REPORT FOR A N A L Y T I C A L CHEMISTS
MICROMETER SPINDLE
Figure 5. A practical example of the Hartley oscillator circuit in a form intended for dielectric constant measurements [Axtmann, R. C , ANAL. CHEM. 2 4 , 783 ( 1 9 5 2 ) ] . Units of capacitances are picofarads
be shown below, the phase-shifted feedback grid voltage is produced only in response to a changing plate voltage. Hence, the feedback signal starts to decay; this causes the grid potential to begin to be less positive t h a n its peak value. This negative-going effect on t h e grid causes the tube current to decrease; the decrease in tube current causes the plate voltage to rise; the positive-going effect at the plate is shifted in phase and fed back to the grid as a negative-going effect; this further decreases the t u b e current, etc. A regenerative chain reaction is once more produced, but this time it causes the tube current to fall to a minimum value. When the tube current is a t its "minimum value, and no longer decreasing, the negative potential of the grid due to the feedback begins to decay away. This is equivalent to a positive-going effect and a new regeneration sets in, producing another current peak. T h u s , t h e tube current oscillates continuously
from a maximum to a minimum value. T h e frequency of this oscillation in t u b e current is determined by the time required for the regeneration to cause the current to build up to its peak and to decrease to its minim u m value. T h i s t i m e is determined by the capacitances, inductances, and resistances present in the circuit, and their configurations relative to each other. For any given oscillator circuit, these p a r a m e t e r s combine to produce a definite time constant, and the oscillations therefore have a definite frequency, called the resonant frequency. Forms of V a c u u m Tube Oscillators
T h e various types of triode oscillators used in chemical instrumentation differ only with respect to the methods employed to feed a portion of the plate output back to the grid. Tickler Oscillator. Figure 3
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VOL. 34, NO. 8, JULY 1 962
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