Earthed Electrode Research Articles

Boiling heat transfer and the electric breakdown of fluorocarbon C2Cl3F3 and C8F16O under a uniform electric field.

The effect of a uniform, alternating current voltage on boiling heat transfer and an electric breakdown under boiling conditions was studied for dielectric liquids C2Cl3F3 and C8F16O, using horizontal and parallel electrodes (the earthed electrode was used as a heating surface) with a small gap. An increase in electrical field intensity increased the nucleation site density and decreased the bubble diameter at departure. In particular, the boiling heat transfer coefficient of the C2Cl3F3 had a maximum valve above 5.0 kV/mm at a gap length of 3.0 mm in terms of the pumping action of the electric field. The electric breakdown voltage decreased remarkably when the bubble was produced from the heating surface, but it did not depend on the heat flux ; and also, it was found to have a similar tendency to Paschen's Law.

Observation of corona phenomena at very high direct voltages

Corona currents of negative polarity flowing from a variety of earthed electrodes of small radii of curvature have been measured up to direct voltages of 1000 kV. The upper, high-voltage electrode has been either a sphere of 2 m diameter or a plane of 130 cm diameter (with curved edges). Over a current range extending from 0.5 μA to 2500 μA the current rises very nearly according to the fairly well established equation i = A(V−V0)V where V0 is the threshold voltage for corona development, an equation applicable to corona in small gaps. Gaps of 25 cm up to 4 m have been studied. The corona is emitted in a series of pulses, generally known as Trichel pulses (1938), each probably lasting less than 1 μsec; the number rises almost linearly with current at least up to a rate of 1 pulse per μs, the maximum rate recorded at 600 kV across a 2.5 m gap. Their amplitude diminishes slightly with increasing voltage, falling from 600 kV to about half their value at 60 kV. Corona currents from earthed points 4 m below a 500 kV DC transmission line can amount to several microamperes.

Effective surface charge densities on two surfaces of polystyrene films charged by friction and corona discharge and their polarity

Effective surface charge densities (σ ∗) were measured for both surfaces of polystyrene (PS) films charged by friction with nylon 6 (Ny) films and corona discharge. In the films charged by friction, the polarity of σ ∗ on charged (S1) and uncharged (S2) surfaces was negative. The absolute value of σ ∗ on S1 was larger than that on S2. After rinsing the charged films with ethyl alcohol, S1 carried negative charge but S2 positive charge. The series of frictional electrification predicts the polarity of charge on PS rubbed by Ny before rinsing. However, it should be noted that the films were polarized distinctly with a small net charge. In temperature dependence of σ ∗, a peak was found at about 40°C and an increase of σ ∗ was noticed in the higher temperature region up to 100°C. They were explained by thermal motion of PS molecules. In corona electrification, the polarity of σ ∗ on both surfaces changed when the distance between the film and an earthed electrode was varied: when negative corona was emitted to the film put directly on the earthed electrode S1 carried negative charge but S2 positive charge. When the film was suspended 3.7 mm above the electrode, S1 and S2 carried negative charges. Charge decay was measured for the films charged by friction and corona discharge and analyzed using a three-exponential-sum equation to obtain relaxation times. Two mechanisms of charge decay were considered: one caused by surface charges and the other by charges injected into surface layer. Thermally stimulated depolarization current was also investigated. All the data showed that the behaviour of charges on S1 was different from that on S2.

An investigation into the conductivity of hydrocarbon liquids

Hydrocarbon liquids have been charged by passage through micro-pore filters. The subsequent conductivity of the charged liquid was measured as a function of the charge density imparted to the liquid, and relaxation rates were also studied. It is shown that the return to an equilibrium charge carrier concentration can be described by the discharge rate of charge carriers at earthed electrodes, and charge carrier regeneration. Differences in the behaviour of gas oil and kerosine can plausibly be explained by differences in charge carrier regeneration rates.

The Effect of Particle Precharging on the Performance of a Fabric Filter Collecting Lead Smelter Dust

Laboratory and field tests have shown that electrostatic precharging can lead to a substantial decrease in the flow resistance of fabric filters. For lead smelter dust, the effectiveness of the precharger is dependent on the relative humidity of the suspending gas. For relative humidities below 5%, severe back corona occurs in the precharger, which results in poor particle charging and little improvement in filter performance. For relative humidities between 5% and 40%, little or no back corona occurs, precharging is effective and results in a reduction of up to 60% in the flow resistance of the fabric filter. The operational range of the precharger can be extended to lower relative humidities by cooling the earthed electrode. The improvement in filter performance can be achieved without the precipitation of large quantities of dust in the precharger.

