The alternating-current transformer is another gift of Faraday to the commercial world.  As is well known, this instrument is a device for raising or lowering electric pressure.  The name is derived from the fact that the instrument is capable of taking in at one pressure the electric energy supplied to it, and giving it out at another pressure, thus transforming it.  Faraday produced the first transformer during his investigations on voltaic-current induction.  The modern alternating-current transformer, though differing markedly in minor details from Faraday’s primitive instrument, yet in general details is essentially identical with it.  The enormous use of both step-up and step-down transformers—­transformers which respectively induce currents of higher and of lower electromotive forces in their secondary coils than are passed through their primaries—­shows the great practical value of this invention.  The wonderful growth of the commercial applications of alternating currents during the past few decades would have been impossible without the use of the alternating-current transformer.

It is an interesting fact that it was not in the form of the step-down alternating-current transformer that Faraday’s discovery of voltaic-current induction was first utilized, but in the form of a step-up transformer, or what was then ordinarily called an induction coil.  As early as 1842, Masson and Breguet constructed an induction coil by means of which minute sparks could be obtained from the secondary, in vacuo.  In 1851, Ruhmkorff constructed an induction coil so greatly improved, by the careful insulation of its secondary circuit, that he could obtain from it torrents of long sparks in ordinary air.  The Ruhmkorff induction coil has in late years been greatly improved both by Tesla and Elihu Thomson, who, separately and independently of each other, have produced excellent forms of high-frequency induction coils.

Induction coils have long been in use for purposes of research, and in later years have been employed in the production both of the Roentgen rays used in the photography of the invisible, and the electro-magnetic waves used in wireless telegraphy.

Roentgen’s discovery was published in 1895.  It was rendered possible by the prior work of Geissler and Crookes on the luminous phenomena produced by the passage of electric discharges through high vacua in glass tubes.  Roentgen discovered that the invisible rays, or radiation, emitted from certain parts of a high-vacuum tube, when high-tension discharges from induction coils were passing, possessed the curious property of traversing certain opaque substances as readily as light does glass or water.  He also discovered that these rays were capable of exciting fluorescence in some substances,—­that is, of causing them to emit light and become luminous,—­and that these rays, like the rays of light, were capable of affecting a photographic plate.  From these properties two curious possibilities arose; namely, to see through opaque bodies, and to photograph the invisible.  Roentgen called these rays X, or unknown rays.  They are now almost invariably called by the name of their distinguished discoverer.