absolutely reliable multiplex wireless telegraphy, is made clear by fig. 68. The oscillation circuits were excited by the same aerial wire. The attachments were coupled direct, but relatively loosely, with the primary circuits, and the latter excited the secondary circuits, the coupling between the two being very loose. The difference between the two effective wave lengths amounted to only about 10 per cent. in the case of a highly unfavourable distance ratio of 10 100 miles.

This method of mounting is unqestionably not the best. Theoretically, the use of only one aerial wire entails joining of same with the various oscillation circuits by different degrees of coupling, in which case losses of energy are inevitable. The best plan-though unfortunately not the simplest in practice-is of course the use of several distinct antennæ for the different oscillation circuits.

Fig. 69 represents the interior of a fully equipped wireless telegraphy station. To the right are the sending apparatus, whilst the receivers are mounted at the left of the long table. From the descriptions already given the reader should feel himself quite at home in this up-to-date telegraph station.

CHAPTER XI.

CONCLUSION.

By strenuous labour in both the scientific and the technical branches, wireless telegraphy has been raised within a single decade, from the smallest beginnings to an indispensable means of communication. This will constitute a lasting memorial of our progressive age, a visible sign of the intelligence and high endeavour, love of work, and capacity of appreciating the deeds of our leading spirits. It also shows how unremitting labour for securing ideal benefits to mankind is not only an ennobling moral influence, but also may lead to practical results outstripping the wildest dreams of imagination. Who would have thought it possible to waft messages across the ocean without any visible means of transmission! Of course progress has been accompanied by a clamour for more, the requirements advanced showing how far we are still from the ideal goal, which appeared so near when the real principles involved were first clearly recognised.

We have seen that at present the work has to be carried on with oscillatory discharge complexes, which even in the most favourable conditions (perfectly loose coupling) produce only relatively slightly damped wave series. The pauses between the sets of discharges are infinitely long in proportion to the time they themselves occupy, the situation being comparable to that of a sea, perfectly quiet all day long except for a single minute's disturbance by waves. In reality, we excite merely a series of short electrical tone impulses which, on account of the wide interval between them, cannot even be regarded as a chain of impulses.

What a difference there would be in the practical utility of electric oscillations were we able to generate and maintain, at will, persistent electric tones of sufficient frequency! Our most modern

alternating generators are cumbrous devices for producing very slow oscillations (about fifty per second), whereas, for wireless telegraphy, a mean frequency of about a million per second is required. Here is a worthy and profitable subject for inventive skill, and one whose solution might lead to perfectly incalculable results.

As was quite natural, the value of wave telegraphy was at first appreciated only in comparison with the range of distance covered; and though this progressively increased, it was not until the introduction of the Braun coupled systems that any imposing dimensions were attained. We have seen that the energy can be brought to exert itself instantly by the use of close coupling, so that vibrations of an explosive character are produced, but that while these can penetrate to great distances, they excite any receiver, whether syntonised or not. As we have seen, it has also become possible to render any amount of energy effective in the form of electrical waves, so that at present stations with a range of 600 miles represent merely ordinary types.

It has already been pointed out that an excess of energy has always to be allowed for in practical wireless telegraphy, because the transmission of the energy varies greatly with the weather. Bright sunshine lessens the action considerably; and a still more unfavourable effect on the transmission of signals is produced by protracted spells of dry weather, during which the air becomes. ionised. On the other hand, when the ionisation has been destroyed by rain, the original efficiency is at once restored. A soft, damp state of the atmosphere is best for the operations of wireless telegraphy, though the ordinary man would be inclined to think the opposite. For these reasons modern stations are always arranged to develop three times the maximum efficiency required for their range.

The explosive emission of energy resulting from close coupling must naturally be attended with great drawbacks when several stations are working simultaneously at short distances apart.

Recently, however, more delicate methods of operating have come into use by the introduction of loose coupling. We have seen how the reduction of the damping through resistance is attained at all points of the oscillation track, especially in the primary sender circuit by subdividing the spark-gap and constructing it of the most suitable length. In the receiver, the secondary system especially has been elaborated into an independent and almost undamped oscillator, to secure the clearest resonance with an effective sender oscillation.