Abstract: Apparatus for measuring the number of turns on a toroidal coil includes a conductive member adapted to receive the coil to be tested; circuitry for energizing the coil to be tested including an oscillator and coupler circuitry connected in circuit between the oscillator and the coil energizing terminals; and circuitry connected to the conductive member including a comparator having first and second inputs and an output. Circuitry responsive to the connection of the conductor member in circuit applies a reference voltage signal to a first input of the comparator; circuitry responsive to the voltage induced in the conductive member by the energized toroidal coil to be tested applies a signal proportional to the induced voltage to the second input of the comparator; and circuitry responsive to the output of the comparator stabilizes the voltage applied from the oscillator to the toroidal coil when the input signals to the comparator are equal.