To dwell too largely upon the importance of leading all lightning conductors down into moist earth, or, as technically called, ‘good earth,’ would be scarcely possible. It would perhaps not be too strong an expression to say that the part of the conductor above ground is a mere appendage to that under ground, the essential function of the whole apparatus--that of dispersing the electric force harmlessly--being accomplished by the subterranean portion. The clear understanding of Benjamin Franklin perceived this at the outset; but after him it seemed as if forgotten for a long time, and the result showed itself in numerous disasters that occurred to buildings protected with conductors, which brought the latter into disrepute with many persons. While, no doubt, in many instances the cause of these disasters was in the bad application of the conductors themselves, their defective character, or their feebleness, still in the great majority the underground connection may be taken to have been in fault. It may be laid down as an absolute certainty that a really good conductor--say, a copper rope from five-eighths to three-quarters of an inch in thickness--cannot possibly fail to carry off the electric force if the lower part reaches moist earth or water. Probably, in nine cases out of ten, whenever a building provided with a conductor is struck by lightning, it is for want of ‘good earth.’
Franklin’s own ideas were very clear on the subject. He laid them down at various times, more particularly when residing in England, during the years from 1764 to 1775, as colonial agent for Pennsylvania. During the latter part of this period he took an active interest in the proceedings of the Royal Society; and this learned body being requested by the Government to give advice regarding the best protection against lightning that could be provided for the great powder magazines at Purfleet, he was nominated into a committee with three other members, William Watson, H. Cavendish, and J. Robertson. The committee drew up a report, dated August 21, 1772, signed by all the members, but known to be written by Franklin alone. Dwelling strongly on the importance of the underground connection, Franklin says in this report: ‘In common cases it has been judged sufficient if the lower parts of the conductor were sunk three or four feet into the ground, till it came to moist earth; but this being a case of great consequence, we are of opinion that greater precaution should be taken. Therefore we would advise that at each end of each magazine a well should be dug, in or through the chalk, so deep as to have in it at least four feet of standing water. From the bottom of this water should rise a piece of leaden pipe to, or near, the surface of the ground, where it should be joined to the end of an upright bar.’ Franklin then goes on to recommend the usefulness of having even more wells than the two, so as to avoid any possibility of failure in protecting the powder magazines. ‘We also advise,’ he says in his report, ‘in consideration of the great length of the buildings, that two wells of the same depth with the others should be dug within twelve feet of the doors of the two outside magazines--that is to say, one of them on the north side of the north building, and the other on the south side of the south building, from the bottom of which wells similar conductors should be carried up.’ It is not on record whether these recommendations were adopted by the Government, but it seems likely that this was the case, as the fear of explosion of powder magazines through a stroke of lightning was very great at the time. Not long before, a magazine had been so destroyed at Brescia, in Italy, with the appalling result of a considerable part of the city being laid in ruins, burying many hundreds of persons. The destruction of the Brescia powder magazine, like all similar events, had, it is scarcely necessary to say, its due effect in spreading a desire for lightning conductors, fear doing what was not effected by foresight.