Page:Popular Science Monthly Volume 63.djvu/565

Rh resonance. Many devices of this kind due to Professor Slaby and others have been suggested and tried but the details are rather too technical to be fully described here.

It will be noticed that the receiving aerial may be arranged in one of two ways—it may be either earthed at the lower end or it may be insulated. It has been claimed that there is a great advantage in earthing the receiving aerial directly in that it eliminates atmospheric disturbances.

We shall allude to this point more particularly later on. Meanwhile it may be mentioned that the receiving arrangements, as a whole, constitute a sensitive arrangement, as shown by Popoff, Tommasina and by all the large experience of Mr. Marconi himself for detecting changes in the electrical condition of the atmosphere, which are doubtless of the nature of electrical oscillations. On the other hand, the receiving arrangements may be perfectly insulated, and some experimentalists have asserted that by this method the greatest freedom is secured from atmospheric disturbances. Amongst the nonearthed arrangements the system invented by Professor F. Braun, of Strasburg, and worked by Messrs. Siemens, of Berlin, may be mentioned.

Professor Braun's arrangements are indicated m the diagram m Fig. 21. In this case, an induction coil is used to create a discharge between two spark balls, and to these two balls are connected the two outer coatings of two condensers, the inner coatings of which are connected together through the primary coil of an air core transformer. The secondary coil of this transformer is connected to two extension wires forming a Hertz resonator, and the length of these wires is so adjusted with reference to the time period of the primary circuit that they resonate to it, the whole length from end to end of the secondary circuit being half a wave length. The receiver, as shown in the diagram, consists of a pair of quarter wave length receiving wires connected through two condensers, which are shortcircuited by the primary coil of an oscillation transformer. The secondary circuit of this last oscillation transformer has two extension wires to it, turned in the same manner, to respond to the primary oscillator; and in the circuit of one of these extension wires is placed a coherer tube, shortcircuited by a relay and a local battery.

It will thus be seen that there is an entire abolition of ground