Lightning Conductors: Their History, Nature, and Mode of Application

by F.C.S. Richard Anderson

The magazine was an oblong building of brick, nine feet long, five feet wide, and six feet high (internal dimensions), and it had a uniform thickness of three bricks. At one end was a heavy iron door, and at the other a lightning conductor, consisting of a copper-wire rope seven-sixteenths of an inch in diameter. The point of the terminal rod was about thirteen feet above the top of the building, and a similar length was carried into the ground and terminated in clayey soil. The conductor was fixed to a pole distant about two inches from the end of the building opposite to that in which the iron door was fixed. It was not connected with the iron door in any way. At the time of the explosion the magazine contained about 2,000 pounds of gunpowder.

Major Majendie, H.M.’s Chief Inspector of Explosives, in his official report ascribed the accident to the fact of the iron door being unconnected with the lightning conductor, and in doing this he was doubtless right, but only to a limited extent. The author of this work visited the colliery shortly after the explosion, and found that the conductor—the weight of which was about one pound per yard—had been fastened to the pole, which was about twenty-one feet high, by two glass insulators, and that the conductor was not connected with the building. On testing the copper rope which formed the conductor, its conductivity was found to be only 39·2 instead of 93 or 94 per cent. The conductor, therefore, was but little better than if it had been made of iron, and, even supposing it had been made of good copper, it was of too small a size. It should have been of double the weight, and not insulated from the pole. In order to be thoroughly efficient it ought to have been brought down the pole, carried through under the roof, down the iron doorpost, and so into the ground.

According to the French theory, that the ‘area of protection’ afforded by a lightning conductor is the space contained within the circular area of a radius double the height of the conductor, the magazine was thoroughly secured, for the conductor was twenty-one feet high, and the building only nine feet long, five feet broad, and six feet high. This case, however, with many others, entirely controverts this theory, and shows very forcibly the fallacy of an argument that at one time was accepted almost as an axiom.

One other case of more recent date may be instanced. At Cromer, in Norfolk, the church—a fine perpendicular building of flint and freestone, having a tower 159 feet high—was damaged by lightning in August 1879. During a thunderstorm the lightning struck one of the pinnacles with considerable force, although on another pinnacle, only twenty-seven feet six inches distant, a good copper conductor, having a diameter of five-eighths of an inch, was fixed. On testing the conductor by means of a galvanometer, it and the earth connection were found to be in thoroughly good order. After what has been said, comment on this last example is needless.

Fig. 22.

The general disposition and adjustment of a lightning conductor demands the greatest care and consideration. No hard and fast rules can be laid down, for each individual case must be studied and elaborated by itself, especially in the instance of large structures, where much depends upon style, outline, and other details. The main point is that every part of the building shall be placed beyond the possibility of being damaged by a disruptive discharge of electricity.

It has been stated previously that the lightning invariably follows the line of least resistance, and that this line may be influenced by the presence of streams of warm vapour, columns of smoke, &c., which, escaping into the air, furnish a ready path for the electric discharge. Consequently it sometimes happens that a building or barn may be struck although it be provided with a lightning conductor. In order to explain this it must be borne in mind that the line of least resistance is not always the shortest line mathematically. The accompanying illustration ([fig. 22]) is an example to the point. It represents a barn furnished with a lightning conductor and filled with new-made hay, which is a better conductor of electricity than the material of which the barn is constructed. This hay is giving off the stream of warm vapour which is pouring out of the opening at the end, and forms an invisible band of conducting matter between the thunder-cloud and the barn, as marked out in the engraving by the dotted lines, the direction of the wind being shown by the arrow and the trees. Under these circumstances the discharge of lightning would naturally follow the path between c and d in preference to the shorter route between a and b, because the former is the line of least resistance between the cloud and the earth. Thus the barn—although furnished with a conductor in good condition—would most likely be set on fire, or otherwise damaged. The same deflection of the lightning-stroke might be caused by a column of smoke, or by the fact of one portion of the building being moistened by the rain and the other kept dry; an occurrence that might easily happen when a strong wind is blowing during a storm.

In order to ensure complete protection, the conductor on the barn should have been carried along the ridge and down the edges of the roof at each gable. By this means the stroke of lightning would have been intercepted.

The engraving on the next page shows a design for the protection of a large detached mansion by means of a multiplication of short points or terminal rods fixed on all the prominent features of the building. The conductor is carried along the ridges in every direction, and down the edges of the roof at each gable. Generally it is sufficient to have two descending conductors, but occasionally the conformation of the building or the nature of the ground renders necessary the use of even more. It is imperative, for obvious reasons, that the descending portion of the lightning conductor shall be carried from the roof to the ground by the shortest possible route, and placed in perfect electrical contact with the earth in the manner to be indicated in a succeeding chapter.