The Hartley oscillator is an electronic oscillator

Electronic oscillator

An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices...

 circuit

Electronic circuit

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...

 that uses an inductor

Inductor

An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...

 and a capacitor

Capacitor

A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

 in parallel to determine the frequency. Invented in 1915 by American engineer Ralph Hartley

Ralph Hartley

Ralph Vinton Lyon Hartley was an electronics researcher. He invented the Hartley oscillator and the Hartley transform, and contributed to the foundations of information theory.-Biography:...

, the distinguishing feature of the Hartley circuit is that the feedback

Feedback

Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

 needed for oscillation is taken from a tap on the coil, or the junction of two coils in series.
A Hartley oscillator is essentially any configuration that uses two series-connected coils and a single capacitor (see Colpitts oscillator

Colpitts oscillator

A Colpitts oscillator, invented in 1920 by American engineer Edwin H. Colpitts, is one of a number of designs for electronic oscillator circuits using the combination of an inductance with a capacitor for frequency determination, thus also called LC oscillator...

 for the equivalent oscillator using two capacitors and one coil).