The new form of energy, for which there are two names—to wit, the Roentgen ray and the X-ray—is radiated from a highly exhausted discharge tube, which may be energized by an induction coil or other suitable electrical apparatus, such as a Holtz or a Wimshurst electrical machine. § 106. The principle underlying the construction of the usual induction (or Ruhmkorff) coil is disclosed in the subject-matter of § § 1, 2, and 3, and is represented in diagram in Figs. 1 and 2 on page 17. It would be well for the amateur or general scientific reader to study these sections carefully, for then he will have all the knowledge that is necessary for understanding the apparatus by which the discharge tube is energized. Of course, he will not comprehend the various mechanical details, nor the many electrical and mathematical relations existing in connection with an induction coil, but he will gain sufficient knowledge to appreciate what is intended when such a device is referred to here and there throughout the book. Since the time of Faraday, Page, and Fizeau induction coils of very large dimensions have been constructed, but none of them probably ever exceeded that built by Spottiswoode, during or about 1875, which was so powerful as to produce between the two electric terminals, in open air, a spark of 42 in. in the secondary current with only 30 small galvanic cells of the Grove type in the primary circuit. The cells are seldom used in this connection at the present time, the same being replaced by the dynamo, and the current being conveniently obtained from the regular incandescent-lamp circuit which may be found in almost any city. Those, therefore, who intend to become better acquainted with the details of the electrical apparatus should study in conjunction with this book some elementary treatise relating particularly to dynamos and electric currents.

The essential element in connection with the generation of X-rays is not the coil nor the dynamo, but the electric discharge, especially when occurring within a rarefied atmosphere, provided within a glass bulb, called the discharge tube throughout the book, but which has usually been called by different names, for example, the receiver of an air pump, or a Geissler tube, when the air is not very highly exhausted, or a Crookes tube (see picture at § 123) when the vacuum is definitely much higher by way of contrast. It has also been called a Hittorff tube, the Lenard tube, and by several other names, according to its peculiar characteristics.

For those who are not acquainted with the nature of the electric charge and discharge, nor with the peculiar and exceedingly interesting phenomena which various investigators have discovered from time to time, nor with the variety of effects according to the nature and the pressure of the atmosphere within the glass bulb, it is exceedingly difficult to understand with any degree of satisfaction the properties, principles, laws, theories, and manner of application of cathode and X-rays. Consequently, the greater part of the book treats of the electric charge and discharge in conjunction with certain kindred phenomena. Primarily, the meaning of the electric discharge may be derived by referring to Fig. 2, page 17, where there is shown an electric spark, indicated by radial lines between the terminals of a fine wire forming the long and fine coil or secondary circuit. Imagine that the wires are at great distances apart. Let them be brought closer and closer together. By suitable tests it will be found, for example, that no current passes through the wire, but when the points are brought sufficiently close together a spark will occur between the two terminals. § 2. Sometimes instead of what is understood as a spark, a brush or glow takes place (§ § 10 and 11), and in fact a numerous variety of effects occur, a general name for all being conveniently termed an electric discharge. Even if no sudden discharge takes place, yet, as when the terminals are far apart, there may be a charge or a tendency, or, as it is technically called, a difference of potential, between the two electrodes, one of which is the cathode and the other the anode. This is comparable to a weight upon one’s hand, tending continually to fall, and always exerting a pressure, and it will fall when the hand is suddenly removed. This is in the nature more of an analogy than of an exact correspondence. A discharge through open air, while adapted to produce a great many curious as well as useful effects, does not act as a generator of X-rays. § 136. Another class of phenomena is obtainable by exhausting the air to a certain extent from a discharge tube, thereby obtaining what is usually called a low vacuum. Such bulbs have been called Geissler tubes. Neither can X-rays be generated therefrom to any practicable extent, but only feebly if at all. § 118. Hittorff, Varley (§ 61a), Crookes (§ § 53 to 61, inclusive), were the first to discover and study the different phenomena that are obtained by diminishing the pressure within the discharge tube to a decrement of several thousand millionths of an atmosphere. This will explain why so many allusions have been made to the Crookes tube, for when the electric discharge is caused to take place in such a high vacuum X-rays are propagated in full strength.

Upon the first announcement of the discovery, electricians, eminent and otherwise, were of one mind in assuming the possibility of obtaining Roentgen rays from other sources than that of the highly evacuated discharge tube. Instead of speculating and theorizing, hosts of crucial tests were instituted, resulting negatively, and it is now safe to conclude that the electric discharge is the only primary source, and it is reasonably safe to assert that the discharge must take place within a highly evacuated enclosure.

The next stage of exhaustion, of no advantage to be considered, is that at which no discharge takes place (§ 25), and neither are any Roentgen rays propagated therefrom.