Now connect the wires of a battery to the binding-posts in contact with the primary coil, and attach two separate wires to the secondary coil binding-posts. Bring these ends near to each other, and a spark will leap across from one end to the other, its size or “fatness” depending on the strength of the battery. The completed apparatus is shown at Fig. 18. In producing a long spark a condenser is an important factor; it is used in series with an induction-coil. There are several forms of condensers, but perhaps the simplest and most efficient is the Fizeau condenser, which is made up of layers of tin-foil with paraffined paper as separators. From a florist’s supply-house purchase one hundred and fifty sheets of tin-foil seven by nine inches, or sheets that will cut to that size without waste; also ten or twelve extra sheets for strips. At a paper supply-house obtain some clear, thin, tough paper about the thickness of good writing-paper. Be careful to reject any sheets that are perforated or have any fine holes in them. The sheets should be eight by ten inches, or half an inch larger all around than those of the tin-foil. The paper must be thoroughly soaked in hot paraffine to make it moisture-proof and a perfect non-conductor. This is done by placing about two hundred sheets on the bottom of a clean tin tray, or photographic developing-dish of porcelain. Don’t use glass or rubber. After placing some lumps of paraffine on the paper, put the tray in an oven so as to dissolve the paraffine and thoroughly soak the paper. Open the oven door and, with a pin, raise up the sheets one at a time, and draw them out of the liquid paraffine. As soon as it comes in contact with cool air the paraffine solidifies and the sheet of paper becomes stiffened. Select each sheet with care, so that those employed for the condenser are free from holes or imperfect places. From pine or white-wood, a quarter of an inch in thickness, cut two boards, eight by ten inches, and give them several good coats of shellac. To build up the condenser, lay one board on a table and on it place two sheets of paraffined paper. On this lay a sheet of tin-foil, arranging it so that half an inch of paper will be visible around the margin. From the odd sheets of tin-foil cut some strips, one inch in width and three inches long. Place one of these strips at the left end of the first sheet of foil, as shown at Fig. 19. Over this lay a sheet of the paraffined paper, then another sheet of the foil. Now on this second sheet of foil lay the short strip to the right end, and so proceed until all the foil and paper is in place, arranging each alternate short strip at the opposite end. Care must be taken to observe this order if the condenser is to be of any use. [Illustration: FIG. 19] [Illustration: FIG. 20]