It is now just about a hundred and fifty years since Benjamin Franklin, as related in his delightful autobiography, wrote a paper "on the sameness of lightning with electricity", and thereby grievously offended the Abbe Nollet, who, being "Preceptor in Natural Philosophy to the Royal Family" of France, could not brook a theory in contradiction of his own. The Abie's opposition was of no avail, and Franklin's discovery was not long without practical results; if electricity could be brought from the clouds by means of a kite, why might not permanent conductors be attached to buildings for the purpose of conveying atmospheric electricity safely to earth, thereby preventing the sudden and terrible havoc so frequently wrought by lightning? Franklin acted on the thought, and the first lightning-conductor was that fixed on his own house in Philadelphia in 1752.

The fact lying at the root of the subject is that some metals are exceedingly good conductors of electricity, and are therefore capable of carrying safely away a quantity of electricity which would be sufficient to melt metals of less conductivity, and which, if allowed to explode, might do considerable damage to life and property.

Copper is the metal almost invariably used for lightning-conductors, its conductivity being three times that of zinc, six times that of iron, and ten times that of lead. As the efficiency of a conductor is in proportion to the conductivity and sectional areas of the metal employed, it follows that an iron con-ductor must have six times the sectional area of a copper one, if it is to be as efficient. There is little doubt that iron soil-pipes occasionally serve as lightning-conductors, as they have the necessary exposed top. earth-connection at the bottom, and continuous metal between, but it is not wise, especially in exposed situations, to trust to the soil-pipe for protection against lightning. Every lofty detached building should be provided with one or more special lightning-conductors of copper, independently of all soil-pipes and the like.

The three essentials of a good lightning-conductor are: (1) the terminal at the top, (2) the metal-conductor or rod, and (3) the earth-connection at the bottom.

The terminal at the top of a lightning-conductor is usually a copper tube, and should be fixed so as to stand clear above the highest part of the building; it must be provided with a number of solid "points", as in Fig. 711, for receiving the discharge of electricity. It is sometimes possible to utilize a metal vane or finial as a terminal, but great care must be taken in making the connection between it and the rod.

The conductor itself may be in the form of a tube, rod, rope, or tape, but the tape is generally considered to be the best, and is now most commonly used. Hopes composed of several strands of copper wire, as shown in figs. 713 and 714. are however. sometimes preferred, and can be obtained of great length. The sectional area of the conductor is regulated by its length, the following sizes being usually considered sufficient: -

Fig. 711.   Terminal for Lighting conductor

Fig. 711. - Terminal for Lighting-conductor.

Length of Conductor

Side of Copper Tape required

Diameter of copper Rope required.

Under 70 feet............

3/4

inch

x

1/8

inch,

or

1 1/2

inch

x

1/16

inch

1/2

inch.

Over 70 feet and under 120 feet.

1

,,

x

1/8

,,

,,

2

,,

x

1/16

,,

3/8

,,

Over 120 feet.............

1 1/2

,,

x

1/8

,,

,,

3

,,

x

1/16

,,

3/4

,,

The conductor must be fixed to the building by means of suitable clips, as shown in figs. 712, 713, and 714. Insulators were formerly placed between the conductor and the wall, but arc never used now. The conductor should be in one length only where possible; if the building is lofty, it may be necessary to use two lengths, but the joint must be perfect, otherwise there will be danger. The connection

Fig. 712.   Sanderson's Copper tape Lighting conductor 1 in. + 1/2 in.

Fig. 712. - Sanderson's Copper-tape Lighting conductor 1 in. + 1/2 in.

Fig. 713. Half  inch copper Lighting conductor, composed of 7 wires.

Fig. 713. Half -inch copper Lighting conductor, composed of 7 wires.

Fig. 714.   Half Inch Copper rope Lightning conductor, composed of 49 wires.

Fig. 714. - Half-Inch Copper-rope Lightning-conductor, composed of 49 wires.

Fig. 715.  Elevation and Section of the Lewis Link.

Fig. 715. -Elevation and Section of "the Lewis Link".

with the terminal must also be perfect; a good arrangement, known as "the Lewis link", is shown in Fig. 715. This is designed for copper tapes; the end of the tape is bent into a loop and placed in the wedge-shaped socket in the lower part of the link: the end of the terminal rod is screwed down on to the tape.

The lower end of the conductor is as important as any other part. It should be connected with a copper plate about a yard square or more, which should be buried 5 or 6 feet deep in moist earth a few yards from the building. It is a good plan to bed the plate in coke or other porous material, and to conduct a portion of the rain-water to it.

The efficacy of a lightning-conductor is not as great as is usually thought. If there are two chimneys of equal height on a building, and a conductor is

1 to one, the other is not safe, unless it is very near the protected chimney. The space which a conductor will protect from direct flashes, is, roughly speakly, a cone having its apex at the top of the terminal and its base four times the height of the conductor; thus, a rod rising 6 feet above the parapet on one side of a flat roof 20 feet square, would only afford protection within a circle of 12 feet radius, and would not therefore protect the whole roof. In a case like this, the conductor should be more elevated and brought to the centre of the roof.

Lightning-conductors, like everything else, may get out of order, and should be periodically tested, in order that defects may be discovered and repaired.

Lofty chimneys ought to be carefully protected, as the carbon of the smoke and soot is a good conductor of electricity. It is for this reason that the "thunder-bolt" so frequently enters a house by way of the chimney. I have myself had one experience of this sort, and certainly do not desire another.