distance from water, either it must
go quite deep or it may be placed in a barrel of charcoal or coke buried
under the surface. These materials will hold whatever water they
receive, and it is a simple matter to wet the soil above such a terminal
from time to time. The plate itself should be of copper and of an area
of at least 25 square feet, including both sides. An old copper boiler,
flattened out, makes a cheap and effective ground plate.
There is no doubt that many buildings have been saved from destruction
by means of properly installed lightning rods, and it is plain that they
are not difficult nor expensive to install.