BatteryResistorCapacitorInductorSwitchWireGroundAntennaHeadphonesDiodeorTransformerCrystal (piezoelec.)Telegraph keySpeakerMicrophoneLampFigure 22.1: Some common schematic symbols. BatteryResistorCapacitorInductorSwitchWireGroundAntennaHeadphonesDiodeorTransformerCrystal (piezoelec.)Telegraph keySpeakerMicrophoneLampFigure 22.1: Some common schematic symbols.