The X-rays will cause the chemicals on the screen to light up and give forth a peculiar green light. If the hand is held against the screen, between the screen and the tube, the X-rays will pass through the hand and cast a shadow on the screen. They do not pass through the bones as easily as they do through the flesh and so will cast a shadow of the bones in the hand on the screen, and if you look closely you will be able to see the various joints, etc. The interrupter on the coil should be carefully adjusted so that the light does not flicker too much. [Illustration: Fig. 174.—How to connect an X-Ray Tube to a Spark-Coil.] If it is desired to take X-ray pictures, a fluoroscope is unnecessary. Turn the tube around so that the X-rays point downward. Shut the battery current off so that the tube is not in operation until everything else is ready. Place an ordinary photographic plate, contained in an ordinary plate-holder, directly under the tube with the gelatin side of the plate upwards. Place the hand flat on the plate and lower the tube until it is only about three inches above the hand. Then start the coil working so that the tube lights up and permit it to run for about fifteen minutes without removing the hand. Then turn the current off and develop the plate in a dark room. It is possible to obtain a very good X-ray photograph of the hand in this manner. Photographs showing the skeleton of a mouse, nails in a board, coins in a purse, a bullet in a piece of wood, etc., are a few of the other objects which make interesting pictures. [Illustration: An X-Ray Photograph of the hand taken with the Outfit shown in Figure 174. The arrows point to injuries to the bone of the third finger near the middle Joint Resulting in a Stiff Joint.] CHAPTER XIII TRANSFORMERS In most towns and cities where electricity for light and power is carried over long distances, it will be noticed that small iron boxes are fastened to the poles at frequent intervals, usually wherever there is a group of houses or buildings supplied with the current. Many boys know that the boxes contain "transformers," but do not quite understand exactly what their purpose is, and how they are constructed. When it is desired to convey electrical energy to a distance, for the purpose of producing either light or power, one of the chief problems to be faced is, how to reduce to a minimum any possible waste or loss of energy during its transmission. Furthermore, since wires and cables of large size are very costly, it is desirable that they be as small as possible and yet still be able to carry the current without undue losses.