and conductors. They do not permeate to the center of the wire, as do normal currents. The surface of a stranded wire is greater in comparison to its cross-section than a solid conductor of the same diameter, and therefore is often employed because it offers less resistance to currents of this sort.

FIG. 24.-Showing how wires are arranged and insulated.

Aluminum wire is very light, and causes very little strain on the pole or cross-arms. It offers more resistance than copper, but some of the larger sizes may be used with equally good results.

Iron wire must never be used, even if galvanized or tinned. It possesses a certain reactance tending to choke off the high frequency currents.

The aerial is always very carefully insulated from its supports and surrounding objects by special insulators, capable of withstanding severe strains, made of a moulded material having an iron ring imbedded in each end.

The wires leading from the aerial to the operating room are called the "rat-tail," or "lead-in." They must be very carefully insulated by leading through a bushing placed in the wall or window of the operating room.

One of the most important factors in a wireless station is the proper earthing arrangement. The usual method is to use large copper plates buried in moist earth, or thrown in the sea. On shipboard it is merely necessary to connect the earth wire to the metallic plates of which the hull of the vessel is built. Amateurs employ the water or gas pipes in the house, the former being preferred. Connections are established by means of a ground clamp.

In the country, where water-pipes are not available, the best way is to bury a sheet of copper three or four feet deep in moist earth.

A very efficient earth can be formed by spreading a large area of chicken wire netting over the ground. This method is the best where the earth is very dry or sandy, and no other way is readily convenient.

CHAPTER III.

THE TRANSMITTING APPARATUS.

The principal instruments composing the apparatus used for sending the wireless messages comprise an induction coil, or in its place a transformer, a key, a spark gap, a condenser, and a helix.

THE CURRENT SUPPLY available will determine the type of the instruments, and whether an induction coil or a transformer is used. Unless current mains for light and power are already installed, it must be generated by an engine and dynamo, or recourse had to batteries. Induction coils may be operated on either direct or alternating current. Dry cells are most commonly employed to furnish

FIG. 28.-Diagram showing how batteries may be arranged in "series" or "series multiple."

the current for small induction coils, but a storage or some form of renewable primary cell, such as the Fuller and Edison, is necessary if the coil is a large one.

When dry cells are used, they should be connected in

FIG. 29.-The power plant of a Marconi transatlantic station, showing engine and generator.

series multiple, as shown in the accompanying diagram. This method of connecting distributes the load, and considerably lengthens the life of the battery.

When the source of current supply is alternating, an induction coil may be operated as a transformer. Both induction coils and transformers are instruments for raising the voltage of the ordinary available current from a comparatively low value, 6-220 volts, to a quantity (15,000

20,000 volts), where it can properly charge the aerial and create a state of strain, or, as it is called in technical parlance, an electro-static field.

Both the induction coil and transformer depend for their operation upon the principles of magnetic induction. In 1831, Michael Faraday, a famous English chemist and physicist, discovered that if a magnet be suddenly plunged into a hollow coil of wire, that a momentary current of electricity is generated in the coil. As long as the magnet remains motionless, it induces no current in the coil, but when it is moved back and forth, it sets up the currents. The source of electrical energy is the mechanical work