The first actual lightning conductor ever constructed was set up by Benjamin Franklin himself, at his house in Philadelphia. Its main object was to protect the house against the effects of thunderstorms; still experiments were so dear to the heart of the great discoverer, that he could not help making trials even with things devoted to other uses. It was in the summer of 1752 that Franklin erected over his house a lightning conductor, made entirely of iron, but with a sharp steel point on the top, the latter projecting seven or eight feet above the roof, while the end was above five feet in the ground. Curious to know whenever an electrical stream was passing through the conductor, he attached to it an ingenious contrivance, by means of which through an electric spark two bells were set in movement as soon as this took place, the greater or lesser noise from them corresponding with the strength of the electrical current. With the aid of this device Franklin was enabled to observe some curious phenomena, which at first puzzled him not a little. ‘I found the bells rang sometimes,’ he informed a friend, ‘when there was no lightning or thunder, but only a dark cloud over the rod; that sometimes, after a flash of lightning, they would suddenly stop, and at other times, when they had not rung before, they would, after a flash, suddenly begin to ring; that the electricity was sometimes very faint, so that when a small spark was obtained, another could not be got for some time after. At other times, the sparks would follow extremely quickly; and once I had a continual stream from bell to bell, the size of a crow-quill. Even during the same gust there were considerable variations.’ By continued watching, Franklin came to make the discovery that the fluctuations in the electrical current were owing to changes and interchanges, in atmosphere and earth, of positive and negative electricity. He held at first that thunder-clouds are usually in a negative state of electricity, but afterwards discovered that they varied from negative to positive during the same storm.