Page:Popular Science Monthly Volume 64.djvu/57

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By Dr. J. A. FLEMING, F.R.S.,


It remains then to consider some of the questions connected with practical Hertzian wave telegraphy and the problem of the limitation of communication. These matters at the present moment very much occupy the public attention, and many conflicting opinions are expressed concerning them.

It may be observed at the outset that the difficulty of dealing with the subject as freely as many desire is that Hertzian wave telegraphy is no longer merely a subject of scientific investigation, but has developed into a business and involves therefore other interests than the simple advancement of scientific knowledge. We can, however, discuss in a general manner some of the scientific problems which present themselves for solution. The first of these is the independence of communication between stations. It is desirable, at the outset, to clear up a little misunderstanding. There is a great difference between preventing the reception of communication when it is not desired by the recipient and preventing it when it is the object of the latter to overhear if he can. It is therefore necessary to distinguish between isolation and overhearing. We may say that a station is isolated when it is not affected by Hertzian waves other than those it desires to receive; but that a station overhears when it can, if it chooses, pick up communications not intended for it, or can not help receiving them against its will.

This distinction is a perfectly fair one. Any telegraph or telephone wire can be tapped, if it is desired, but unless there is some fault on the line, no station will receive a message against its desires. Moreover, it may be noted that there are penalties attaching to tapping a telegraph wire, and at present there are none connected with the misappropriation of an ether wave.

We shall therefore consider in the first place the methods so far proposed for preventing any given receiver from being affected by Hertzian waves sent out from other stations, except that of those from which it is desired to receive them. The first method is that which has been called the method of electrical syntony, and consists in adjusting the electrical capacity and inductance of the various open and closed