Take two pieces of bare copper wire about eight inches long and bend them at right angles. Place them in the secondary terminals of a spark-coil as in Figure 171. Bend them so that the vertical portions are about one-half of an inch apart at the bottom and one inch apart at the top. Start the coil working, and the sparks will run up the wires from the bottom to the top and appear very much like the rungs in a ladder. X-Rays Most young experimenters are unaware what a wonderful and interesting field is open to the possessor of a small X-ray tube. Small X-ray tubes which will operate satisfactorily on an inch and one-half spark-coil may be obtained from several electrical supply houses. They usually cost about four dollars and a half. With such a tube and a _fluoroscope_ it is possible to see the bones in the human hand, the contents of a closed purse, etc. The tube is made of glass and contains a very high vacuum. The long end of the tube contains a platinum electrode called the _cathode_. The short end contains two electrodes called _anodes_, one perpendicular to the tube and the other diagonal. The tube is usually clamped in a wooden holder called an X-ray tube stand. The tube should be so adjusted that the X-rays which are reflected from the diagonal anode will pass off in the direction shown by the dotted lines in Figure 174. The fluoroscope is a cone-shaped wooden box fitted with a screen composed of a sheet of paper covered with crystals of a chemical called platinum-barium-cyanide. [Illustration: Fig. 172.—An X-Ray Tube.] The opposite end of the box is fitted with a covering of felt or velvet which shuts off the light around the eyes and nose when you look into the fluoroscope and hold it tightly against the face. A fluoroscope may be purchased complete, or the platinum-barium-cyanide screen purchased separately and mounted on a box as shown in Figure 173. The two anodes of the tube should be connected, and led to one terminal of a spark-coil capable of giving a spark at least one and one-half inches long. Another wire should be led from the cathode of the tube to the other terminal of the coil. [Illustration: Fig. 173.—Fluoroscope.] When it is desired to inspect any object, such as the hand, it must be held close to the screen of the fluoroscope and placed between the latter and the tube in the path of the X-rays. The X-rays are thrown forth from the tube at an angle of 45 degrees from the diagonal anode. Look into the fluoroscope and it should appear to be filled with a green light. If not, the battery terminals connected to the primary of the coil should be reversed, so as to send the current through in the opposite direction.