FROM THE CURRENT LITERATURE

DYNAMIC ELECTROMETER*)

If one does not count certain detectors of a special type used in work with ion beams, then for the measurement of very small currents or small charges at the present time electrometers of two types are used—mechanical and tube electrometers. Among the merits of mechanical electrometers should be counted their high stability, slight susceptibility to the influence of external fields, and unsurpassed sensitivity (for example, that of the Hoffmann electrometer). Their shortcomings are complexity and, in stationary installations, the need for special measures to ensure complete mechanical rest. Tube electrometers, in contrast, are instruments mechanically quite “rough.”

Their specific shortcomings, both when special tubes are used and, to an even greater degree, when ordinary tubes are used in the “electrometer” regime, consist in instability of the zero and in a strong susceptibility to the influence of external electromagnetic fields.

Instability of the zero is the most unpleasant factor, greatly hindering work with the instrument. If in a mechanical electrometer, where this phenomenon, of course, also occurs, the shift of zero is due mainly to two factors—the change with time of the contact potential difference and the presence of elastic stresses in the suspension—then in a tube electrometer there are immeasurably more such factors. Apart from instability of the power sources, these are chiefly grid currents of various origin[^1], nonconstancy of the cathode emission, and also the influence of changes in contact potentials. The drifting of the zero makes operation of tube electrometers insufficiently stable long before the level of fluctuation noise is reached.

A third type of electrometer has been proposed—the so-called “dynamic capacitor electrometers,” which, while making use of the advantages of tube amplification of weak currents, at the same time do not have the basic shortcomings of ordinary tube electrometers. Altogether several attempts to construct such instruments are known; two of them date from 1932, and one from 1941[^2]. These models, however, were made insufficiently perfect and by far did not realize the possibilities of instruments of this type. Therefore of interest is the description of two new designs of a “dynamic electrometer”[^3] contained in the papers reviewed. The principle of operation of the “dynamic capacitor electrometer,” the general scheme of which is shown in the figure, consists in the fact that the measured constant or slowly varying charge supplied to the instrument is converted into an alternating emf by means of a capacitor \(C_e\) with periodically varying capacitance. All further amplification proceeds as amplification of alternating

) H. Palevsky, R. Swank and R. Grenchik, R.S.I. 18, 298 (1947); S. Scherbatskoy, T. Gilmartin and G. Swift, R.S.I. 18*, 415 (1947).

DYNAMIC ELECTROMETER

currents. This ensures: 1) elimination of the influence, on stability, of all factors connected with the power supplies and tubes; 2) high stability of the amplification factor of the tube circuit; 3) reduction of the level of fluctuation noise owing to a substantial narrowing of the pass band and the choice of the most favorable operating frequency; 4) provision of a high active input resistance of the instrument without the use of special electrometer tubes; 5) weakening of the shielding requirements, which here do not go beyond the shielding requirements for a normal low-frequency amplifier; in particular, the need for special shielding of the supply batteries, which is obligatory in the usual circuit of a tube electrometer, is eliminated; 6) thanks to the latter circumstance, the portability of the instrument is increased and a considerably greater flexibility of the construction is achieved; 7) finally, the instrument becomes considerably less sensitive to mechanical shocks, in particular as a result of the weakening of the influence of the microphonic effect of the tubes.

The most important part of the instrument is the “dynamic capacitor,” with periodically varying capacitance. In one of the early works mentioned it was made in the form of a cylindrical capacitor with a rotating electromotor drive. The simplest and most expedient is the vibrational type of dynamic capacitor, in which the movable plate oscillates relative to the fixed plate with a frequency equal to its own frequency. The oscillations are excited by an electromagnet, fed from the a.c. mains or from a special tube generator of the required frequency. The movable plate is made either in the form of a steel reed, or—in the most portable model—in the form of a round thin steel diaphragm (diameter 50 mm, thickness 0.18 mm). The basic properties of the instrument—stability and, to a considerable degree, sensitivity—depend on the quality of the dynamic capacitor.

The sensitivity is determined by the ratio of signal to noise at the output of the instrument. Leaving aside certain additional causes of noise, such as the microphonic effect, the sensitivity of the tube electrometer must be regarded as limited by the level of fluctuation noise—the thermal fluctuations at the input and the shot effect in the first amplifier tube. With a sufficiently high operating frequency of the dynamic electrometer (the frequency of vibration of the capacitor) and with a sufficiently large time constant for the noise, the shot effect can be made small in comparison with the thermal fluctuations. In this case the permissible grid current of the first tube may be three orders of magnitude higher than the maximum grid current permissible for electrometer tubes ($10^{-12}\ a$ instead of $10^{-15}\ a$). The sensitivity limit of the electrometer, due only to thermal fluctuations, is then determined by the well-known expression:

\[ Q_0 = \sqrt{ul'C_e} = 450e \cdot \sqrt{C_e}, \]

where $e$ is the electron charge, $C_e$ is the mean capacitance of the dynamic capacitor in $\mu\mu F$, and the pass band of the amplifier is taken to be infinite. The smallest capacitance $C_e$ in one of the instruments described was equal to

25 μμF, which gives for \(Q_0\) the value \(2250e\), or \(3.6 \cdot 10^{-16}\) coulomb. This corresponds to the sensitivity of the Hoffmann electrometer (see, for example, \({}^3\)). In practice, such sensitivity was not achieved in any of the manufactured specimens of the electrometer. In designing it, according to the authors, the aim was not so much to obtain record parameters as to create instruments that were sufficiently sensitive, but at the same time simple and not too “delicate.” As a result, the instruments described in the article had an average noise level of the order of 50 μV, or \(Q_0 \simeq 1 \div 2 \cdot 10^{-15}\) coulomb. The time constant ranged from 0.2 to 5 seconds.

