It may occur to readers to inquire why the oscillating currents are not passed direct to a galvanometer. The answer is that because they are oscillating a very sensitive galvanometer is not possible. True, the Duddell thermo-galvanometer has been mentioned in this connection, but although it is a beautiful instrument it cannot compare for delicacy with the direct-current galvanometers. The latter are easily a _hundred thousand times_ more sensitive. But the trouble can be overcome by "rectifying" the oscillating currents, by passing them through a "unidirectional" conductor--one, that is, which passes current one way only. These remind one of a turnstile as installed at certain public places, which let you out but will not let you in unless you pay. In fact they will not let you _in_ at all. In like manner "rectifiers" will only allow those currents to pass which are flowing in one direction, and so they cut out every alternate oscillation, thus producing something very like continuous current, which can be detected by the very delicate galvanometers which are usable where continuous currents are concerned, or more often by a telephone receiver. The rectifying conductors are in many cases crystals, hence these detectors are called "Crystal Detectors." Carborundum is a favourite for this purpose. And that brings us to the important question of the secrecy of wireless communication, and the measures taken to prevent confusion from the number of independent messages flying through the air at the same time. This can be largely achieved by the aid of resonance. Trains of waves flung out by one antenna may strike several other antennæ, but unless the latter are in tune with the sending apparatus they will probably not be affected appreciably. Let one of them, however, be in tune, and it will pick up easily the message which is not noticed by the others. It is as if three people watching a distant lamp were affected by a form of colour-blindness which rendered them practically blind to all colours except one. Suppose one could see red only, the other blue and the third yellow. A light sent through a blue glass being robbed of all rays except the blue ones would be visible only to the man who could see blue. The man who could see blue would, in like manner, be quite blind to light sent through red or yellow glass. Each of them, in fact, could be signalled to quite independently of the others by simply sending him rays of the colour to which his eyes were sensitive. In precisely the same way each wireless receiver is or can be made most sensitive to waves of a particular length and practically blind to all others. The operator can adjust his apparatus for certain prearranged wave-lengths, and so he can communicate with secrecy to stations whose wave-length he knows. The change, of course, is made by altering the capacity, or inductance, or both. The instruments can be so calibrated that it is quite easy to make the alteration.