The cathode follower technique for obtaining the guard potential can be compared ... the parallel combination of 0.5 ~F and 10 Mn. We have, of course, arranged ...
HighImpedance Guarded Voltmeter Don Latham Citation: Rev. Sci. Instrum. 42, 535 (1971); doi: 10.1063/1.1685158 View online: http://dx.doi.org/10.1063/1.1685158 View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v42/i4 Published by the AIP Publishing LLC.
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NOTES 2. 10
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a copper tube for tube T 2 the same sample holder has been used by us for ionic thermocurrentlO measurements. The authors wish to thank Dr. Ajit Ram Verma for encouragement and keen interest in this work.
4
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R. G. Breckenridge, J. Chern. Phys. 16, 959 (1948). J. S. Cook and J. S. Dryden, Australian J. Phys. 13,3260 (1960). 8 S. C. Jain and K. Lal, Proc. Phys. Soc. (London) 92, 990 (1967); Crystal Lattice Defects 1, 165 (1970); J. Phys. C. [Proc. Phys. Soc., (London)] 3, L39 (1970). 4 K. Lal, PhD. thesis, University of Delhi, Delhi, 1969. • S. C. Jain, K. Lal, and V. Mitra, J. Phys. C. [Proc. Phys. Soc. (London)] (to be published). 6 K. Lal and D. R. Pahwa (to be published). 7 F. Jona and G. Shirane, Ferroelectric Crystals (Pergamon, London, 1962). 8 R. W. Dreyfus and A. S. Nowick, Phys. Rev. 126, 1367 (1962). 9 J. H. Beaumont and P. W. M. Jacobs, J. Chern. Phys. 45, 1496 (1966). 10 C. Bucci, R. Cappelletti, R. Fieschi, G. Guidi, and L. Pirola, Nuovo Cimento Suppl. 4, 607 (1966). 1
10
2
2
FREOUENC'(
(c/au)
High-Impedance Guarded Voltmeter*
FIG. 2. Loss tangent vs frequency isothermal for an NaCI crystal do~ed with 15 ppm of cobalt. The measurements were made at 90 C.
spacer Ts is fixed to the top Tp with an epoxy resin to make the connection vacuum tight. Epoxy resin is also used to make the junction of the upper end of rod R3 and the Teflon spacer T. vacuum tight. A suitable coaxial connector can be used to connect the electrodes to a bridge or other measuring system. A thermocouple Th is connected to the lower electrode L to measure the temperature of the specimen. The thermocouple leads are taken out through a Teflon spacer T's fixed to the top Tp with an epoxy resin. A small furnace F is wound around the tube T2 to heat the specimen. Water circulation copper coils C are wound around T 2 to cool the O-ring and also the top Tp. Before making the measurements, the electrode assembly is taken out by lifting the tube T 1 • The specimen having conducting electrodes (e.g., Aquadag) on both surfaces is placed on the lower electrode L. The upper electrode is screwed down to form a good electrical contact with the upper surface of the specimen. After the specimen is placed between the electrodes, the tubes T 1 and T 2 are assembled and measurements are taken. The sample holder has been used for studying the dielectric loss in uncolored and colored pure and doped alkali halide crystals.3- 6 Figure 2 shows a typical tano vs frequency plot for an NaCI crystal containing 15 ppm of Co. The measurements were made at 90°C. The crystal was kept in a vacuum of "-' 10-3 Torr. This arrangement has also been successfully used for electrical conductivity measurements in vacuum or under inert gas atmosphere up to 700°C.8,4,6 In fact the temperature can be increased up to 1000°C. The frequency of measurement can be varied from a few hertz to a few megahertz. By choosing
DON LATHAM
University of Miami, School of Marine and Atmospheric Science, Division of Atmospheric Science,. Coral Gables, Florida 33124 (Received 2 November 1970)
A
PROGRAM for conducting measurements of atmospheric electrical parameters at the air-sea interface required a low power drain, large dynamic range, guarded voltmeter with an extremely high input impedance. Previous guarded instruments of this typel - 4 use electron tubes in essentially a cathode follower configuration to develop the necessary guard potential, or a motor driven potentiometer servosystem. These devices, however, demand considerable power and were deemed unsuitable for this application. The cathode follower technique for obtaining the guard potential can be compared to the use of a series regulator. Power is always dissipated in the regulating element, and if the source voltage is large, insuring large span, and the nominal operating point is a small fraction of the span (typically 10%), the series regulator is evidently a poor choice from an efficiency standpoint. Far greater efficiency can be obtained through use of a switching regulator. This device delivers pulses of the unregulated voltage to a smoothing circuit, usually a second order filter, although a first order filter may be used at reduced efficiency. The action of the switching regulator may be represented in block form as a "bangbang" servomechanism (see Fig. 1, top). Our application of this concept is shown in Fig. 1, bottom. The potential at a point in the atmosphere is matched to a solid state electrometer amplifier with a 50 ~Ci polonium probe [we will not discuss this matching technique here (see Ref. 1)]. The amplifier has an error
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536
NOTES TABLE
1. Guarded voltmeter characteristics.
Span
TMlon Insulator
RL2
Guard, ±900 V. Output, ±9 V Dynamic range Depends on readout device; with 10-bit AID is 53 dB. Hysteresis ±4 V (frequency dependent). Linearity Better than 0.1% full scale at recorder output Frequency response dc to 0.1 cps (3 dB) at recorder output Input impedance > 1014 fl shunted by 3 pF Input offset current
