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. ------------------------------------------------------------------------ CHAPTER I ------- 1. FARADAY’S EXPERIMENT, 1831. SECONDARY CURRENT BY INDUCTION. _Experimental Researches, Proc. Royal. So. 1841._—In brief, the experiment involved the elements illustrated in the accompanying diagram, Fig. 1, p. 17; a ring made of iron; upon the ring, two coils of copper wire, suitably insulated from each other and from the iron; a galvanometer included in circuit with one coil, and an electric battery of ten cells placed in circuit with the other coil. He found that upon breaking or completing connection with the battery, the needle was powerfully deflected. Without entering into further detail, it is important, however, to notice that he did not perform any experiments tending to establish the principle of increase of E. M. F. by making the very slight change now known to be necessary. § 2. 2. PAGE’S EXPERIMENT, 1838. ELECTRIC SPARK BY INDUCED CURRENT. _Pynchon_, p. 427. Dr. Page performed an experiment in which the primary coil was but a few feet in length, while the secondary coil was 320 ft. He included, in the primary circuit, only a few cells of battery. The manner in which he first caused rapid interruptions of the circuit of the primary coil was by the use of what may be called a coarse file, Fig. 2, p. 17. He discovered that the E. M. F. during the rapid interruption was so much increased over that of the small battery, that an electric spark would pass between the secondary terminals without first bringing them into contact with each other. § 6. The result of these experiments was not only the generation of a current of high E. M. F. from a generator of low E. M. F., but also a current of great quantity as compared with currents obtained from frictional and influence machines, whose complete history is found in Mascart’s work on Electricity.