Characteristics of propagating electrostatic discharges on dielectric films

Spark discharges that occur as a spherical, earthed electrode approaches the charged surface of an insulating sheet lying on an earthed base change in character as the initial surface charge density increases. Above a certain charge level the discharge no longer remains localised in the region directly opposite the electrode, but propagates vigorously over large areas. The threshold charge density for such propagation was found to vary with the film thickness. The criterion governing the threshold behaviour is discussed from a mechanistic viewpoint.

Incendivity of sparks from surfaces of electrostatically charged liquids

Hydrocarbon liquids prone to electrostatic charging may accumulate a considerable charge in the liquid of a tank being filled eventually leading to a spark in the possibly flammable atmosphere of the tank. As an important step in the assessment of the hazard, this paper studies the discharges between an earthed metal electrode and the surface of a charged liquid in particular with respect to the incendivity of the occurring sparks in a propane/air atmosphere which may be taken as representative of the tank atmosphere containing hydrocarbon vapour. Using a variety of spherical electrodes with diameters ranging from 10 mm to 40 mm as well as ellipsoidal electrodes of 50/25 mm and 30/15 mm dia. ignition could be obtained when the charge transfer in the sparks was raised to approximately the same level of the order of 0.1 μC. To achieve this it was necessary to charge the liquid surface to a potential of 58 kV for spherical and 62 kV for ellipsoidal electrodes. Based on these results and making some allowance for a safety margin it is advocated that a surface potential exceeding ca. 40 kV should be regarded as potentially hazardous in tank filling operations.

The ignition of vapour and droplets by liquid-to-metal sparks

The flow of hydrocarbon product can generate a charge in the liquid sufficient to lead eventually to a spark in the ullage space of a tank being filled. Depending on the volatility of the product, the ullage space may contain an inflammable mixture with air in which the fuel ranges from wholly vapour to almost wholly liquid droplets. As an aid to the assessment of hazard in fuel handling operations, this paper describes the discharges between earthed metal electrodes and a charged product surface that have ignited a small number of such mixtures. When all the fuel was in vapour form a spark charge transfer of 0.1 μC caused ignition and this could be drawn by a partially immersed, 8-mm-diameter rod or a 50 mm × 25 mm-diameter ellipsoidal electrode suspended 15 mm above a surface 15 charged to 60 kV. At 65% of the fuel as droplets, a charge transfer of 1 μC was incendive and at 95% as droplets, 2 μC caused ignition.

Role of gas breakdown in the charging and discharging of macron clouds

When a pile of dust is dispersed by a blast of air, a charged cloud is formed. The action of gas breakdown due to intense space-charge fields generated on dispersal is shown to explain a surprising invariance in the net charge of the cloud with flow velocity, particle size and material. When the cloud subsequently discharges to a small earthed electrode, the associated energy is of the order of that required to initiate a chemical explosion (this may be of significance in supertankers, where discharges from a charged mist are suspected to be a possible cause of explosions). The collision of particles in an electric field is shown to be an effective charging mechanism during dispersal, and later in the creation of a dust 'plasma'. Electrical oscillations of approximately the calculated ‘plasma’ frequency (103−104 Hz) are observed.

RF biasing through capacitive collector to target coupling in RF diode sputtering

An RF sputtering system which is suitable for deposition of both conductive and nonconductive thin films by bias sputtering is described. The substrate bias is obtained by capacitive collector to target coupling, the capacitance of the collector dark space and an external capacitor being used for voltage division. The influence of target voltage and sputtering pressure on bias generation is considered. When using RF discharge no electrode is in direct electric contact with the plasma, and a systematic error is introduced by measuring the bias voltage as collector DC voltage relative to the target's earthed counter electrode, which error is discussed. Data are presented on the temperature coefficient of resistance in bias-sputtered nickel films showing similar dependence on target voltage and on bias voltage.

An investigation of the behaviour of drops and drop-pairs subjected to strong electrical forces

A numerical model is described which simulates irrotational, incompressible flow on a computer. It has been applied to the problems of the deformation of uncharged drop-pairs separated in an electric field of critical strength and isolated drops charged to the Rayleigh limit in the absence of an electric field. In the case of pairs of drops of radius R , separated by an initial distance X in an electric field equal to that predicted by Brazier-Smith (1971) to cause disruption two types of interaction were identified. For values of X/R less than about 1.2 the drops deform and their surfaces accelerate towards each other and make contact. For X/R greater than about 1.2 the drops deform, a concavity appears at the near poles and then the near surfaces assume a conical profile of angle equal to that predicted by Taylor (1964). The subsequent issuance of liquid from these jets could not be studied with the present model. The computations predict that an isolated drop carrying the critical charge calculated by Rayleigh (1882) will deform, while retaining an approximately spheroidal shape, until the axial ratio achieves a value of about 2.5, whereupon cones possessing the Taylor angle are formed at each end of the drop, from which liquid will issue in the form of a jet. Experiments were performed in which uncharged water drops of radius R and surface tension T were directed towards a highly polished, earthed electrode at a shallow angle in an electric field of strength E . Each drop experienced the same electric forces as would result if the earthed electrode were removed and replaced by an identical drop twice as far away. Stroboscopic photographs of drops and their optical image in the polished electrode illustrated their deformation and eventual disintegration. The experimentally determined relation between normalized disintegration field E(R/T) 1/4 and both X/R and the elongation a/b agreed well with theory over a wide range of separations.

