The Operation of Reactors and Condensers.--A reactor is a single coil of wire wound on an iron core, see Fig. 90 and A in Fig. 91, and it should preferably have a large inductance. The reactor for the plate and grid circuit of a wireless telephone transmitter where one or more tubes are used as modulators as shown in the wiring diagram in Fig. 90, and the filter reactor shown in Fig. 92, operate in the same way.

When an alternating current flows through a coil of wire the reversals of the current set up a counter electromotive force in it which opposes, that is reacts, on the current, and the higher the frequency of the current the greater will be the reactance. When the positive half of an alternating current is made to flow through a large resistance the current is smoothed out but at the same time a large amount of its energy is used up in producing heat.

But when the positive half of an alternating current is made to flow through a large inductance it acts like a large resistance as before and likewise smooths out the current, but none of its energy is wasted in heat and so a coil having a large inductance, which is called an inductive reactance, or just reactor for short, is used to smooth out, or filter, the alternating current after it has been changed into a pulsating direct current by the rectifier tubes.

A condenser also has a reactance effect on an alternating current but different from an induction coil the lower the frequency the greater will be the reactance. For this reason both a filter reactor and filter condensers are used to smooth out the pulsating direct currents.

[CHAPTER XX]

HOW TO MAKE A RECEIVING SET FOR $5.00 OR LESS

In the chapters on Receptors you have been told how to build up high-grade sets. But there are thousands of boys, and, probably, not a few men, who cannot afford to invest $25.00, more or less, in a receiving set and would like to experiment in a small way.

The following set is inexpensive, and with this cheap, little portable receptor you can get the Morse code from stations a hundred miles distant and messages and music from broadcasting stations if you do not live too far away from them. All you need for this set are: (1) a crystal detector, (2) a tuning coil and (3) an earphone. You can make a crystal detector out of a couple of binding posts, a bit of galena and a piece of brass wire, or, better, you can buy one all ready to use for 50 cents.

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Wireless Receptor, the size of a Safety Match Box. A Youthful Genius in the person of Kenneth R. Hinman, Who is only twelve years old, has made a Wireless Receiving Set that fits neatly into a Safety Match Box. With this Instrument and a Pair of Ordinary Receivers, He is able to catch not only Code Messages but the regular Broadcasting Programs from Stations Twenty and Thirty Miles Distant.

The Crystal Detector.--This is known as the Rasco baby detector and it is made and sold by the Radio Specialty Company, 96 Park Place, New York City. It is shown in Fig. 96. The base is made of black composition and on it is mounted a standard in which a rod slides and on one end of this there is fixed a hard rubber adjusting knob while the other end carries a thin piece of phosphor-bronze wire, called a cat-whisker To secure the galena crystal in the cup you simply unscrew the knurled cap, place it in the cavity of the post and screw the cap back on again. The free end of the cat-whisker wire is then adjusted so that it will rest lightly on the exposed part of the galena.