Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

THE MARCONI MAGNETIC DETECTOR.

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.

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.

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.

The terminals of both the primary and secondary are extended to binding posts mounted on the case of the instrument.

Fig. 114. Method of Joining Ends of Band.

The core or revolving band is made by winding 100 strands of No. 36 silk covered soft iron wire between two small pegs, placed a distance apart, equal to twice the circumference of the oval formed by the two pulleys. The wire is all wound in the same direction. It should be carefully removed from the pegs and kept taut while it is slightly twisted, doubled, and then further twisted into a rope or cord. The ends are threaded together with a separate piece of insulated wire, into a link which will pass easily through the primary tube.

Fig. 115. Pulley.

The pulleys around which the band revolves are made of hard wood. They are 4 inches in diameter and 3/8 inch thick and have a V-shaped groove cut in the edge. In order to minimize friction and wear, it is advisable to fit them with a bearing which may be made out of brass tubing and a couple of washers. One of the pulleys is geared to a clockwork motor so that the band makes a complete revolution about once every two minutes. An old eight day clock may be adapted for this purpose, or, what is much better, the motor from an old phonograph.

Fig. 116. Pulley Bearings.

The horseshoe magnet is mounted with its north pole pointing towards and nearly touching the middle of the outside of the secondary. The south pole is placed opposite the end of the primary tube which is on the side towards which the band is revolving, that is, the band in revolving passes first over the north pole and then over the south. Two magnets are sometimes used with their north poles together in the center of the secondary, and a south pole opposite each end of the primary tube.

Fig. 117. Circuit of Magnetic Detector.

The commercial instrument is placed in a glass covered case which protects it from dust and injury. The clockwork motor is concealed in the lower part of the case. This is a good plan but an experimental detector may be merely mounted on a flat wooden base as illustrated in Fig. 112.

The secondary terminals are connected directly to the telephone receivers while the primary leads to the aerial and the ground. When the detector is started up it should make a very slight hissing sound in the telephone receivers as the band passes slowly through the coils. This shows the instrument to be in good working order and ready for the reception of signals.

The Audion.—Dr. Lee DeForest was led by the flickering of a sensitive gas flame to investigate whether or not it would respond to Hertzian vibrations as well as to those of heat and sound. His experiments led to the invention of the audion, a peculiar instrument making use of ionized gas for its operation.

The audion consists of an incandescent lamp having a metallic filament, on either side of which are a grid and a plate made of nickel. When the filament is lighted it throws off ions which act as a relay to high frequency oscillations passing between the plate and the grid. A properly constructed audion is exceedingly sensitive and produces very loud tones in the telephone receivers. It has the further advantages of entire absence of adjustment except the governing of the battery voltage, and is capable of extremely fine tuning.

Fig. 118. Fleming Oscillation Valve.

Fleming originated the oscillation valve illustrated in Fig. 118. It consists of an ordinary incandescent lamp with a carbon filament, having a metal cylinder, C, placed around the filament, but attached to an independently insulated platinum wire sealed in the glass. When the lamp is lighted by passing a current through the filament, the incandescent carbon liberates negative ions. If oscillations are then set up in a circuit which includes a pair of sensitive telephone receivers and is formed by connecting the negative terminal of the filament with the platinum cylinder, negative electricity will be enabled to pass from the filament to the cylinder but not in the opposite direction, and so sounds will be produced in the telephone receivers. High frequency oscillations themselves could not be made to pass through the telephone receivers because of the choking action of the iron cores of the electromagnets.

Fig. 119. Flame Audion.

The simple but sensitive form of detector illustrated in Fig. 119 is not of practical value for commercial work, but is very interesting as the progenitor of the audion, and provides a good field for amateur investigation. Its only drawback is that the gas flame is very difficult to keep steady and every flicker registers as a sound in the telephone receivers.

A Bunsen burner using coal gas furnishes the flame, and a salt of an alkaline metal heated in the flame, the ions. The hydroxides of caesium, potassium and sodium give the best results in the order named.

The salt is contained in a piece of trough-shaped platinum foil, about 3/8 inch long and 1/16 inch wide. This trough is made the cathode or negative of the telephone circuit and placed in the outer oxidizing flame just above its juncture with the interior reducing flame and must be kept incandescent. The upper electrode or anode is a piece of platinum wire about 1/16 inch above the trough.

Fig. 120. Circuit of Flame Audion.

The arrangement and construction of the detector is clearly indicated by the drawing so that it is unnecessary to go into details. The block, E, which fits on the tube of the Bunsen burner, is made of fiber. Two double binding posts, D, are fastened to E to support the rods, R, which are fitted at the tops with binding posts, B, into which the electrodes may be clamped.

Twelve dry cells are connected with a multiple point switch so that an electromotive force of 6-18 volts, varying in steps of one cell at a time, may be secured. The flame is best provided with a mica chimney to protect it from drafts. By keeping plenty of salt in the trough and carefully adjusting the voltage, this detector may be made marvelously sensitive.

[3]	The different detectors in order of their sensitiveness are electrolytic, perikon, magnetic, silicon, carborundum.

[4]	Peacock ore or bornite, which consists of about 60 parts of copper, 14 parts of iron and 26 parts of sulphur, may be substituted for the chalcopyrites with excellent results.