Page:Popular Science Monthly Volume 64.djvu/66

62 A lightning conductor or long vertical rod of the same height as the transmitting aerial is set up at the receiving station, and at a point six or nine feet from the ground a circuit is taken off, consisting of a wire loosely coiled in a spiral, the length of which is nearly equal to, although a little shorter than, the height of the vertical wire above the point of connection. The outer end of this loose spiral is connected to one terminal of the coherer tube, and the other terminal of the coherer is connected to the earth through a condenser of rather large capacity. The terminals of this last condenser are short-circuited by a relay and a single cell. When the adjustments are properly made, it is claimed that the receiver responds only to waves coming from its own syntonized or tuned transmitter. In this case, the length of the receiving aerial above the point of junction with the coherer circuit is one quarter the length of the wave. A variation of the above arrangements consists in making this lateral circuit equal in length to one half of a wave, and connecting the coherer to its center through a condenser to the earth. The outer end of this lateral circuit is also connected to the earth (see Fig. 24).

Dr. Slaby claims that this arrangement is not affected by atmospheric electricity, and that the complete and direct earthing of the aerial and also in the second arrangement, of the receiver of the outer end of the lateral conductor, conduces to preserve the receiver immune from any electrical disturbances except those having a period to which it is tuned.

A method has also been arranged by him for receiving on the same aerial two messages from different transmitting stations, simultaneously. In this case, two lateral wires of different lengths are connected to the receiving aerial, and to the outer end of each of these is connected a coherer tube, the other end of which is earthed through a condenser. One of these lateral wires is made equal or nearly equal in length to the aerial and the other is made longer to fulfil the following condition. If we call H the height of the receiving aerial above point of junction of the lateral wires, then the length of one lateral wire is made equal to H, and the height of the aerial is adjusted to be equal to one quarter of the wave-length of one incident wave. The other lateral wire may then be made of a length equal to one third of H and it will then respond to the first odd harmonic of that wave, of which the fundamental is in syntony with the vertical wire. By suitably choosing the relation between the wave lengths of the two transmitting stations, it is possible to receive in this manner two different