"Dip meter" can also refer to an influential early commercial expert system called Dipmeter Advisor; or may refer to an instrument that measures the magnetic dip angle of Earth's magnetic field, the field line angle in a vertical plane. Grid dip oscillator (GDO), also called grid dip meter, gate dip meter, dip meter, or just dipper, is a type of electronic instrument that measures the resonant frequency of nearby unconnected radio frequency tuned circuits. It is a variable-frequency oscillator that circulates a small-amplitude signal through an exposed coil, whose electromagnetic field can interact with adjacent circuitry. The oscillator loses power when its coil is near a circuit that resonates at the same frequency. A meter on the GDO registers the amplitude drop, or "dip", hence the name. Dip oscillators have been widely used by amateur radio operators for measuring the properties of resonant circuits, filters, and antennas. They can also be used for transmission line testing, as signal generators, and for measuring inductance and capacitance of components. Measurement with a GDO is called "dipping" a circuit. Principle of operation Central to the dip meter is a high-frequency variable-frequency oscillator with a calibrated tuning capacitor and matching interchangeable coils, as shown in the circuit diagram on the right. Resonance is indicated by a dip in amplitude of the signal within the GDO, by a meter on the device. When the oscillator's exposed coil is in the vicinity of another resonant circuit, the coupled pair behaves as a low-Q transformer whose coupling is most effective when their respective resonant frequencies match. The degree of coupling affects the frequency and amplitude of oscillation in the dip meter, which is sensed in any of several ways, the simplest and most usual of which is a built-in microammeter. The distance between the coil and the tested circuit needs to be adjusted carefully so that the GDO amplitude is significantly affected by the coupled circuit, but its frequency is not. History Grid dip oscillators were first developed in the 1920s and were built with vacuum tubes. The devices displayed the amplitude of the tube's grid current, hence the "G" in GDO. Modern dip meters are solid-state devices, and are sometimes called gate dip oscillators or emitter dip oscillators in reference to the analogous part of the transistor whose current is measured instead of a vacuum tube grid. Solid-state versions of the grid dip oscillator are more