A COMBINED OSCILLATOR, AMPLIFIER, AND POWER SUPPLY FOR CONDUCTANCE BRIDGES W. F. LUDER, E. F. McCARREN, JR., and A. A. VERNON Northeastern University, Boston, Massachusetts
INTHE past it has been customary to provide separate power supplies for the oscillator and amplifier used with bridges for measuring the conductance of electrolytic solutions.' This isolation was believed to be necessary to prevent direct pickup from oscillator t o amplifier. Also considerable separation (at least ten feet) between the oscillator and the rest of the equipment was rec~mmended.~ Wishing t o combine all our equipment into one compact setup, and also to adapt it to use miniature tubes in place of the old types, we decided to subject the oscillator, amplifier, and power supply t o a thorough investigation. Our &st combination was elaborately shielded, with the oscillator, amplifier, and power supply each completely enclosed and separated by sheet iron from each other and from the bridee. Decou~linefilters were placed in the B+ leads to both the bseiiiator and the amplifier. Although this combination was entirely successful, we suspected that it was more complicated than necessary. Accordingly, we set out t o simplify the combination LUDER, W. F , J . Am. Chem. Soc., 62,89 (1940). LUDER,W. F.,Rev. Sci. Inst., 14, 1 (1943).
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without impairing its operation. Figure 1 is a photograph of the result; Figure 2 gives the wiring diagram. In Figure 1, the bridge controls are contained in the nearer chassis; the farther one carries the combination oscillator, amplifier, and power supply. S o shielding is employed in the combination other than that provided by the aluminum chassis and by proper location
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permitting the bridge t o be employed in measuring high resistances, including the resistance of the solvent. However, with the usual components the Wagner ground cannot be balanced when a ratio of more than 10 to 1 is used. Thus, except for measuring solvent resistance to one or two significant figures, ratios higher than 10 to 1 should not be attempted. For measuring solvent resistance we usually replace the two 1000-ohm ratio arms by 10,000- and 100-ohm resistors to obtain a 100 t o 1 ratio. With measuring resistances up to 110,000 ohms this permits an approximate measurement up t o 11,000,000 ohms. Besides the use of plug-in ratio arms, and of the shielding provided by the chassis, we have found another feature desirable:' a small variable condenser across the cell leads. Its maximum capacity should be slightly higher than the minimum capacity of the large condenser across the measuring resistances. With both condensers a good resistance may be used in place of the cell to check the accuracy of the bridge. The bridge control chassis is fitted with two shielded cables about a foot long and ending in plugs for connection to the oscillator and amplifier; also with the cell and measuring resistance leads; and with a ground wire, from the point on the chassis to which the Wagner ground connection is made, over to a binding post bolted directly to the other chassis. The leads from the shields of the measuring resistances are brought to the same binding post. With this equipment the oscillator and amplifier are on the same chassis, not more than a foot apart; and the bridge controls and measuring resistances are not much more than a foot from either. Yet after much testing and three years of use we have found no direct pickup, and no errors as large as those inherent in the measuring resistances. Although the equipment is designed for research use, we are also using ten sets (with less expensive resistance decade boxes) for physical chemistry laboratory experiments. Because the apparatus is well suited to student use, we should like to recommend it to replace that described by us in an earlier article in THIS
of the parts on it. Only one decoupling filter is used: the one shown in the B+ lead of the oscillator in Figure 2. The combination is mounted on a standard 7- X 15-inch aluminum chassis. The oscillator outnut iack is located at the left front of this chassis; the am~iifier input jack is a t the right. The amplifier output jack is on the right-hand end toward the hack, close to the output transformer. Under the chassis, the 0.75 henry iron-core audio choke and the oscillator transformer must be located as far as possible from the amplifier input transformer. The choke and the oscillator transformer are at the extreme left, the input transformer a t the extreme right; they are mounted at right angles to each other. The rest of the layout, with the power supply between the oscillator and the amplifier, is shown in the two figures. The amplifier output transformer (extreme right), the power transformer, and the power chokes are mounted on top of the chassis. The oscillator is a modernized, and slightly modified, version of one we have used for many years.' Its regeneration control, fitted with a slotted shaft, is mounted behind the oscillator tube. For two reaeons this control must be adjusted very carefully. If i t is not properly adjusted the harmonic content of the 1000-cycle output will be too high,' and its voltage may he so high as to cause heating in the cell. With two 1000-ohm ratio arms in the bridge the voltage can vary with the setting of the control from less than 0.5 volt to well over 5 volts; when one of the plug-in ratio arms is replaced by a 10,000-ohm resistor the output voltage of the oscillator may be much higher than 5 volts. If a vacuum-tube-voltmeter is used, the regeneration control can be set and locked at 0.5 volt, while the bridge is operating to measure a resistance of about 1000 ohms. However, numerous checks have shown that the control may be set by ear a t a value between 0.5 and 1 volt, as follows. With the bridge operating slightly off balance t o measure a resistance of about 1000 ohms, turn the control clockwise until the 1000-cycle note is heard in the phones. Slowly turn the control back until the note just dies out; then very slowly turn it clockwise again until oscillation just begins. Occasionally, with a different load in the bridge or when the tube is changed the oscillator will stop oscillating; however, readjustment of the regeneration control as just described will start it. The two stages provided by the high-mu twin-triode amplifier permit balancing the bridge easily to 0.0170, when the gain control is fully advanced, using headphones in a fairly quiet room3 Although hum from the power supply is audible it is not loud enough to interfere with balancing the bridge. For the bridge we have used the circuit of Jones and Josephs,%ith the controls mounted on a 7- X 12-inch aluminum chassis as shown in the photograph. General Radio plug-in resistors are used for ratio arms, thus
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' LUDER,W. F., A N D A. A. VERNON,J. CHEM.EDUC.,17, 229 (1940). 0.1
Despite the application of electric eyes, and even oscilloscopes, by some users of conductance bridges, we feel that headphones give the most satisfactory results. 4 JONES,G., A'D R. C. JOSEPAS,J . Am. Chem. Soc., 50, 1049 (1928).
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