Etude d'une methode microdynamometrique pour la mesure de la concentration des poussieres

The microdynamometric aerosol detector is a variable condenser formed by a stainless steel foil and the conducting detector body. The 10 μm thick blade is aerodynamically stabilized very close to the earthed electrode. Under the impact of particles brought to near sonic velocities in a nozzle, the lamella moves slightly and causes a change in capacitance. Depending on the relative dimensions of the blade and the nozzle, especially the length-width ratio of the latter, it is possible: 1. (1) to bring almost all the particles to the same velocity and thus have an instrument which is independent of particle size between 1 and 20 μm diameter. In this case, the detector is a sensing device with an output nearly proportional to the mass concentration, from 15 mg/m 3 on. 2. (2) to accelerate the particles selectively in a relatively short nozzle, thus obtaining a response which is particle size dependent. If the mass concentration is otherwise known (e.g. by the simultaneous use of another sensor operated by mode 1 above), then the mean mass diameter (at known or assumed form of the distribution) may be inferred. With a RMS voltmeter as indicating instrument, this simple and inexpensive aerosol sensor forms a stable detecting system which is readily adaptable to extreme climatic conditions (high temperatures, pressures, etc.). The theoretical principles underlying the operation of this sensor are briefly explained.

Improved spark-gap voltmeter

The field in the conventional sphere gap is appreciably influenced by the surrounding earthed surfaces, and the expected accuracy of the sphere-gap voltmeter may be increased by modifying the configuration of the earthed electrode so as to make the field less dependent on the distance to these surfaces. Accordingly it is proposed that the conventional earthed sphere and supporting shank be replaced by a hemisphere placed on the ground plane, and that the upper sphere be made movable to vary gap length. Alternatively, the distance of the hemisphere above the ground can be varied if this is strongly preferred; for example, if difficulties arise in providing a motor driven high-voltage electrode. Use of the proposed electrode arrangements allows a considerable reduction in cost, especially when requirements of laboratory space are taken into account, and renders voltmeters using sphere diameters of 1 m and above very much simpler to use.Calculations of field factor and measurements of flashover voltage confirm that a sphere-hemisphere gap is affected very much less than a sphere gap by variations in the clearance to walls and large pieces of equipment near the spark gap. It is therefore possible to work with smaller clearances than are required by BS 358 and IEC52 without increasing the dependence of the voltmeter reading on the value of the clearance. Tests show that the sphere-hemisphere gap has a flashover voltage about 5% lower than the sphere gap when the gap length is equal to the sphere radius, and the difference is smaller as the gap length is reduced. A correction factor k is proposed to convert sphere-gap calibrations to sphere-hemisphere gap calibrations, where k = (1.015 − 0.06g), g ≥ 0.25, and g is the ratio of gap length to sphere radius.Two quite different means of supporting the top sphere of the sphere-hemisphere gap have been proposed, and both give entirely satisfactory performance when used with alternating, direct or impulse voltages.

Incendivity of discharges from electrostatically charged plastics

Electrostatic charge densities of 1·1-2·3 nc cm-2 can be generated over areas of polyethylene (polythene) sheet by rubbing. The nature and magnitude of electric discharges from the polyethylene sheet to an earthed electrode are a function of the relative humidity of the atmosphere and of the size of the electrode. The discharges change from corona to spark type if the radius of a hemispherical electrode is increased from 1 to 10 mm; with the latter-sized electrode the sparks can contain up to 0·23 μC of charge. Such sparks, which have an equivalent electrical energy of 0·67-0·92 mJ, can ignite flammable mixtures of coal gas, methane, acetone, methanol, toluene, cyclohexane and dioxane with air. For a fixed electrode system the incendivity of a spark is directly related to the electric charge in it.

Characteristics and breakdown initiation of triggered spark gaps with uniform applied field at very high voltages

The characteristics of the trigatron spark gap, in which the main electrodes are shaped to give an approximately uniform applied field, have been investigated. It is shown that this form of trigatron spark gap operates over a wider range of voltage and with shorter and more consistent time lags to breakdown than conventional trigatrons having spherical or hemispherical electrodes. The luminous phenomena which occur during breakdown initiation have been studied using an image convertor as an electro-optical shutter. Breakdown occurs in three stages. Firstly, a burst of corona-like discharge, sometimes with a bright stem at the origin, was initiated at the earthed electrode and was directed towards the opposite electrode. Secondly, the corona-like discharge was followed by a bright filamentary discharge (referred to as a leader stroke), usually starting from the anode and propagating across the gap towards the cathode. Finally, an even brighter discharge occurred (referred to as a main stroke) starting from the cathode and tracing back the route of the leader stroke towards the anode. Leader-stroke velocities fell within the range 5×107−6×108cm/s, depending on experimental conditions. Main-stroke velocities were about 108−109cm/s. It is shown that the phenomena occurring during breakdown can be explained by a mechanism of the streamer type.

