Most important time scales now in use, primary standards which keep them, and frequency comparison methods used between laboratories are described. Equations for a Meacham type oscillator are developed, finding the stability coefficients relative to phase and modulus of the amplifier. — The oscillator is amplitude controlled by a d.c. amplifier to reduce the power in the crystal to a level smaller than 1 X 10⁻⁶ watt. — The equivalent circuits for a cristal, near both resonances, are derived, including resistance, equivalent Q and influence of a series trimming capacitor. — Measurements of the unit used, with a Q-meter and in the oscillator circuit itself, are described, showing excellent agreement between them. — Different methods of temperature control are described, and the one used in our case, as well as temperature measurements, are analyzed in detail.- The stability achieved is better than 10⁻³°C for 15°C variation of environment temperature. — Frequency measurement are described, and results for the long term stability of the crystal are given.