The next very important characteristic of an electrometer is the stability of the zero. An instrument may have very high sensitivity, but if during measurement its zero does not remain fixed, this sensitivity cannot be used.

In an electrometer of the type described, the principal source of zero instability is the contact potential difference at the input. If this quantity remained constant, it would produce a certain zero charge on the plates of the dynamic capacitor. Therefore any changes in the contact potential difference entail a “drift” of the zero with the input short-circuited. With the input open, the ionization current is an additional source of drift, arising from ionization of the air in the space between the plates (the cause being penetrating radiation or accidental radioactive contamination in the material of the capacitor). The field of the contact potential difference deposits ions on the capacitor plates, producing a current between them. Finally, mechanical or electrical residual stresses in the insulators may also serve as sources of small currents causing zero displacement. The design and technology of the dynamic capacitor were carefully worked out with a view to combating all these causes. The magnitude and stability of the contact potential difference depend strongly on the condition of the contacting surfaces. Mechanical damage to the surface, scratches, or roughness on it are inadmissible. As the authors point out, the influence of such factors as, for example, small islands of contamination on an already cleaned surface of the capacitor leads to distortion of the waveform of the generated emf and to the appearance of peaks on it. The working surfaces of the dynamic capacitor were treated as follows: after polishing to a mirror finish and washing with alcohol, the steel plates were immediately immersed in an electrolytic bath, and a layer of gold \(0.025—0.050\) mm thick was deposited on them by double deposition. After washing and drying, a second layer of gold was applied by cathodic sputtering. As quickly as possible after this, both plates of the capacitor were mounted in their place in the unit, and, in order to prevent oxidation and accidental deposition, the assembly was carried out in a stream of nitrogen. After the assembly was completed, air was pumped out of the hermetic casing in which the capacitor was placed, and the space was filled with argon at atmospheric pressure. The walls of the casing and all the metallic parts located inside it were also carefully cleaned, and in some cases gold-plated.

As a result of this procedure, the contact potential difference between the plates could be reduced to a value not exceeding 20 millivolts. The constant value of this difference was not established immediately, and its “drift” one week after assembly of the capacitor still amounted to 0.1 millivolt per day.

The remaining part of the dynamic-electrometer circuit consists of a low-frequency amplifier (designated in the figure as \(Y\)), an oscillation generator feeding the electromagnet (if the mains is not used directly), and a detector with one or another output indicator. A characteristic feature of the designs cited in the articles under review is the use-

application in amplifiers of a large negative-feedback coefficient in DC. As is known, this makes the amplifier insensitive either to considerable changes in the EMF of the power-supply sources or to replacement of tubes, and thus both these factors cease to affect the sensitivity and stability of the dynamic electrometer.

In order for the electrometer to give not only the magnitude but also the sign of the charge being measured, a phase-sensitive detector is placed at the output of the low-frequency amplifier. In the most portable model, where a microammeter is used as the indicator, the phase-sensitive detector is a pentagrid tube, to one of whose control grids the amplified EMF of the “signal” is applied, and to the other an oscillation from the generator feeding the electromagnet; thus the phase of the signal EMF is taken into account. In some models, a directly recording instrument is placed at the output. In this case, a two-phase selsyn motor is used as the phase-sensitive detector. The figure schematically shows a model of the first type. \(R_f\) is the feedback resistance, \(C_c\) the coupling capacitor, and \(R_0\) the decoupling resistance, necessary so that, with the charge source connected, the charge on the dynamic capacitor does not change when the capacitance oscillates. \(C_c\) and \(R_0\) are mounted in the casing of the dynamic capacitor.

Electrometers of the type considered have found application in work with ionization chambers. In particular, they have been used to measure the radioactivity of rocks directly in boreholes.\(^4\) An electrometer with a chamber can be lowered into a borehole to a great depth, while continuous automatic recording is carried out on the surface of the earth.

It seems to us that the idea of a “dynamic capacitor electrometer” deserves every attention.

B. Bagaryatskii

CITED LITERATURE

1. Tsarev, UFN, 35, issue 2, p. 251 (1948).
2. R. Gunn, Phys. Rev., 40, 307 (1932); Kirkpatrick, RSI, 3, 430 (1932); H. Le Caine and J. Waghorne, Can. J. Research, 19, 21 (1941).
3. B. Zipprich, Phys. Zeits., 37, 36 (1936).
4. Scherbatskoy and Fearon, Phys. Rev., 70, 90 (1946).