FIG. 48. Galena Detector.
The Perikon Electra detector is a very sensitive form of the regular Perikon detector fitted with a micrometer adjustment.
The Galena detector consists of a crystal of that material to which contact is made by means of a fine wire spring exerting very light pressure.
FIG. 49. Audion Detector.
Crystal detectors act as rectifiers and change the alternating currents Into direct currents, which will pass through the telephone receivers. Minerals used for this purpose are said to possess unilateral conductivity, that is, they conduct currents better in one direction than the other and act much the same as a valve which allows water to flow in one direction, but not in the other.
Another well known detector of the "valve" type is that known as the Audion, consisting of a small incandescent lamp containing a small grid and plate of nickel. When the lamp is lighted by connecting a battery to the filament, a flow of ions passing from the hot filament through the grid to the plate is set up. The grid and plate form part of the receiving circuit containing the telephones. The flow of ions carries the oscillatory currents from the grid to the plate, but does not allow them to pass back again. In this manner, the alternating oscillatory currents are converted into direct currents, which will pass through the telephone receivers.
FIG. 50. Carborundum Detector.
The Carborundum detector, as its name implies, is a device making use of the unilateral conductivity of carborundum. This form of detector is very sensitive and has been employed for a number of years in all the installations of the United Wireless Telegraph Co.