[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 deg..  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.