. The principles underlying radio communication . , so that part of the alternating magnetic flux of Ppasses through S. There are three principal styles of oscilla-tion transformer, the double helix, hinged coil, and flat spiraltypes. See Figs. 197, 198, 199. In order to have a low resist-ance the conductor is usually a copper ribbon of large surface,or edgewise-wound copper strip. The amount of coupling, orthe mutual inductance, between them is varied by moving oneor both of the pair. Connections are made to such coils bymovable clips, so that any desired amount of self-inductancemay be used.
. The principles underlying radio communication . , so that part of the alternating magnetic flux of Ppasses through S. There are three principal styles of oscilla-tion transformer, the double helix, hinged coil, and flat spiraltypes. See Figs. 197, 198, 199. In order to have a low resist-ance the conductor is usually a copper ribbon of large surface,or edgewise-wound copper strip. The amount of coupling, orthe mutual inductance, between them is varied by moving oneor both of the pair. Connections are made to such coils bymovable clips, so that any desired amount of self-inductancemay be used. The hot-wire ammeter is used for measuring the current inthe antenna circuit. For merely tuning to resonance a low-resistance lamp such as a small flashlight lamp may be used inplace of the hot-wire ammeter, the maximum current being indi-cated by the maximum brightness of the lamp filament. If the 372 TEA NSMITTINO APPARATUS. current is too great for the lamp, it should be shunted by aShort length of wire. The ammeter or lamp may be short cir-. Pig. 197.—Double helix oscillation transformer; coils separated axially. cuited except when actually needed in order to keep the resist-ance of the antenna circuit low. ADJUSTMENTS OF TRANSMITTING SETS. 373 162. Direct Coupled Transmitting Set.—Direct instead of in-ductive coupling may be used between the closed circuit andthe antenna circuit, as in Fig. 200. (Direct coupling was ex-plained in Sec. 119.) One inductance coil is all that is the contacts shown, as much or as little of the inductance asdesired can be used in either circuit. In order to tune to somewave lengths it may be necessary to have an additional coil in
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