Nitro-Explosives: A Practical Treatise | Page 6

lessens the chance of matches or
steel implements being taken upon the danger area. Changing houses,
one for the men, and another for the girls, should also be provided. The
tools used upon the danger area should, whenever the building is in use,
or contains explosives, be made of phosphor bronze or brass, and brass
nails or wooden pegs should be used in the construction of all the
buildings.
[Illustration: FIG. 2.--MELSENS SYSTEM OF LIGHTNING
CONDUCTORS.]
~Lightning Conductors.~--The Explosive Substances Act, 38 Vict. ch.
17, clause 10, says, "Every factory magazine and expense magazine in
a factory, and every danger building in a magazine, shall have attached
thereto a sufficient lightning conductor, unless by reason of the
construction by excavation or the position of such magazine or building,

or otherwise, the Secretary of State considers a conductor unnecessary,
and every danger building in a factory shall, if so required by the
Secretary of State, have attached thereto a sufficient lightning
conductor."
The exact form of lightning conductor most suitable for explosive
works and buildings has not yet been definitely settled. Lightning-rod
engineers favour what is known as the Melsens system, due to
Professor Melsens, of Brussels, and Professor Zenger, of Prague, but
first suggested by the late Professor Clerk-Maxwell. In a paper read
before the British Association, Clerk-Maxwell proposed to protect
powder-magazines from the effects of lightning by completely
surrounding or encasing them with sheet metal, or a cage of metallic
conductors. There were, however, several objections to his system as he
left it.
Professor Melsens[A] has, while using the idea, made several important
alterations. He has multiplied the terminals, the conductors, and the
earth-connections. His terminals are very numerous, and assume the
form of an aigrette or brush with five or seven points, the central point
being a little higher than the rest, which form with it an angle of 45°.
He employs for the most part galvanised-iron wire. He places all
metallic bodies, if they are of any considerable size, in communication
with the conducting system in such a manner as to form closed metallic
circuits. His system is illustrated in Fig. 2, taken from Arms and
Explosives.
[Footnote A: Belgian Academy of Science.]
This system is a near approximation to J.C. Maxwell's cage. The
system was really designed for the protection of powder-magazines or
store buildings placed in very exposed situations. Zenger's system is
identical with that of Melsens, and has been extensively tried by the
Austrian military authorities, and Colonel Hess has reported upon the
absolute safety of the system.
[Illustration: Fig. 3.--FRENCH SYSTEM OF LIGHTNING
CONDUCTORS.]

The French system of protecting powder-magazines is shown in Fig. 3,
where there are no brush terminals or aigrettes. The French military
authorities also protect magazines by erecting two or more
lightning-rods on poles of sufficient height placed close to, but not
touching, the walls of the magazine. These conductors are joined below
the foundations and earthed as usual.
In the instructions issued by the Government, it is stated that the
lightning-rods placed upon powder-mills should be of such a height,
and so situated, that no danger is incurred in igniting the powder-dust
in the air by the lightning discharge at the pointed rod. In such a case a
fork or aigrette of five or more points should invariably be used in
place of a single point.
[Illustration: FIG. 4a.--GOVERNMENT SYSTEM OF LIGHTNING
CONDUCTORS FOR LARGE BUILDINGS.]
[Illustration: FIG. 4b.--GOVERNMENT SYSTEM OF LIGHTNING
CONDUCTORS FOR SMALL BUILDINGS.]
In Fig. 4 (a and _b_) is shown the Government method for protecting
buildings in which explosives are made or stored. Multiple points or
aigrettes would be better. Lord Kelvin and Professor Melsens favour
points, and it is generally admitted that lightning does not strike
buildings at a single point, but rather in a sheet; hence, in such cases, or
in the event of the globular form being assumed by the lightning, the
aigrette will constitute a much more effective protection than a single
point. As to the spacing of conductors, they may, even on the most
important buildings, be spaced at intervals of 50 feet. There will then
be no point on the building more than 25 feet from the conductor. This
"25-feet rule" can be adhered to with advantage in all overground
buildings for explosives.
Underground magazines should, whenever possible, also be protected,
because, although less exposed than overground buildings, they
frequently contain explosives packed in metal cases, and hence would
present a line of smaller electrical resistance than the surrounding earth
would offer to the lightning. The conductor should be arranged on the

same system as for overground buildings, but be applied to the surface
of the ground over the magazines.
In all situations where several conductors are joined in one system, the
vertical conductors should be connected both at the top