III Dry Battery for Use in Audion Circuits, and also Storage Battery 27 IV Radiotron 42 V Variometer and Variable Condenser of the General Radio Company. Voltmeter and Ammeter of the Weston Instrument Company 91 VI Low-Power Transmitting Tube, U V 202 106 VII Photographs of Vibrating Strings 155 VIII To Illustrate the Mechanism for the Production of the Human Voice 170 IX Western Electric Loud Speaking Receiver. Crystal Detector Set of the General Electric Co. Audibility Meter of General Radio Co. 203 X Audio-Frequency Transformer and Banked-Wound Coil 218 XI Broadcasting Equipment, Developed by the American Telephone and Telegraph Company and the Western Electric Company 235 XII Broadcasting Station of the American Telephone and Telegraph Company on the Roof of the Walker-Lispenard Bldg. in New York City where the Long-distance Telephone Lines Terminate 250 LETTERS OF A RADIO-ENGINEER TO HIS SON LETTER 1 ELECTRICITY AND MATTER MY DEAR SON: You are interested in radio-telephony and want me to explain it to you. I'll do so in the shortest and easiest way which I can devise. The explanation will be the simplest which I can give and still make it possible for you to build and operate your own set and to understand the operation of the large commercial sets to which you will listen. I'll write you a series of letters which will contain only what is important in the radio of to-day and those ideas which seem necessary if you are to follow the rapid advances which radio is making. Some of the letters you will find to require a second reading and study. In the case of a few you might postpone a second reading until you have finished those which interest you most. I'll mark the letters to omit in this way. All the letters will be written just as I would talk to you, for I shall draw little sketches as I go along. One of them will tell you how to experiment for yourself. This will be the most interesting of all. You can find plenty of books to tell you how radio sets operate and what to do, but very few except some for advanced students tell you how to experiment for yourself. Not to waste time in your own experiments, however, you will need to be quite familiar with the ideas of the other letters.