cm. of crystalline material above adhered to the sample tube so well that the expansion during the melting of the new zone produced high pressures. The problem did not occur with the straight-chain hydrocarbons, and may or may not be serious in a given case depending on the substance and the dimensions of the sample tube. Joncich and Bailey (2) have solved this problem by using polytetrafluoroethylene sample tubes. Such tubes would be unsuitable for the present apparatus because rigidity is required for proper rotation, dependable functioning -of the shaft seals, and protection of the final crystal from deformation. The problem was eliminated by the arrangement shown in Figure 2. A tapered glass plug, sliding loosely and supported by a spring, forms the end of the sample space. The molten zone initially extends down to the point where the annular space between the
tube and the plug attains its minimum width, but not far beyond. If the sample requires added space for each new zone, the plug moves downward gradually and the spring is compressed. The initiation of crystal growth in the narrow annular space may encourage formation of a single crystal, as does the pointed tip frequently used on sample containers in the Hridgman method. To avoid excessive pressures no rapid change in length of the molten zone, such as that which occurs on restarting the apparatus after a power failure, should be permitted except a t the end of the traverse. If the solid adhers to the sample tube, the multiple-zone oscillating-heater technique of Sloan and McGowan (4) could in principle be used with a fluid carrier of many compartments. Such an arrangement would present mechanical problems because of the large total frictional torque of all the shaft
seals. If the solid does not adhere well, the body of solid between two liquid zones might tend to rise or sink and disrupt the continuity of the process. In this case a multiplicity of zones would be undesirable. ACKNOWLEDGMENT
The author thanks Harold Johnson for his loyal assistance throughout the development and testing of this apparatus and C. P. Saylor for helpful discussion. LITERATURE CITED
(1) Baum, F. J., Rev. Sci. Znstr. 30, 1064 (1959). ( 2 ) Joncich, M. L., Bailey, D. R., ANAL. CHEM.32, 1578 (1960). (3) Lawson, W. D., Nielsen, S., "Preparation of Single Crystals," Butterworths, London, 1958. (4) Sloan, G. J., NcGowan, N. H., Rev. Sci. Znstr. 34,60 (1963).
High Precision Conductivity Bridge Robert L. Wershaw and Marvin C. Goldberg, U. S. Geological Survey, Denver, Colo. 80225
r
N THE DEVELOPMENT of methods for
the measurement of solubilities of pesticides in water, a conductivity bridge of very high precision for measuring small differences in conductivity was required. Commercially available instruments could not meet all of the required specifications. Therefore, the bridge which is described below was designed and built in our laboratory. A schematic diagram of the bridge circuit is given in Figure 1. The ratio arms of the bridge are formed by a variable tap autotransformer (ESI Model IlT72.4 or Gersch 1000 Series Inductive Divider). A standard cardwound resistor of low capacitance and inductance, and a Jones conductivity cell make up the comparison arms of the bridge circuit. The two leads from the oscillator are connected directly to the comparison arms of the bridge. The resistances of the leads to the inductive divider are, therefore, effectively eliminated from the comparison arms of the bridge and are incorporated into the ratio arms of the bridge, where the resistance of the leads is negligible compared with the input impedance of the divider (approximately 1,000,000 ohms at 1000 cycles/second). An appreciable error is eliminated by this type circuit, for it was found that errors of the order of 50-100 p.p.m. in ratio measurements resulted from having the oscillator connections at binding posts '1 '1M)
ANALYTICAL CHEMISTRY
of the divider. A complete discussion of four terminal resistance measurements is given by Thomas ( 2 ) . The quadrature correction is made with two adjustable air capacitors connected across the over-windings of the autotransformer.
This conductivity bridge offers the following advantages over the Jones Bridge ( I ) which has been used almost universally up to this time for high precision conductivity measurements : the measurements are made using a fourterminal resistance measuring circuit
CONSTANT TEMPERATURE
r - 1 2
Figure 1 .
