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Consideration is given to the various electrical quantities associated with the equivalent network of a quartz crystal which might be used to assess the Â¿activityÂ¿ that the crystal will exhibit in an oscillator circuit. It is concluded that the most generally serviceable criterion is afforded by measuring the anti-resonant impedance of the parallel combination of mounted crystal and a standardized value of capacitance typical of that encountered in current oscillator designs. This is especially true for the high-frequency thickness-shear-mode plates which at present constitute the bulk of the output of the industry, since these plates are practically always used in circuits in which this anti-resonant impedance determines directly the amplitude of oscillation. The relations between the various possible criteria of activity are investigated. The following pieces of equipment are described:Â¿ 1. A flexible laboratory instrument capable of measuring the series-resonant or anti-resonant impedance of any crystal response in the frequency range 50 kc/sÂ¿20 Mc/s. 2. A simpler laboratory instrument capable of measuring the anti-resonant impedance of any response in a restricted frequency band around selected spot frequencies. 3. An instrument designed for factory or other routine testing, in which the parallel dynamic resistance of the dominant crystal response is measured by a substitution method in an oscillator circuit. This instrument is suitable for frequencies above 3 Mc/s. 4. An instrument similar to No. 3 but slightly more complex, for the frequency range 80 kc/sÂ¿3 Mc/s.