There are three methods of generating the electrical discharge which produces the rays. The commonest method is that in which the Ruhmkorf coil is used. This coil is what is now known as a transformer, and consists of one coil of a few turns of coarse wire, which is connected to a battery or other source of electricity, and of another coil surrounding the first of a great number of turns of fine wire. Any sudden change of the battery current produces an electric pressure or electro-motive force at the ends of the fine coil of wire. By this simple arrangement of two coils we can thus exalt a current of low pressure to one of high electro-motive force. A battery current which can barely produce an electric spark of one hundredth of an inch at the ends of the coarse coil can cause a spark of eight inches or more at the terminals of a fine coil. In the second method one uses an ordinary electrical machine in which the glass plates are supplanted by rubber ones, which are run at a high rate of speed. Both of these methods have their advocates. The use of the Ruhmkorf coil is the most universal. The third method consists in charging a number of Leyden jars by a storage battery and in discharging these one after another, so as to obtain a high electro-motive force. This method is a very flexible one. I can experiment with my apparatus over a range of electric pressure extending from twenty thousand units to three million. The electrical discharge produced by three million units or volts is over six feet in length. The apparatus for discharging the Leyden jars or condensers in series is represented in Fig. 2. [Illustration: FIG. 2.--Apparatus for producing electrical sparks six feet in length.] A fourth method, first used by Professors Norton and Goodwin, of the Massachusetts Institute of Technology, consists in discharging a quantity of electricity through the coarse coil of a Ruhmkorf coil. This method obviates the necessity of a mechanical break to interrupt the battery current which is employed to excite the current in the coarse coil of this apparatus. I have experimented with more powerful quantities of electricity than have been hitherto used. The accompanying photograph gives an idea of the magnitude of the quantity which I can use to excite the X rays. It represents the discharge, burning a fine iron wire, and it makes a noise resembling the crack of a pistol. Now, this discharge can be used in a variety of ways to excite various transformers in order to produce the best conditions for exciting the X rays. The method of using this powerful discharge to excite a transformer seems at present the most promising one in seeking the best conditions for obtaining rays of high penetrating power. There is still another method of obtaining the rays yet in its infancy--the simplest method of all, for no apparatus is required.