It consists of a small crystal of carborundum clamped tightly between two carbon electrodes. It may be used with or without a battery. The battery is preferred.
The Magnetic detector is a very sensitive device utilizing the changes in the magnetic state of iron, which are caused by rapidly oscillating currents. If a core of iron wires be placed in a varying magnetic field, the magnetization of the iron will lag behind the magnetizing force on account of hysteresis or "magnetic friction."
FIG. 51. Marconi Magnetic Detector
But if a rapidly oscillating current is passed through a coil surrounding the iron, a sudden change in magnetization occurs, sufficient to induce an E. M. F. in a second coil surrounding the core and thus operate a telephone receiver in series with this coil.
The usual form of magnetic detector consists of a belt of fine iron wires passing over two pulleys which are driven by clockwork. A pair of permanent magnets supply the field which induces a continuously varying magnetization in the moving core. The core passes through the centre of a double coil, one part of which is connected to the telephone receivers and the others to the aerial and ground.
LESSON TWENTY-THREE. TUNING COILS.
The tuning coil is a device consisting of a large number of turns of wire wound in the form of a cylinder and provided with one or more sliding contacts which can be brought into touch with any one of the turns at will in order to increase or decrease the electrical length or period of the circuit to suit the incoming waves.
FIG. 52. Double Slide Tuning Coil.