=Lectures of Professor Lodge.=--But, in the month of March, 1888, two lectures were given before the Society of Arts, in London, by Professor Oliver Lodge, in which this theory was directly challenged, and attacked with cogent arguments, supported by striking and original experiments. These lectures gave rise to an animated controversy, which culminated in a formal discussion at the recent meeting of the British Association in Bath. The discussion was carried on with great spirit, and most of the leading representatives of physical and mechanical science took an active part in it. The greater portion of this volume was printed off before the meeting of the British Association took place. But the discussion on the theory of lightning conductors seemed to me so interesting and important that I thought it right, in the form of an Appendix, to give some account of the questions at issue, and of the opinions expressed upon them. Professor Lodge maintains[41] that the received theory of lightning rods is open to two objections. First, it takes account only of the conducting power of the lightning rod, and takes no account of the phenomenon known as self-induction, or electrical inertia. Secondly, it assumes that the whole substance of a lightning rod acts as a conductor, in all cases of lightning discharge; whereas there is reason to believe that, in many cases, it is only a thin outer shell that really comes into action. I will deal with these two points separately. =The Effect of Self-Induction.=--When an electric discharge begins to pass through a conductor, a momentary back electro-motive force is developed in the conductor, which obstructs its passage. This phenomenon is called by some self-induction, by others electrical inertia; but its existence is admitted by all. Now, when a flash of lightning, so to say, falls on a lightning rod, the back electro-motive force developed is very considerable; and it may offer so great an obstruction that the discharge will find an easier passage by some other route, such as the stone walls and woodwork, and furniture of the building. According to this view, the obstruction which a flash of lightning encounters in a conductor consists partly of the resistance of the conductor, in the ordinary sense of the word resistance, and partly of the back electro-motive force due to self-induction. The sum of these two Professor Lodge calls the _impedance_ of the lightning rod; and he considers that the impedance may be enormously great, even when the resistance, in the ordinary sense, is comparatively small.