FIG. 8. If a cross section of the aerial and atmosphere could be made in the same manner that an apple is sliced with a knife and the waves held stationary, they would appear as above.
These waves follow the contour of the earth, and so may cross mountains and valleys, and travel anywhere. They radiate from the aerial like the ripples from a pebble in a pool of water, in gradually enlarging circles. If a cross section of the aerial and atmosphere could be made in the same manner that an apple can be sliced with a knife, and the waves held stationary long enough to see them, they would appear as in Fig. 8. The curved lines represent the lines of strain induced by the oscillations. Each group of lines represents a wave. It will be noticed as they radiate farther from the aerial that they become larger and spread out.
FIG. 9.—Under the same conditions, but when viewed from above, the appearance would be that of a series of concentric circles.
The electromagnetic waves have the power of exciting oscillations in a conductor on which they impinge. This is made use of for the purpose of receiving the messages. When the waves strike the aerial of a distant station they set up high frequency oscillations, which are usually too weak to make their presence known except with the aid of a sensitive device, called a detector.
FIG. 10.—A simple receiving arrangement. The detector rectifies the oscillatory currents passing from the aerial to the ground so that they will flow through the telephone receiver and register as sound.
The most prominent type of detector in use to-day is a crystal of silicon, iron pyrites, zincite or certain other minerals. The mineral is placed between two contact points, one or both of which are adjustable so that the most sensitive portion of the mineral may be selected. A telephone receiver is connected across the terminals of the detector. When the electromagnetic waves from the transmitting station strike the aerial of the receiving station, they set up therein a series of high frequency oscillations, corresponding to the Morse signals emitted from the transmitter. The oscillations flow back and forth through the aerial and ground, striking the mineral detector on their journey. The high frequency oscillations are alternating currents, because they reverse their direction many thousand times per second. Such a current will not pass through the telephone receiver, because the little magnets contained therein exert a choking action on alternating currents of high frequency and effectually block their passage. The mineral detector acts as a valve, allowing the current to pass through in one direction, but not permitting it to return or go in the opposite direction. The result is a series of impulses flowing in one direction only, and therefore called a direct current. Such a current will flow through a telephone receiver and produce a motion of the diaphragm which imparts its motion to the surrounding air, the result being sound waves audible to the ear. By varying the periods during which the key is pressed and the oscillations are being produced, according to a prearranged code, the sounds in the receiver may be made to assume an intelligible meaning.
CHAPTER II. THE MEANS FOR RADIATING AND INTERCEPTING ELECTRIC WAVES. AERIAL SYSTEMS. EARTH CONNECTION.
Every radiotelegraphic station may be summed up as comprising these elements: first of all, certain appliances collectively forming the transmitter and serving to create the waves; secondly, the receiving apparatus, whose function is to detect the signals of some far-distant sending station, and lastly, an external organ called the aerial, or antenna, consisting of a huge system of wires elevated high in the air above all surrounding objects, either vertically or sloping, or partly horizontal and partly vertical, which radiates or intercepts the electromagnetic waves, accordingly as the station is transmitting or receiving.