182 
THE CULTIVATOR. 
June, 
Lightning Rods. 
Our correspondent, E. L. of Greenville, Tennessee, 
requests information on the essential requisites of a 
good lightning conductor to his barn, 90 by 54 feet—a 
subject on which he has found conflicting theories, and 
contradictory information. 
This subject is often unnecessarily involved in a 
great deal of mystery, and as a consequence many 
useless directions are given. We shall therefore very 
briefly point out what is requisite in a good conductor, 
that our correspondent (as well as others) may know 
precisely what is wanted. 
1. Sharp points at the top, to draw off the electric 
fluid silently. When a rounded knob is made to ap¬ 
proach a highly electrified body,—as every one familiar 
with the use of the electric machine knows,—the fluid 
passes to the knob in the form of an explosion, as- soon 
as the knob approaches within a certain distance. This 
distance is longer as the charge is greater. But if a 
sharp point is made to approach the charged body, the 
fluid is drawn off silently in a stream, with no explo¬ 
sion whatever, and at a greater distance than its pas¬ 
sage could be effected between balls or blunt surfaces. 
Hence the very great importance of sharp points at the 
upper end of the rod—drawing off the lightning from 
a highly charged cloud, safely and silently, while the 
cloud is yet at such a distance from the building as to 
preclude the danger of striking. Where, however, the 
cloud is unusually loaded with electricity, and ap¬ 
proaches very rapidly, the point may be insufficient to 
drain the whole, and an explosion, producing heavy 
thunder, may take place. This may be prevented, at 
least in a great degree, by several points at the upper 
extremity of the rod, all of which may operate together. 
The same effect will be produced whether these points 
are within an inch of-each other or a foot asunder. 
Some years ago, we examined a rod having but a sin¬ 
gle point, just after a terrific clap of thunder which 
followed instantaneously the flash, and we found that 
the point, which was tipped with thin silver, had been 
melted, iron and all, into a ball the size of a lead bullet. 
Portions of the building were very slightly splintered; 
and without the rod it would have been doubtless torn 
to pieces. Had there been several points, it is proba¬ 
ble none would have been melted; and possibly the 
explosion might have been less severe.- As it was, the 
largest cannon would have been a mere pop-gun to it. 
It is not necessary that the point be tipped with sil¬ 
ver, gold or platinum ; if filed as sharp and bright as 
a needle, it will remain so mapy years, and never be¬ 
come dull enough to cause damage. 
It is said that soon after the Revolutionary war broke 
out between England and America, and after Dr 
Eranklin had made his electric discoveries, certain of¬ 
ficials of the British government, unwilling to ac¬ 
knowledge their indebtedness to the Yankee, substitut¬ 
ed knobs for points, which brought ■ out the following 
epigram :— 
While you, great George, to show affront, 
Your sharp conductors change to blur"’ 
The nation’s out of joint; 
Franklin a wiser course pursues, 
And all your thunder fearless views, 
By keeping to the point. 
2. A sufficient height above the roof of the build¬ 
ing. It is said that lightning has never been known 
to strike within a distance from the rod, twice as great 
as the height of the rod above the building. Or in 
other words, a rod will protect a circular space whose 
diameter is four times as great as the height of the 
point. A barn forty feet long would therefore be safe 
with a rod at the middle ten feet above it; or with a 
rod at one end twenty feet above; or with a rod at 
each end, ten feet above. A barn 90 feet long, like 
that of our Tennessee correspondent, would require a 
rod. at the center, or at each end, 22 feet high above 
the roof. Or, a rod might be placed 22£ feet from each 
end, only eleven feet high, and be equally efficacious. 
These two rods might be connected by a horizontal one 
to a larger one at the center running down into the 
earth., 
3. A continuous connection throughout. The best 
rods are welded together in one piece. But where this 
is inconvenient, the two parts may be screwed together 
with each end into one common nut; or they may be 
placed side by side and rivited, or screwed together. 
The connection is as complete, so far as thunder and 
lightning are concerned, by two surfaces in actual con¬ 
tact merely, as if welded together. The only precau¬ 
tion in this case being to secure the connection from 
any accident which might after a lapse of years pro¬ 
duce a separation, and which would immediately make 
the rod worse than no rod at all, by attracting the fluid, 
without a provision for its escape to the earth. 
4. Entering the earth several feet at the bottom. 
However perfect every part of the rod may be, it will 
do more harm than good, unless the fluid can pass off 
with perfect facility and become dissipated in the earth. 
Moist earth is always a pretty good conductor; dry 
earth is a poor conductor. It is therefore highly re¬ 
quisite that the rod descend deep enough to enter the 
permanent^ moist stratum, or the portion that rema ins 
moist independently of the drouth of summer. This 
will of course vary in different places; but as a gene¬ 
ral rule, a hole at least six feet deep should be dug 
for this purpose. It will greatly assist the dissipation 
of the fluid by placing in the bottom of the hole, in 
contact with the rod, at least half-a-bushel of common 
charcoal, which is an excellent conductor. 
Some rods, perfect in other respects, become worse 
than useless by not sinking deep enough. Some years 
ago, a meeting-house was struck with lightning, al¬ 
though furnished with what appeared to be an excel¬ 
lent conductor. The building was injured, and the 
earth torn up around it. On examination, the lower 
end was found to enter the earth but a short distance, 
and the season being very dry, and the soil destitute of 
moisture, no escape was afforded for the electric fluid 
5. The size of the rod. should be sufficient to carry 
off any discharge without becoming heated, as would 
be the case if too small. Three fourths of an inch 
would doubtless be enough in any case whatever. The 
portion above the building must horvever be stiffer, to 
withstand stormy winds, especially if it rise many feet 
above the roof. We have found the best way by weld¬ 
ing together three or four pieces of different sizes, 
placing the larger below. This gives the rod a gradual 
taper to the top. The larger portion should extend 
down far enough to admit of thorough stiffening. 
6. The proper mode of fastening to the building, 
is very imperfectly understood. The electric fluid al¬ 
ways takes the best conductor, and will not pass into 
wood or other imperfect conductor, so long as it can 
find a ready escape through metal. The discharge 
from a machine severe enough to knock a man sense¬ 
less, may pass unfelt through a metalic rod held in the 
hand, provided the escape through the rod is perfect 
and unimpeded. So, there is no danger of the fluid 
passing into the bnilding through wooden supports, so 
long as there is a ready communication to the moist 
earth below. Nothing can be more absurd than the 
common practice of passing the conductor through 
