TABLE III Circuit of Fig. 112 Number Inductance in Wave length with antenna of of turns. mil-henries. 0.0001 mf. 0.0002 mf. 14 0.04 120 170 20 0.07 160 220 28 0.12 210 290 36 0.18 250 360 44 0.25 300 420 56 0.38 370 520 75 0.60 460 650 In the secondary circuit there is only one capacity, that of the variable condenser. If it has a range of values from about 0.00005 mf. to 0.0005 mf. your coil of 60 turns and 0.42 mf. permits a range of wave-lengths from 270 to 860 m. Using half the coil the range is 150 to 480 m. With the larger condenser the ranges are respectively 270 to 1220 and 270 to 670. For longer wave-lengths load with inductance. Four times the inductance will tune to double these wave-lengths. [Footnote 11: If you can afford to buy, or if you can borrow, ammeters and voltmeters of the proper range you should take the characteristic yourself.] LETTER 22 HIGH-POWERED RADIO-TELEPHONE TRANSMITTERS MY DEAR EXPERIMENTER: This letter is to summarize the operations which must be performed in radio-telephone transmission and reception; and also to describe the circuit of an important commercial system. To transmit speech by radio three operations are necessary. First, there must be generated a high-frequency alternating current; second, this current must be modulated, that is, varied in intensity in accordance with the human voice; and third, the modulated current must be supplied to an antenna. For efficient operation, of course, the antenna must be tuned to the frequency which is to be transmitted. There is also a fourth operation which is usually performed and that is amplification. Wherever the electrical effect is smaller than desired, or required for satisfactory transmission, vacuum tubes are used as amplifiers. Of this I shall give you an illustration later.