Measurement of electric fields at electrode surfaces

A new technique for determining the field at the surface of the earthed electrode of a gap subjected to an impulse voltage is described. An example is given of the results obtained when the technique is applied to the study of the field variation during the initial stages of spark breakdown in a rod-plane gap, showing, in particular, the influence of space charges.

The impulse breakdown voltage and time-lag characteristics of long gaps in air II. The negative discharge

Oscillographic measurement and time-resolved photographic recording have been used to examine the breakdown of rod/rod, rod/sphere and rod/plane gaps with long-duration negative impulse potentials of up to 1 MV crest. As in the positive discharge, the breakdown process is found often to cease for considerable periods on the impulse wave tail. Breakdown then occurs after a long time lag for gaps composed of rod cathodes and rod or small sphere anodes. Where the anode is large, no long time lags are observed. Measurement of the variation of breakdown strength with time during the impulse duration has been made by the superimposition of a second impulse upon the first, following a controllable delay. The results show a minimum probability of breakdown initiation after 15 μs of the applied impulse, and a subsequent slow recovery. It is suggested that this confirms the effect of the initial corona phase in causing space charge fouling of the gap. The breakdown voltage of rod/rod gaps is shown to decrease by up to 30 % when the impulse wave front is reduced from 0.50 to 0.06 μs. The breakdown strength with negative impulses is thus less than with positive impulses for wave fronts faster than 0.20 μs. For the wave form 0.50/2000 μs, the breakdown voltage is found to depend more critically upon the dimensions of the earthed electrode than is the case with positive impulses. The breakdown voltage for a rod/25 cm sphere gap is 73 % greater than for a rod/rod gap, yet also 15 % greater than for a rod/plane gap. For gap lengths between 25 and 65 cm, the breakdown voltage of the rod/plane gap is found to increase with decreasing wave front duration. The variation of the negative impulse corona with rate of potential rise and with crest voltage is examined photographically. Many of the observed variations of breakdown voltage and time-lag characteristics can be accounted for in terms of the impulse corona phase. The time-resolved photography of the negative discharge over a wide range of conditions suggests that in the range of gap lengths investigated the formation of the positive leader rather than the negative leader is the necessary forerunner of sparkover.

Study of electric breakdown of liquid dielectrics using Schlieren optical techniques

The work is a contribution to the understanding of the mechanism of electric breakdown in liquid dielectrics. The liquid dielectric chosen throughout the experiments was n-hexane, because an extensive literature exists on the breakdown of this material. The applied voltage was a rectangular pulse of different durations supplied from a 125 kv, five-stage Marx-Goodlet impulse generator. A novel rotary multiple electrode containing eight pairs of electrodes was incorporated in a simple and efficient fractionation-filtration unit. A protection circuit, incorporating an inverter and a trigatron, limited the duration of breakdown current. A series of photographs of the state of n-hexane under the applied electric field was taken by Schlieren technique, using a spark light-source and a still camera. A statistical survey of the results shows the formation of a region of very low refractive index at the cathode. It is suggested that this is due to streamers with a very large number of branches, which propagate towards the earthed electrode. The rate of growth increases with voltage and above a certain voltage, if enough time is allowed, the stepped streamers bridge the electrode gap and breakdown of the liquid results.

Zero Error in Induction Flowmeters Employing a Permanent Magnet

A source of zero error in permanent magnet flowmeters is discussed. The error results from a redistribution of the potential produced at any earthed electrode connected to an experimental animal. It can occur when the blood vessel or connecting tubes are clamped to obtain a baseline of zero flow, and can be eliminated by using matched electrodes of low impedance, together with an amplifier employing matched input circuits of high impedance.

Development of the Spark Discharge

SOME time ago, one of us1 photographed the long electric spark with a rotating camera and showed that a preliminary discharge occurred prior to the main spark, just as in the case of the lightning discharge studied by Prof. B. F. J. Schonland and others. The original photographs of the spark between a pointed high-voltage electrode and a plane earthed electrode showed the course of this leader discharge for about 20 per cent of the inter-electrode spacing the luminosity diminishing with increasing distance from the high-voltage electrode. The time interval between the leader and the main stroke corresponded to the time-to-sparkover as recorded on the oscillograph. Leader strokes preceded positive and negative discharges.