Where high frequencies are dealt with it isn't necessary to have iron cores because the effect is large enough without the help of the atomic loops. And even if we wanted their help it wouldn't be easy to obtain, for they dislike to turn so fast and it takes a lot of power to make them do so. We know that fact because we know that an iron core increases the inductance and so chokes the current. For low frequencies, however, that is those frequencies in the audio range, it is usually necessary to have iron cores so as to get enough effect without too many turns of wire. The fact that iron decreases the inductance and so seriously impedes alternating currents leads us to use iron-core coils where we want high inductance. Such coils are usually called "choke coils" or "retard coils." Of their use we shall see more in a later letter where we study radio-telephone transmitters. LETTER 21 YOUR RECEIVING SET AND HOW TO EXPERIMENT MY DEAR STUDENT: In this letter I want to tell you how to experiment with radio apparatus. The first rule is this: Start with a simple circuit, never add anything to it until you know just why you are doing so, and do not box it up in a cabinet until you know how it is working and why. Your antenna at the start had better be a single wire about 25 feet high and about 75 feet long. This antenna will have capacity of about 0.0001 m. f. If you want an antenna of two wires spaced about three feet apart I would make it about 75 feet long. Bring down a lead from each wire, twisting them into a pigtail to act like one wire except near the horizontal part of the antenna. [Illustration: Fig 110] Your ground connection can go to a water pipe. To protect the house and your apparatus from lightning insert a fuse and a little carbon block lightning arrester such as are used by the telephone company in their installations of house phones. You can also use a so-called "vacuum lightning arrester." In either case the connections will be as shown in Fig. 111. If you use a loop antenna, of course, no arrester is needed. At first I would plan to receive signals between 150 meters and 360 meters. This will include the amateurs who work between 160 and 200 m., the special amateurs who send C-W telegraph at 275 m., and the broadcasting stations which operate at 360 m. This range will give you plenty to listen to while you are experimenting. In addition you will get some ship signals at 300 m. [Illustration: Fig 111] To tune the antenna to any of the wave lengths in this range you can use a coil of 75 turns wound on a cardboard tube of three and a half inches in diameter. You can wind this coil of bare wire if you are careful, winding a thread along with the wire so as to keep the successive turns separated. In that case you will need to construct a sliding contact for it. That is the simplest form of tuner.