Circuit diagram of conductivity bridge
BATH
rather than a two-terminal one as used in the Jones 13ridgeJ therefore, minimizing lead resistance errors; the two comparison arms of the bridge are immersed in a constant temperature bath to eliminate heating and temperature errori; this bridge is capable of greater llrecihion than the Jones I3ridge because of the high resolution ( 0 . 0 0 0 0 2 ~ ~of) the inductive divider used in i t ; the circuit may be readily modified to make high precision capacitance and inductance measurements: and it is considerably less expensive than a Jones Bridge. EXPERIMENTAL
I n operation, a resistor of similar value to the resistance of the cell is placed in the circuit between points d and B (Figure 1) and the cell is connected between points C and D . Leads
from the oscillator and leads from the divider are connected directly to the points A and D . A balance is obtained by adjusting the divider and the capacitors for minimum response of the null detector. I n most of our studies the oscillator frequency was 1000 cycles/ second; however, any frequency from 50 cyclesjsecond to 10,000 cycles/'second may be used. For measurements of the highest precision, the resistance of the lead between B and C must be known. This lead resistance may be evaluated by replacing the conductivity cell with a resistor of known value; the circuit is balanced with the detector connected to the point B ; the detector lead is then moved to the point C and the bridge is balanced again. The difference in the two ratio measurements multiplied by the total resistance between A and D gives the resistance of the link between B and C. T o verify the accuracy of this bridge,
a calibrated resistor was substituted for the conductivity cell. Agreement between values of the resistor ratios measured at the Xational Bureau of Standards Laboratory and those measured in our laboratory was *0.2 p.p.m, ACKNOWLEDGMENT
The authors acknowledge the cooperation and guidance of Thomas L. Zapf and Patrick H. Lowrie, Jr., of the National Bureau of Standards, and Claude R. Daum and Garth Ghering of the U. S. Geological Survey. LITERATURE CITED
(1) Jones, G., Josephs, R. C., J . Am.
Chem. SOC.50, 1949 (1928). (2) Thomas, J. L., KBS Circular 470 (1948). PUBLICATIOK authorized by the Director, U. S. Geological Survey, Washington, D. C.
A Time Base Generator with Digital Readout of Nuclear Magnetic Resonance Line Positions J. M. Purcell and J. A. Connelly, Eastern Utilization Research and Development Division, U. S. Department of Agriculture, Philadelphia, Pa. 191 18 a n X-Y recorder as a T readout device with Varian HR-60 high resolution nuclear magnetic resHE USE OF
onance (NMR) spectrometers is well established. I n this application the X-axis is driven by an external time base. Spectral calibration is normally accomplished by the sideband technique ( I , 8 ) . Line positions are measured by first obtaining a scale factor between the frequency scale and linear distance. The linear distance is measured from each N M R line to the reference line. The value of the linear distance multiplied by the scale factor yields the line position in terms of frequency (cycles per second), Several disadvantages are inherent in this method of measuring N M R line positions, not the least of which is the amount of time involved. The chief characteristic which affects the accuracy of the NMR line positions is the linearity of the external time base and! or of the recorder. Commercial recorders generally possess linearity of the order of 0.1 to 0.2% of full scale. If the linearity of the external time base is greater than the recorder linearity (considering only the readout device), the accuracy of the line positions is limited only by the linearity of the recorder. The inherent nonlinearity of the slidewire accounts at least in part for the nonlinearity of the recorder. The device described herein provides a more accurate and convenient method for measuring N M R line positions
through the use of an operational amplifier in two modes of operation. The accuracy of N M R line position measurements can be increased in two ways: improving time base linearity and avoiding slidewire nonlinearity. Consider first time base linearity which may be defined as constancy in the time rate of change of a ramp voltage. An excellent method for the production of a linear ramp voltage is the integration of a constant voltage. Assuming in principle that this method will yield a precisely linear ramp voltage, one can see that in pract,ice the linearity of the time base depends upon the specifications of the electronic devices used. We have designed a time base generator of improved linearity using a solid state voltage source with a low temperature coefficient and a high gain, chopperstabilized operational amplifier. Consider now the inherent nonlinearity of recorder slidewires. S o n linearity is the result of the fact that the resistance of t'he wire per unit, distance is not constant. Therefore, when one attempts to correlate voltage with linear distance, using another scale which possesses a linearity different from that of the slidewire, the result is a certain unavoidable error. This error can be avoided by driving the, pen through the same distance along the same portion of slidewire by means of an accurate voltage source and then correlat'ing that voltage with distance. The necessit.y of measuring distance on
the chart paper is thereby eliminated, since for a given recorder range the voltage necessary to reach a given point on the slidewire is now known. This voltage can be correlated with the quantity plotted along the abscissa. However, to achieve the greatest possible accuracy in measuring line positions the measurement should be made before the chart has been removed from the recorder. By employing this principle we have devised a method which provides a fast digital readout of N M R line positions in terms of frequency. EXPERIMENTAL
The device consists of a combination of two modes of operation of an operational amplifier (Dymec Model DY2460A-M1 with Model DY-2461A-bI5 blank plug-in; 2-p.p.m. d.c. gain accuracy including linearity)-i.e., integration of a constant voltage and +1 gain amplification. A schematic representation is given in Figure 1. A photographic view of the time base generator constructed in this latoratory is given in Figure 2. For the integration mode the time constant is 100 seconds. Since stability of the time constant is important, a 10-megohm metal film resistor and a 10-pf. polystyrene capacitor were used. Proper connections for the Dymec 2460A operational amplifier can be found in the Dymec instruction manual. Ramp voltage linearity depends upon the voltage source stability. T o proVOL. 37, NO. 9, AUGUST 1965
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