The second condition of a lightning conductor is, that it should be made of such material, and of such dimensions, as to offer an easy passage to the greatest flash of lightning likely to fall on it; otherwise it might be melted by the discharge, and the lightning, seeking for itself another path, might force its way through bad conductors, which it would partly rend asunder, and partly consume by fire. Copper is now generally regarded as the best material for lightning conductors, and it is almost universally employed in these countries. If it is used in the form of a rope, it should not be less than half an inch in diameter; if a band of copper is preferred--and it is often found more convenient by builders--it should be about an inch and a half broad and an eighth of an inch thick. In France it has been hitherto more usual to employ iron rods for lightning conductors, but since iron is much inferior to copper in its conducting power, the iron rod must be of much larger dimensions; it should be at least one inch in diameter.[30] The third condition is that the lightning conductor should be continuous throughout its whole length, and should be placed in good electrical contact with the earth. This is a condition of the first importance, and experience has shown that it is the one most likely of all to be neglected. In a large town the best earth connection is furnished by the system of water-mains and gas-mains, each of which constitutes a great network of conductors everywhere in contact with the earth. Two points, however, must be carefully attended to--first, that the electrical contact between the lightning conductor and the metal pipe should be absolutely perfect; and, secondly, that the pipe selected should be of such large dimensions as to allow the lightning an easy passage through it to the principal main. If no such system of water-pipes or gas-pipes is at hand, then the lightning rod should be connected with moist earth by means of a bed of charcoal or a metal plate not less than three feet square. This metal plate should be always of the same material as the conductor, otherwise a galvanic action would be set up between the two metals, which in course of time might seriously damage the contact. Dry earth, sand, rock, and shingle are bad conductors; and, if such materials exist near the surface of the earth, the lightning rod must pass through them and be carried down until it reaches water or permanently damp earth. =Mischief Done by Bad Conductors.=--If the earth contact is bad, a lightning conductor does more harm than good. It invites the lightning down upon the building without providing for it, at the same time, a free passage to earth. The consequence is that the lightning forces a way for itself, violently bursting asunder whatever opposes its progress, and setting fire to whatever is combustible.