AN INEXPENSIVE VACUUM TUBE OSCILLATOR FOR STUDENT USE IN MEASURING THE CONDUCTANCE OF ELECTROLYTES D. E. HULL University of Minnesota, Minneapolis, Minnesota
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radio supply houses under the name of radiofrequency chokes. The values of the inductances chosen in this circuit are Ll = L = 60 millihenries, and Ls = 30 millihenries. (These are the catalog ratings, and not the actual values of the inductances as the coils are used in this apparatus.) The three coils are stacked one above the other on a soft iron rod abont 4 mm. in diameter; the iron bas the function of increasing the mutual inductance between the coils and bringing the frequency down into the audible range. The condenser Ceshould have a capacity of 0.25 microfarad to give a frequency of the order of 1000 cycles per second. It is essential that the right terminal from the coil L, be connected to the grid of the oscillator tube. Perhaps the simplest way to determinetle correct connection is to try bot6 possible directions. In only one direction will oscillations be produced. The third coil is provided to bring out the alternating current in a circuit insulated from the power supply so that the various parts of the Wheatstone bridge.may be safely handled while the oscillator is in operatibn. A resistance R2of abont 1000 ohms is placed in series with-this coil in order to prevent the oscillations from being choked out when small resistances are being measured in the bridge. The intensity of the sound produced by this arcuit is so great that one cannot comfortably wear a pair of OUTPUT headphone attached directly across the output. The sensitivity of the adjustment of the bridge with this sults and without any change in the circuit. (3) It is alternating current source is greatest for the measurerugged and dependable in operation. (4) It gives an ment of resistances of about 1000 ohms, where the point intensity of sound sufficient for use without any am- of balance can be located within 0.1 per cent. T h e plifier, and makes possible a high sensitivity in the range through which resistances can be measured with an accuracy of one per cent. or better extends from 10 bridge setting when used in a quiet room. The circuit is represented in the diagram. The 1223 to 10,000 ohms. If the set is to be used with direct tube serves as a rectifier and the 76 as an oscillator. current, the right-hand terminal in the diagram should The heater elements of both tubes require 0.3 ampere; be connected to the positive pole, although no damage they are connected in series with a resistance RI of would result from accidentally reversing the connec300 ohms (and 25 watts) to the 110-volt source. The tion. The total cost of the parts required to construct this positive potential attained by the cathode of the rectifier is steadied by an electrolytic condenser C, of 25 oscillator is less than five dollars a t c u r r e ~ tmarket microfarads. The induction coils are obtained from prices.
HIS paper describes a source of alternating current for students' use in measurements of conductance whicb combines certain advantages not possessed by circuits heretofore published. The circuit shown below has the following characteristics. (1) It is easily constructed from a few standard radio parts which are inexpensive and readily obtained. (2) It operates without the use of any batteries, from an ordinary 110-volt line. Either 'alternating. or direct current may be used with equally good re-
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