The overhead part of the apparatus employed to collect the electric waves is called the antennæ, and in the various commercial forms of wireless apparatus this feature differs. The general principle, however, is the same, and in [Figs. 15], [16], [17], and [18] some simple forms of construction are shown.
Great care must be taken to properly insulate the rod, wire, or fingers of these antennæ, so that the full force of the vibration is carried directly down to the coherer and sounder or receiver. For this purpose, porcelain, glass, or gutta-percha knobs must be employed.
In [Fig. 15] the apparatus consists of an upright stick, a cross-stick, and a brace, or bracket, to hold them in proper place.
Porcelain knobs are made fast to the sticks with linen string or stout cotton line. Then an insulated copper wire is run through the holes in the knobs, and from the outer knob a rod of brass, copper, or German-silver, or even a piece of galvanized-iron lightning-rod, is suspended. Care should be taken to see that the joint between rod and wire is soldered so as to make perfect contact. Otherwise rust or corrosion will cause imperfect contact of metals, and interrupted vibrations would be the result. The upright stick should be ten or fifteen feet high, and may be attached to a house-top, a chimney, or on the corner of a barn roof.
Another form of single antenna is shown in [Fig. 16]. This is a rod held fast in a porcelain insulator with cement. The insulator, in turn, is slipped over the end of a staff, or pole, which is erected on a building top or out in the open, the same as a flag-pole. Near the foot of the rod, and just above the insulator, a conducting-wire is made fast and soldered. This is run down through porcelain insulators to the apparatus.
If the pole is erected on a house-top it may be braced with wires, to stay it, but care must be taken not to have these wires come into contact with the rod, or conducting-wire.
TYPES OF ANTENNÆ
Another form of antennas is shown in [Fig. 17], where rods are suspended from a wire which, in turn, is drawn taut between two insulators. The insulators are held in a framework composed of two uprights and a cross-piece of wood.