Condensers for spark telegraph transmitters can be ordinary Leyden jars or glass plates coated with tin or copper foil and set into a frame, or they can be built up of mica and sheet metal embedded in an insulating composition. The glass plate condensers are the cheapest and will serve your purpose well, especially if they are immersed in oil. Tuning coils, sometimes called transmitting inductances and oscillation transformers, are of various types. The simplest kind is a transmitting inductance which consists of 25 or 30 turns of copper wire wound on an insulating tube or frame. An oscillation transformer is a loose coupled tuning coil and it consists of a primary coil formed of a number of turns of copper wire wound on a fixed insulating support, and a secondary coil of about twice the number of turns of copper wire which is likewise fixed in an insulating support, but the coils are relatively movable. An oscillation transformer (instead of a tuning coil), is required by government regulations unless inductively coupled.
The Vacuum Tube Telegraph Transmitter.--This consists of: (1) a source of direct or alternating current, (2) a telegraph key, (3) a vacuum tube oscillator, (4) a tuning coil, and (5) a condenser. This kind of a transmitter sets up sustained oscillations instead of periodic oscillations which are produced by a spark gap set. The advantages of this kind of a system will be found explained in [Chapter XVI].
The Wireless Telephone Transmitter.--Because a jump-spark sets up periodic oscillations, that is, the oscillations are discontinuous, it cannot be used for wireless telephony. An electric arc or a vacuum tube sets up sustained oscillations, that is, oscillations which are continuous. As it is far easier to keep the oscillations going with a vacuum tube than it is with an arc the former means has all but supplanted the latter for wireless telephone transmitters. The apparatus required and the connections used for wireless telephone sets will be described in later chapters.
Useful Information.--It would be wise for the reader to turn to the [Appendix], beginning with page 301 of this book, and familiarize himself with the information there set down in tabular and graphic form. For example, the first table gives abbreviations of electrical terms which are in general use in all works dealing with the subject. You will also find there brief definitions of electric and magnetic units, which it would be well to commit to memory; or, at least, to make so thoroughly your own that when any of these terms is mentioned, you will know instantly what is being talked about.
[CHAPTER II]
PUTTING UP YOUR AERIAL
As inferred in the first chapter, an aerial for receiving does not have to be nearly as well made or put up as one for sending. But this does not mean that you can slipshod the construction and installation of it, for however simple it is, the job must be done right and in this case it is as easy to do it right as wrong.
To send wireless telegraph and telephone messages to the greatest distances and to receive them as distinctly as possible from the greatest distances you must use for your aerial (1) copper or aluminum wire, (2) two or more wires, (3) have them the proper length, (4) have them as high in the air as you can, (5) have them well apart from each other, and (6) have them well insulated from their supports. If you live in a flat building or an apartment house you can string your aerial wires from one edge of the roof to the other and support them by wooden stays as high above it as may be convenient.
Should you live in a detached house in the city you can usually get your next-door neighbor to let you fasten one end of the aerial to his house and this will give you a good stretch and a fairly high aerial. In the country you can stretch your wires between the house and barn or the windmill. From this you will see that no matter where you live you can nearly always find ways and means of putting up an aerial that will serve your needs without going to the expense of erecting a mast.
Kinds of Aerial Wire Systems.--An amateur wireless aerial can be anywhere from 25 feet to 100 feet long and if you can get a stretch of the latter length and a height of from 30 to 75 feet you will have one with which you can receive a thousand miles or more and send out as much energy as the government will allow you to send.