substituting a thin iron wire for the stout copper wire at first
employed, a very curious result was obtained. The wire chosen was of
the same length as the copper, but had a resistance about 1,300 times
as great; its resistance being, in fact, 33.3 ohms. Nevertheless, in
this experiment, when the B knobs were at a distance of 1.43 inches,
no spark passed, which showed that the discharge always followed the
line of the conductor, and therefore that the conductor offered less
obstruction than 1.43 inches of air. The knobs were then brought
gradually nearer and nearer; and it was not until the distance was
considerably reduced that the sparks began to pass between them. When
the distance was exactly 1.03 inches, the discharge sometimes passed
between the knobs, and sometimes through the conductor; this was,
therefore, the _critical distance_, in the case of the iron wire. Thus
it appeared that the obstruction offered to the discharge by the iron
wire was much less than that offered by the copper, the one being equal
to a resistance of only 1.03 inches of air, the other to a resistance
of 1.43 inches.
It does not appear that Professor Lodge undertakes to offer any
satisfactory explanation of this result. He has come to the conclusion,
from his various experiments, that, in the case of a sudden discharge,
difference of conducting power between fairly good conductors is a
matter of practically no account; and that difference of sectional area
is a matter of only trifling account. But he does not see why a thin
iron wire should have a _smaller_ impedance than a much thicker wire of
copper. He proposes to repeat the experiments so as to confirm or to
modify the result, which for the present seems to him anomalous.[42]
=The Outer Shell only of a Lightning Rod Acts as a Conductor.=--As a
consequence of self-induction or electrical inertia, Professor Lodge
contends that a lightning discharge in a conductor consists of a
series of oscillations. These oscillations follow one another with
extraordinary rapidity--there may be a hundred thousand in a second,
there may be a million. Now it has been shown that, when a current
starts in a conductor, it does not start at once all through its
section; it begins on the outside, and then gradually, but rapidly,
penetrates to the interior. From this he infers that the extremely
rapid oscillations of a lightning discharge have not time to penetrate
to the interior of a conductor. The electricity keeps surging to
and fro in the superficial layer or outer shell, while the interior
substance of the rod remains inert and takes no part in the action. A
conductor, therefore, will be most efficient for carrying off a flash
of lightning if it present the greatest possible amount of surface; a
thin, flat tape will be more efficient than a rod of the same mass; and
a number of detached wires more efficient than a solid cylinder. As for
existing lightning conductors, the greater part of their mass would,
in many cases, have no efficacy whatever in carrying off a flash of
lightning.
=The Discussion.=--The discussion at the meeting of the British
Association was opened by Mr. William H. Preece, F.R.S., Electrician
to the Post Office, who claimed to have 500,000 lightning conductors
under his control. He expressed his conviction that a lightning rod,
properly erected and duly maintained, was a perfect protection against
injury from lightning; and in support of this conviction he urged
very strongly the report of the Lightning Rod Conference. This report
represented the mature judgment of the most eminent scientific men,
who had devoted years to the study of the question; and he wished
particularly to bring before the meeting their clear and decisive
assertion--an assertion he was there to defend--that “there is no
authentic case on record where a properly constructed conductor failed
to do its duty.”
The new views put forward by Professor Lodge were based, in great
measure, on his theory that a lightning discharge consisted of a series
of rapid oscillations. But this theory should be received with great
caution. It seemed to be nothing more than a deduction from certain
mathematical formulas, and was not supported by any solid basis of
observation or experiment. Besides, there were many facts against
it. They all knew that a flash of lightning magnetized steel bars,
deranged the compasses of ships at sea, and transmitted signals on
telegraph wires. But such effects could not be produced by a series of
oscillations, which, being equal and opposite, would neutralize each
other. It was alleged that these rapid oscillations occurred in the
discharge of a Leyden jar. That might be true, and probably was true;
but they were not dealing with Leyden jars, they were dealing with
flashes of lightning. If there was any analogy between the discharge of
a Leyden jar and a flash of lightning, it was to be found, not in the
external discharge employed by Professor Lodge in his experiments, but
in the bursting of the glass cylinder between the two coatings of the
jar.
Lord Rayleigh thought the experiments of Professor Lodge were likely
to have important practical applications to lightning conductors. But
though these experiments were valuable as suggestions, they did not
furnish a sufficient ground for adopting any new system of protection.
