The Hartley oscillator is made up of an LC tank circuit which develops oscillations. In addition to the LC tank circuit this circuit contains an amplifier unit made up using a BJT. This controls the oscllations that are fed back from the tank circuit. The tank cirucit is made up of two inductors split in the middle like a single coil being center-tapped. A capacitor is connected in parallel to it. The oscillating frequency of the Hartley oscillator is given by f = 1/(2* pi * sqrt ( Lt * C)) where Lt is the combined inductance of the two inductors.