It is very necessary that the pellets be kept dry, as otherwise a loud singing and hissing noise, due to the decomposition of the water, will render the reception of signals very difficult.
When an oscillatory discharge takes place through a coil of wire surrounding a needle, it magnetizes the needle in a totally different manner from a voltaic current. The needle will have several poles throughout its length, many of them reversed. Rutherford applied this phenomenon to the detection of electrical oscillations, but it remained for Marconi to improve it and give the magnetic detector its existing form. This type of detector is very sensitive, free from all adjustment and is not made inoperative by the heavier discharge of the transmitter during each period of sending.
[Illustration: Fig. 112. Marconi Magnetic Detector.]
A small transformer is provided with a core composed of a band or cord of iron wires in the form of an endless belt which passes around two pulleys kept in motion by a clockwork motor. The band revolves in the field of a strong horseshoe magnet and passes directly over the poles after issuing from the transformer bobbin, so that the portion approaching the bobbin are constantly in a state of increasing magnetism. The actual operation is based upon the property of iron called hysteresis, for the magnetism of the core lags behind that of the permanent magnet and is of a different degree from what it ought to be, in view of its position in the vicinity of the permanent magnet. The moment the oscillations pass through the primary coil of the transformer, this lag is set free and the magnetism assumes its full value. The change in magnetism induces a current in the secondary, which registers as a sound in the telephone receivers.
[Illustration: Fig. 113. Details of Transformer.]
The primary coil is made up of a single layer of No. 36 B. S. gauge silk covered wire wound on a thin walled glass or hard rubber tube. The ends of the tube, which is 2 inches long and 1/4 inch external diameter, are slightly flared so as not to chafe the band. The primary is thoroughly shellacked and covered with a single layer of paper.
The secondary is also of No. 36 B. S. silk covered wire and is wound between two disks of hard rubber, 1/4 inch thick and 1 1/2 inches in diameter, placed 3/8 inch apart in the center of the secondary and the intervening space wound full of wire.