It was only by experience with lightning conductors themselves that the
question could be finally settled.
Sir William Thomson hoped for great fruit from the further
investigation of self-induction in the case of sudden electrical
discharges. He warmly encouraged Professor Lodge to continue his
researches; but he expressed no decided opinion on the question at
issue. Incidentally he observed that the best security for a gun-powder
magazine was an iron house; no lightning conductor at all, but an iron
roof, iron walls, and an iron floor. Wooden boards should, of course,
be placed over the floor to prevent the danger of sparks from people
walking on sheet-iron. This iron magazine might be placed on a dry
granite rock, or on wet ground; it might even be placed on a foundation
under water; it might be placed anywhere they pleased; no matter what
the surroundings were, the interior would be safe. He thought that was
an important practical conclusion which might safely be drawn from the
consideration of these electrical oscillations and the experiments
regarding them.
Professor Rowland, of the Johns Hopkins University, America, said that
the question seemed to be whether the experiment of Professor Lodge
actually represented the case of lightning. He was very much disposed
to think it did not. In the experiment almost the whole circuit
consisted of good conductors; whereas, in the case of lightning, the
path of the discharge was, for the most part, through the air, and
therefore it might be an entirely different phenomenon. The air being a
very bad conductor, a flash of lightning might, perhaps, not consist of
oscillations, but rather of a single swing. Moreover, it was not at all
clear that the length of the spark, in the experiment, could be taken
as a measure of the obstruction offered by the conductor. Professor
George Forbes was greatly impressed with the beauty and significance of
Professor Lodge’s experiments, but he did not think the result so clear
that they should be warranted in abandoning the principles laid down by
the Lightning Rod Conference.
M. de Fonvielle, of Paris, supported the views of Mr. Preece. He cited
the example of Paris, where they had erected a sufficient number
of lightning conductors, according to the received principles, and
calamities from lightning were practically unknown. He suggested that
the Eiffel Tower, which they were now building, and which would be
raised to the height of a thousand feet, would furnish an unrivalled
opportunity for experiments on lightning conductors.
Sir James Douglass, Chief Engineer to the Corporation of Trinity House,
had a large experience with lighthouse towers. The lightning rods on
these towers had been erected and maintained during the last fifty
years entirely according to the advice of Faraday. They never had a
serious accident; and such minor accidents as did occur from time to
time were always traced to some defect in the conductor. They had now
established a more rigid system of inspection, and he, for one, should
feel perfectly safe in any tower where this system was carried out.
Mr. Symons, F.R.S., Secretary to the Meteorological Society, had taken
part in a discussion on lightning conductors as long ago as 1859. It
had been a hobby with him all his life to investigate the circumstances
of every case he came across in which damage was done by lightning,
and the general impression left by his investigations entirely
coincided with the views just expressed by Sir James Douglass. He had
been a member of the Lightning Rod Conference, and was the editor of
their report; and he wished to enter his protest against the idea of
rejecting all that had hitherto been done in connection with lightning
conductors on the strength of mere laboratory experiments.
Professor Lodge, in reply, said he could perfectly understand the
position of those who held that a lightning rod properly fitted up
never failed to do its duty, because, whenever it failed, they said
it was not properly fitted up. The great resource in such cases was
to ascribe the failure to bad earth contact. He thought a good earth
contact was a very good thing, but he could not understand why such
extraordinary importance should be attached to it. A lightning rod
had two ends--an earth end and a sky end--and he did not see why good
contact was more necessary at one end than at the other. If a few sharp
points sticking out from the conductor were sufficient for a good sky
contact, why were they not sufficient also for a good earth contact?
Besides, though a bad earth contact might explain why a certain amount
of disruption should take place at the earth where the bad contact
existed, he did not see how it accounted for the flash shooting off
sideways half-way down the conductor. Again, what does a bad earth
contact mean? If an electrical engineer finds a resistance of a
hundred ohms, he will rightly pronounce the earth contact to be very
bad indeed. But why should the lightning flash leave a conductor
with a resistance of a hundred ohms in order to follow a line of
non-conductors where it encounters a resistance of many thousand ohms?
