JOURNAL OF CHEMICAL EDUCATION
A SIMPLE 1000-CYCLE OSCILLATOR ALEXANDER P. MARION Queens College, Flushing, New York
ANELECTRONIC
1000-cycle oscillator characterized by a minimum of parts, good frequency stability and wave form, and sufficient output for most measurements should be a useful instrument in the physical chemistry laboratory. The circuit diagram and description for such an oscillator are given below. The simplicity and lack of critical adjustments have been made a t the expense of power output vhich still remains sufficient, however, to permit reasonably accurate determination of resistance or of the conductance of a solution by means of a Kohlrausch slide \vire bridge or student type potentiometer. A single, octal-base tube of the dual-triode type, a 6 SN 7 GT, was employed as a cathode-coupled oscillator. Since satisfactory operation was obtained a t low plate potentials, a six-volt storage battery was used for both filament and plate power, thereby making the unit independent of the a.-c. line. None of the components is unusually expensive, so
Oufpu'
/+
-yfJ---o
6v RI = 0.1 megohm, 1 watt
R2 = 3000 ohm, 1 watt L, = 4 henry CI
= 0.1 mfd. C2 = 0.01 mfd.
that the apparatus can be constructed from radio parts of ordinary commercial tolerances. The frequency is dependent upon the LlC2circuit, but in this frequency range the capacitance can be varied by a factor of about ten and oscillation will be sustained. Consequently, an inductance approximately equal to that indicated will serve, although if exactly 1000 cycles per second is desired, it may be necessary t o build up a different value of CZfrom small paper or mica condensers. Too low a value of cathode resistor Rzresults in poor wave form while the output drops off with increasingly high values of this resistor. A compromise value is 3000 ohms, alt,hough greater output may be realized with some other resistance if an oscilloscope is used for checking. It.can be seen that the external circuit is in series with Cz so that if any appreciable capacitance is introduced between the output termina1s;the frequency of oscillation will chame. Loading als? affects the frequency. In an attempt to minimize these factors the primary of a transformer was used as the inductance and the load connected to the secondary. (Actually one-half of the primary of a Thordarson type 54S94 transformer was shunted by a 0.007-mfd. condenser and the external circuit was connected between the 0 to 15-ohm terminals.) Some improvement was noted but not enough to warrant adoption. The wave form was very good when viewed on an oscilloscope, although some distortion was introduced by the iron-core choke. The h-equency stability proved to be excellent, for no drift was evident during several hours of operation. Since the oscillator does not produce an air signal, the null point is more easily detected, for the-disturbing background noise of the microphone hummer is absent. Determinations were made of noniuductive resistances by use of a bridge circuit with good accuracy. Conductance measurements of solutions of potassium chloride, hydrochloric acid, and acetic acid a t several dilutions gave goodresults. Apparently the capacitance of the conductivity eel! is negligible for no change in frequency could be noted. The assistance and criticism of Robert Williamson of the Physics Department of Queens College is gratefully acknowledged.
