A tuning coil of the simplest kind consists of a single layer of copper wire wound on a cylinder with an adjustable, or sliding, contact, but for sharp tuning you need a loose coupled tuning coil. Where a single coil tuner is used a fixed condenser should be connected around the telephone receivers. Where a loose coupled tuner is employed you should have a variable condenser connected across the closed oscillation circuit and a fixed condenser across the telephone receivers.

When listening-in to distant stations the energy of the received wireless waves is often so very feeble that in order to hear distinctly an amplifier must be used. To amplify the incoming sounds a vacuum tube made like a detector is used and sometimes as many as half-a-dozen of these tubes are connected in the receiving circuit, or in cascade, as it is called, when the sounds are amplified, that is magnified, many hundreds of times.

The telephone receiver of a receiving set is equally as important as the detector. A single receiver can be used but a pair of receivers connected with a head-band gives far better results. Then again the higher the resistance of the receivers the more sensitive they often are and those wound to as high a resistance as 3,200 ohms are made for use with the best sets. To make the incoming signals, conversation or music, audible to a room full of people instead of to just yourself you must use what is called a loud speaker. In its simplest form this consists of a metal cone like a megaphone to which is fitted a telephone receiver.

About Transmitting Stations--Getting Your License.--If you are going to install a wireless sending apparatus, either telegraphic or telephonic, you will have to secure a government license for which no fee or charge of any kind is made. There are three classes of licenses issued to amateurs who want to operate transmitting stations and these are: (1) the restricted amateur license, (2) the general amateur license, and (3) the special amateur license.

If you are going to set up a transmitter within five nautical miles of any naval wireless station then you will have to get a restricted amateur license which limits the current you use to half a kilowatt [Footnote: A Kilowatt is 1,000 watts. There are 746 watts in a horsepower.] and the wave length you send out to 200 meters. Should you live outside of the five-mile range of a navy station then you can get a general amateur license and this permits you to use a current of 1 kilowatt, but you are likewise limited to a wave length of 200 meters. But if you can show that you are doing some special kind of wireless work and not using your sending station for the mere pleasure you are getting out of it you may be able to get a special amateur license which gives you the right to send out wave lengths up to 375 meters.

When you are ready to apply for your license write to the Radio Inspector of whichever one of the following districts you live in:

First District..............Boston, Mass.
Second " ..............New York City
Third " ..............Baltimore, Md.
Fourth " ..............Norfolk, Va.
Fifth " ..............New Orleans, La.
Sixth " ............. San Francisco, Cal.
Seventh " ............. Seattle, Wash.
Eighth " ............. Detroit, Mich.
Ninth " ..............Chicago, Ill.

Kinds of Transmitters.--There are two general types of transmitters used for sending out wireless messages and these are: (1) wireless telegraph transmitters, and (2) wireless telephone transmitters. Telegraph transmitters may use either: (a) a jump-spark, (b) an electric arc, or (c) a vacuum tube apparatus for sending out dot and dash messages, while telephone transmitters may use either, (a) an electric arc, or (b) a vacuum tube for sending out vocal and musical sounds. Amateurs generally use a jump-spark for sending wireless telegraph messages and the vacuum tube for sending wireless telephone messages.

The Spark Gap Wireless Telegraph Transmitter.--The simplest kind of a wireless telegraph transmitter consists of: (1) a source of direct or alternating current, (2) a telegraph key, (3) a spark-coil or a transformer, (4) a spark gap, (5) an adjustable condenser and (6) an oscillation transformer. Where dry cells or a storage battery must be used to supply the current for energizing the transmitter a spark-coil can be employed and these may be had in various sizes from a little fellow which gives 1/4-inch spark up to a larger one which gives a 6-inch spark. Where more energy is needed it is better practice to use a transformer and this can be worked on an alternating current of 110 volts, or if only a 110 volt direct current is available then an electrolytic interrupter must be used to make and break the current. A simple transmitting set with an induction coil is shown in Fig. 2.

A wireless key is made like an ordinary telegraph key except that where large currents are to be used it is somewhat heavier and is provided with large silver contact points. Spark gaps for amateur work are usually of: (1) the plain or stationary type, (2) the rotating type, and (3) the quenched gap type. The plain spark-gap is more suitable for small spark-coil sets, and it is not so apt to break down the transformer and condenser of the larger sets as the rotary gap. The rotary gap on the other hand tends to prevent arcing and so the break is quicker and there is less dragging of the spark. The quenched gap is more efficient than either the plain or rotary gap and moreover it is noiseless.