He accepted the statement of Mr. Preece that his whole theory depended
on the existence of oscillations in the lightning discharge; but there
was good reason to believe they existed, because they were proved to
exist in the discharge of a Leyden jar. Mr. Preece objected that an
oscillating discharge could not produce magnetic effects, as a flash of
lightning was known to do. He confessed he was unable to explain how
an oscillating discharge produced such effects;[43] but that it could
produce them there was no doubt whatever, for the discharge of a Leyden
jar produces magnetic effects, and we have ocular demonstration that
the discharge of a Leyden jar is an oscillating discharge.
As to the assurances we had received from electrical engineers that
a properly fitted lightning conductor never fails, he should like to
ask them how the Hotel de Ville, in Brussels, had been set on fire by
lightning on the 1st of last June. The system of lightning conductors
on this building had been erected in accordance with the received
theory, and had been held up by writers on the subject as the most
perfect in Europe. Unless some explanation were forthcoming to account
for its failure, we could no longer regard lightning conductors as a
perfect security against danger.
The President of Section A, Professor Fitzgerald, in bringing the
discussion to a close, observed that one result of this meeting would
be to give a new interest to the phenomena of static electricity and
its practical applications. He was inclined himself to think that the
experiments of Professor Lodge were not quite analogous to the case of
a flash of lightning. In comparing the discharge of a Leyden jar with
a flash of lightning they should look for the analogy, not so much in
the external discharge through a series of conductors, but rather,
as Mr. Preece had observed, in the bursting of the glass between the
two coatings of the jar. As regarded the oscillations in a Leyden jar
discharge, he did not think such oscillations were at all necessary
to account for the phenomena observed in the experiments. Many of
the results which Professor Lodge seemed to think would require some
millions of oscillations per second would be produced by a single
discharge lasting for a millionth of a second. Improvements, perhaps,
were possible in our present system of lightning conductors, but
practical experience had shown, however we might reason on the matter,
that, on the whole, lightning conductors had been a great protection to
mankind from the dangers of lightning.
=Summary.=--I will now try to sum up the results of this interesting
discussion, and state briefly the conclusions which, as it seems to
me, may be deduced from it. First, I would remind my readers that a
lightning rod has two functions to fulfill. Its first function is to
promote a gradual, but rapid, discharge of electricity according as it
is developed, and thus to prevent such an accumulation as would lead
to a flash of lightning. Its second function is to convey the flash
of lightning, when it does come, harmless to the earth. Now, the new
views advanced by Professor Lodge in no way impugn the efficiency of
lightning rods as regards their first function; and it is evident that
the greater the number of lightning rods distributed over a given area,
the more perfectly will this function be fulfilled. This is a point of
great practical importance which seemed to me, in some degree, lost
sight of during the progress of the discussion.
Secondly, it was practically admitted by the highest authorities that
the experiments and reasoning of Professor Lodge afford good grounds
for reconsidering the received theory of lightning conductors as
regards their second function--that of carrying the lightning flash
harmless to the earth. But there was undoubtedly a general feeling
that it would be rash to set aside, all at once, the received theory
on the strength of laboratory experiments made under conditions widely
different from those which actually exist in a lightning discharge.
Experiments are wanted on a larger scale; and, if possible, experiments
with lightning rods themselves.
Thirdly, the testimony of electrical engineers who have had large
experience with lightning conductors seems almost unanimous that a
lightning conductor erected and maintained in accordance with the
conditions prescribed by the Lightning Rod Conference gives perfect
protection. It was certainly unfortunate that the Hotel de Ville, in
Brussels, which was reputed the best protected building in Europe,
should have been damaged by lightning just two months before the
discussion took place; but no certain conclusion can be drawn from
this catastrophe until we know exactly the conditions under which it
occurred.
So the matter stands, awaiting further investigation.
FOOTNOTES:
[41] See his Lectures, published in the _Electrician_, June 22, June
29, July 6, and July 13, 1888.
[42] See paper read at the meeting of the British Association, in Bath,
1888, published in the _Electrician_, page 607. September 14.
[43] See a very ingenious hypothesis, to account for this phenomenon,
suggested by Professor Ewing in the _Electrician_, p. 712. October 5,
1888.
Transcriber’s Notes
Errors and omissions in punctuation have been corrected.
Page 11: “continuous inpression” changed to “continuous impression”
Page 41: “full conconducting power” changed to “full conducting power”
Page 44: “it base” changed to “its base”
Page 58: “follow one an-another” changed to “follow one another”