Telefunken Company has constructed models illustrating the modus operandi of the full-size apparatus.

Figs. 77 and 78 represent sender and receiver, but without aerial conductor, whilst fig. 79 illustrates a complete sender with slow-radiating aerial fitting, the latter consisting of a vertical

copper wire, with terminal capacities attached top and bottom

(aerial-wire-counter-capacity). In the centre of the aerial conductor is mounted a coil for coupling purposes, and also a variable

self-induction (large bare wire coil) to regulate the wave length of the aerial conductor.

By the use of loose coupling in sender and receiver, these models can be employed to demonstrate not only the transmission of wireless messages, but also the selective or multiplex wireless telegraphy with three different wave lengths.

The foregoing review of the successive development of wireless telegraphy will have clearly illustrated the enormous advance that has been made in a comparatively short time. For a while it seemed threatened with grave danger. In consequence of fantastic reports the general public was led to overestimate the new means of communication to such an extent that the days of the cable were looked upon as over. Naturally a reaction set in, and hot enthusiasm was replaced by its antithesis. Nevertheless, the strenuous labour of professionals, and the recognition that wireless telegraphy has its natural limitations, have rescued it from discredit and directed public opinion into a healthy channel of active interest. Wireless telegraphy is destined to supplement and not displace the cable, and its aid will be the more valuable since the cost of installation and maintenance is lower than that of cable work. In fact, for a range of 600 miles the prime cost of a wireless installation is at most only one-tenth, and the cost of maintenance one-third that of a cable system. The special field of wireless telegraphy is that in which communication is to be established as quickly as possible, either temporarily or between movable points.

The last stages of development have (to use an expression of Hertz) placed us in the position of no longer being compelled to make in electricis music with wooden clappers, for we now have a whole range of tones at our disposal for the purpose.

We are still far from the goal, achieved in acoustics, of uniting great intensity with the highest resonance possibilities, and for the present have to be content with the choice between the two.

Such higher aims can only be realised by close co-operation of science and technology, which is in no field so essential as in wireless. telegraphy.

BIBLIOGRAPHY.

GENERAL.

Abraham, M., Ann. der Physik, Bd. 66, p. 435, 1898.

Braun, F., Drahtlose Telegraphie durch Wasser u. Luft, Leipzig, 1901. Ann. d. Physik, Bd. 8, p. 165, 1902.

Über drahtlose Telegraphie und neuere physikal. Forschungen. Rektoratsrede, 1905.

Bjerkness, V., Ann. der Physik, Bd. 55, p. 121, 1895.

Dönitz, J., Elektrotechn. Zeitschrift, p. 920, 1903. (Der Wellenmesser— The Ondameter.)

Drude, P., Ann. d. Physik, Bd. 53, p. 721, 1894.

1

Bd. 16, p. 116, 1905.

Physikal. Zs., No. 23, p. 745, 1905.

Eichhorn, G., Dissertation, Zürich, 1901.

Die drahtlose Telegraphie, Veit & Co., Leipzig, 1904.

Feddersen, Pogg. Annalen, Bd. 103, 108, 112, 113, 116.

Fleming, J. A., Cantor Lectures on Hertzian Wave Telegraphy, London,

1903.

Proc. Roy. Soc. London, 71, p. 347. Helmholtz, H., Die Erhaltung der Kraft.