The Hartley uses a tuned tank circuit with a tapped inductor in the feedback loop. That means it's inductively coupled. One side connects to the amplifier input and the other to the amplifier output. It uses a tuning capacitor. The Hartley and the Colpitts are two hobbist Oscillator circuits you'll come across a lot in educational texts. the advantage is that they are simple. the disadvantage is that you cant output a pure sinewave for testing, it's loaded with harmonics.
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