[Illustration: _By permission of_] [_C. H. F. Muller._ FIG. 41.--Diagram showing parts of X-Ray Tube.] An induction coil is generally used to supply the high-tension electricity required for the production of the Röntgen rays. For amateur or experimental purposes a coil giving continuous 4-inch or even 3-inch sparks will do, but for medical work, in which it is necessary to take radiographs with very short exposures, coils giving sparks of 10, 12, or more inches in length are employed. An electrical influence machine, such as the Wimshurst, may be used instead of an induction coil. Very powerful machines with several pairs of plates of large diameter, and driven by an electric motor, are in regular use for X-ray work in the United States, but in this country they are used only to a very small extent. A Wimshurst machine is particularly suitable for amateur work. If a screen is to be used for viewing bones through the flesh a fairly large machine is required, but for screen examination of such objects as coins in a box, or spectacles in a case, and for taking radiographs of these and other similar objects, a machine giving a fairly rapid succession of sparks as short as 2 inches can be used. Of course the exposure required for taking radiographs with a machine as small as this are very long, but as the objects are inanimate this does not matter very much. For amateur X-ray work the arrangement of the apparatus is simple. The tube is held in the required position by means of a wooden clamp attached to a stand in such a way that it is easily adjustable. Insulated wires are led from the coil or from the Wimshurst machine to the tube, the positive wire being connected to the anode, and the negative wire to the cathode. With a small Wimshurst machine light brass chains may be used instead of wires, and these have the advantage of being easier to manipulate. For medical purposes the arrangements are more complicated, and generally a special room is set apart for X-ray work. If the connexions have been made correctly, then on starting the coil or the machine the tube lights up. The bulb appears to be sharply divided into two parts, the part in front of the anti-cathode glowing with a beautiful greenish-yellow light, while the part behind the anti-cathode is dark, except for lighter patches close to the anode. The Röntgen rays are now being produced. The illumination is not steady like that of an electric lamp, but it consists of a series of flickers, which, with powerful apparatus, follow one another so rapidly as to give the impression of continuity. If the connexions are wrong, so that the negative wire goes to the anode instead of to the cathode, the bulb is not divided in this way, but has patches of light almost all over. As soon as this appearance is seen the apparatus must be stopped and the connexions reversed, for the tube is quickly damaged by passing the discharge through it in the wrong direction.