The Barre daily times. (Barre, Vt.) 1897-1959, 1907-04-02
HOLLOW LIGHTNING RODS,
Here IL One That Broke When the
Current Defended by It.

Current Defended by 1t.
In Nature, London, July 3, 1906,
page 230, Prof. j. "A, Pollock Of the
University Of sydney, n. S. w. des-
cribes a lighting stroke passing down a
hollow rod used as q lighting conductor
and crushing 1t inward, AS the copper
was about one millimeter thick con
siderable force would be required to do
this, but ais the diameter of the rod
IL not given nothing more definite can
be stated MS to the necessary pressure.
II is well known that lightning flash
involves the sudden expansion of the
air in the direct line Of discharge, thus
producing an explosive effect, which
is doubtless the origin Of the thunder.
The intensity Of this effect may be
greatly reinforced by the sudden con-
version into vapor Of any masses of
water or other liquid that may lie in
the path. In this way the bursting
asunder OF the limbs and bodies Of
trees and the boring Of the trenches in
the ground il explained.
The collapse Of the copper tubular
conductor is attributed BY Prof. Pollock
to compression produced by the electro
dynamic action Of the current when the
tube was hot and plastic. An equally
| plausible explanation is offered by Or.
| Irving Iang1nuir of New York who has
been investigating the dissertation Of
I gases and vapors around highly heated
wires, as set forth in the following
letter Of Aug, ~;
1 In reply to your letter of Aug. is,
would say that it is not possible that
! the dissertation of wster yapor on the
surface Of hollow lightning rod could
account for pressures sufficient to make
the rod collapse. AT the melting point
OF copper (10s4 degrees CO the dis;
ISOLATION OF water yapor under atmos-
pherie pressure is only 0.o05 per cent,
and even at the melting point of plati-
mum t1710 degrees CD II Is less than
1 / per cent. Under higher pressure the
| dissertation, even if complete, would
| increase the pressure only 50 per cent,
a small amount compared 10 the pres.
sure due to the heating Of the gases
around the rod.
would suggest as an explanation Of
1 the phenomenon the practically instan-
famous (explosive) vaporization Of q
| layer Of water ON the outside surfaee
Of the tubular rod Even thin film
/ If converted rapidly enough into steam
could produce very great pressure.
Even without the presence Of a filn
:. of water the pressure might be pro.
duced by the explosive expansion Of the
air near the outer surface of the rod
II is probable that the air inside the
tube would not be heated 50 rapidly
or to such a high temperature as the
air outside the tube, because the cur.
rent through the rod, being oscillary
would practically sbe confined to tht
outer layers.
In a later letter, dated Sept. is, DR.
Langmuir remarks:
| Prof. Pollocks explanation IA inter
estinQ - ! do know whether hs
made any calculation 70 see whether
I the electro magnetic forces would b,
Of the order OF magnitude required Il
crush copper tube. The problem in
terested me, 80 / endeavored to MAKE
such a calculation. According to Pocket
| w. Kohlrausch and L. Weber, the cur
rent in lightning discharge is, on th'
average, 10000 ampheres, with a maxi
mum value Of 10000 anpheres. Taking
this last figure as the basis for the cal
eulation and assuming the outside dia
meter Of the rod to be one centimeter
and the inside diameter O.s centimeter
II find that the electromagnetic fore
on the outside of the tube would b
equivalent to pressure Of AZ pound
per square inch. This would cause com
pressive stresses in the tube Of 2ii
pounds per square inch. This is cal
culated on the assumption that th'
current flows uniformly throughout tn
cross section of the copper of the tube
Ii the current IS limited 70 the outsid
layers because Of It's oscillatory char
, acter. the pressure would be about /
, per cent. less.
! a similar calculation, worked out 70
the case that the outer diameter Is 0.
centimeter and the inner diameter 0.
ccntimeter, gives for the pressure SO
pounds per square inch. and for tn
stress in the copper $40 pounds pc
square inch, With an osciilatory cui
: rent the pleasures would be about
per cent. less than these.
IT hardly seems probable that sue
: small pressurea lasting for such yer
/. short time as that during which th
J discharge lasts could produced the co
lapse of the rod. Copper does not b.
come very soft when heated below II
melting point, 80 we would not expeC
II to yield to pressure of such srna
magnitude
OF course the data on which tn
!. above calculation ARE based are to
uncertain and the results come out 10
. nearly TO the order OF magnitude nece.
; sary for the crushing Of the rod to er
4 able onn TO prove conelusiyely that III
electro. magnetic force i not the caus
+ Of the collapse. The pressure "a DIR,
& portional to the square Of the eurren
4 70 that a current Of 10000 ampher,
4 would produce four times the effect
calculated above Monthly Weatht
4 Review.
4 The old newspaper machines in mi
/ No. Of the International Paper Cc
az Bellows Falls are being taken o,
and replaced With equipments Tor NAL