Wireless Telegraph Construction For Amateurs

by Alfred Powell Morgan

CHAPTER IV. INDUCTION COILS.

Some means of charging the condenser which produces the oscillatory discharge is necessary. An induction coil is the most practical for the amateur.

The induction coil consists of a primary coil of wire wound around a central iron core and surrounded by a secondary coil consisting of many thousand turns of carefully insulated wire. The primary coil is connected to a source of direct current which also includes an interrupter to "make" and "break" the current in rapid succession. Every "make" of the circuit and consequent magnetization of the core induces a momentary inverse current in the secondary, and every "break" and corresponding demagnetization a momentary direct current. Normally, the induced currents would be equal, but by means of a condenser shunted across the interrupter the circuit when "made" requires considerable time for the current and magnetization of the core to reach a maximum value, while when broken the demagnetization and current drop are nearly instantaneous. The value of the induced electromotive force in a circuit varies as the speed at which the magnetic lines of force cut the circuit, and so the induced e.m.f. at "break" is thus rendered high enough to leap across a gap in the shape of sparks.

The formulas connected with induction coils depend upon conditions which are never met in actual practice and cannot be relied upon. To construct a coil of a given size, it is necessary to use dimensions obtained empirically. Therefore it is well for the amateur to stick closely to lines and hints which are given here or which appear in some up to date book on induction coil building.

Fig. 25. Diagram of an Induction Coil.

For a long time the induction coil was an expensive, inefficient instrument, until wireless telegraphy demanded of it more rigid and efficient design and construction. It was the aim of manufacturers to produce the longest possible spark length with a minimum amount of secondary wire. As a result of this demand, wireless coils are now made with a core of large diameter and give heavier and thicker sparks. The secondary in this case is short and uses wire of large cross section in order to reduce the resistance and minimize the heating.

No one part of an induction coil may be developed to its maximum efficiency without seriously influencing and lowering the efficiency of the other parts. The following suggestions regarding the construction are given that they may prove a useful guide to the amateur coil builder. The parts will be considered in their natural order of construction.

Core.—Some experimenters not quite familiar with the principles of magnetism reason that if an induction coil were provided with a closed core as the transformer, the efficiency of the coil would be materially increased. But this is not so, for the magnetization and demagnetization of the iron cannot take place rapidly enough in a closed core when an interrupted direct current is employed in place of an alternating current.

The core of an induction coil is therefore always straight. For the same reason, it is never solid but is made up of a bundle of soft iron wires in order that rapid changes in magnetism may take place. The wires are always of as high a permeability[¹] as possible so as to create a strong magnetic field. Swedish or Russian iron of a good quality is the best, as its hysteresis[²] losses are small. The smaller the diameter of the wire the less will be the eddy current losses and heating, but the permeability is also rendered less and the core will not be so effective, as the amount of iron is thereby decreased and the oxidized surface increased. No. 22 B. S. gauge wire is the best size for the average core.