. The principles underlying radio communication . ectron tube as a detector see the next chapter. Section 194. An inductively coupled circuit is shown in Fig. 239-a, whichhas been found to be particularly adapted to wave lengthsshorter than 400 meters and also to give good results on longerwave lengths. In this circuit it is essential that the variable RECEIVING CIRCUITS. 427 inductances Li and Lz shall be continuously variable. Thename variometer is sometimes applied to a form of con-tinuously variable inductance often used with this inductance Li may be of the usual type, va
. The principles underlying radio communication . ectron tube as a detector see the next chapter. Section 194. An inductively coupled circuit is shown in Fig. 239-a, whichhas been found to be particularly adapted to wave lengthsshorter than 400 meters and also to give good results on longerwave lengths. In this circuit it is essential that the variable RECEIVING CIRCUITS. 427 inductances Li and Lz shall be continuously variable. Thename variometer is sometimes applied to a form of con-tinuously variable inductance often used with this inductance Li may be of the usual type, variable bysteps. Condenser C2 may be variable by steps, and is not neces-sarily continuously variable. Ci and Li are adjusted to approxi-mately the wave length of the incoming signal, but it is notessential that the primary be accurately tuned. It is, however,essential that the secondary be very accurately tuned, and in-ductance 1/2 must be very carefully adjusted. The inductanceLz in the regenerative circuit (see Sec. 199, p. 487) should also be. Fig. 239-a.—Tuned plate regenerative tuning circuit, using continu-ously variable inductances. carefully adjusted until maximum response is secured in thetelephone receivers. This same circuit can be used for receiving continuous wavesby the autodyne method (see Section 205, p. 503) by adjust-ing Lz so that the tube is in the oscillating condition. Forcontinuous waves the best signal will usually be received whenLz is adjusted just above the point when the tube is in theoscillating condition. For spark reception, this circuit willusually give best results when Lz is adjusted just below thepoint where the tube is in the oscillating condition. This same type of circuit can also be used for reception witha coil antenna. (See Section 151.) In this case the antennaand receiving transformer shown in Fig. 239-a are not present;that is, in Fig. 239-a the part of the figure to the left of points 428 RECEIVING APPARATUS. A and B is deleted. Th
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