Sphere Gap Research Articles

A Tripping Circuit for a Multi-stage Surge Generator

The sphere gap in the first stage of an existing multistage surge generator has been replaced by two equal gaps in series by introducing a third sphere. By applying an impulse to the centre sphere when the gaps are charged nearly to the breakdown voltage, breakdown can be initiated at a desired instant. This tripping impulse is generated by a thyratron becoming ionized when a low voltage impulse is applied to its grid, and simultaneously to the sweep circuit of the oscillograph. On account of the inherent time delay in the ionization of a thyratron, the sweep circuit is initiated before the surge generator. A delay cable in the measuring circuit is thus rendered unnecessary. Visual or automatic methods of controlling the output voltage of the generator can be used.

Calibration of uniform-field spark-gaps for high-voltage measurement at power frequencies

A calibration is given for spark-gap electrodes designed to ensure that the spark path lies in a uniform field, and it is shown that for all three sizes of electrodes investigated, covering a range of 9 to 315 kV (peak), the sparkover voltages are within 0.2% of the values given by the empirical lawV=24.22S+6.082√SkV (peak) at 20°C and atmospheric pressure of 760 mm Hg, where S is the electrode spacing in centimetres.All observations were made with vertical gaps having the lower electrode connected to earth, and comparative data were obtained for a sphere gap with electrodes 25 cm in diameter. The effects of surrounding objects and of faulty alignment of the electrodes were determined experimentally, and on the basis of these results a number of recommendations are made, giving the limits within which the spark-over voltage is defined by the above equation.

Depth Dose Measurements for 100-, 120-, and 135-kv. Roentgen Rays

The present trend toward larger x-ray doses in the treatment of superficial cancer makes the determination of the amount of radiation delivered to the underlying healthy tissue of greater importance. Such calculations, however, have been limited by the lack of complete depth dose tables for low and intermediate voltages, although numerous data are available for special technics or conditions (1, 5, 7, 10, 12). The present paper is a report on our phantom measurements for half-value layers from 1.0 to 8.0 mm. A1 (0.04 to 0.44 mm. Cu). Equipment and Measuring Instruments The apparatus was chosen with the aim of duplicating the conditions used clinically and consisted of a shockproof, oil-cooled x-ray tube energized by a half-wave, two-valve 140-kv. generator. The x-ray tube was mounted in a “rayproof” housing, and the total inherent filtration of the pyrex tube wall, oil, and bakelite window was equivalent to approximately 0.5 mm, aluminum. Comparative measurements were carried out, also, with an air-cooled shockproof tube having an inherent filtration of approximately 1 mm, aluminum. The tube peak kilovoltage was determined by means of a 12.5-cm. sphere gap which had previously been checked at constant potential against a Taylor high-voltage resistance unit. The milliamperage was maintained at a constant value by means of a stabilizer of the saturated core type. The radiation was measured with two bakelite thimble chambers coated on the inside with aquadag and having inside diameters of 0.52 cm. and 0.72 cm. respectively, with volumes of 0.40 c.c, and 1.27 c.c, Measurements at the surface and down to 5 cm. depth were made with both chambers, while at depths below 5 cm. the larger chamber only was used. A well shielded electrometer of the Wulf type was permanently connected to the chamber used. Choice of Phantom Material Preliminary experiments were made to determine the best phantom material for the ray qualities used. Water, being the main constituent of human tissue, has been preferred by several earlier investigators. It has, however, practical disadvantages. It is art electric conductor, which prevents its use with ionization chambers of the Failla extrapolation type (4) and, furthermore, since it is a liquid, it is difficult to obtain layers of definite thickness or cross section. Of the many solid substances used as phantom material, “masonite presdwood” has been the most satisfactory. “Masonite presdwood” is a wood fiber board manufactured by exploding wood chips by high pressure steam. Presdwood phantom measurements were first made by Quimby and collaborators (9), who found that masonite of unit density gave the same depth dose as water for 200-kv. radiation. In a previous paper (3) we reported that higher depth doses were obtained with masonite than with water when 40- to 50-kv. radiation was used, as applied in “contact” therapy. The absolute difference, however, was small, about 3 per cent at 1.0 em. depth, as the short target skin distance, 2.0 cm., caused a low depth dose for either material in accordance with the inverse square law.

The measurement of impulse voltages by means of small sphere gaps

The erratic behaviour of small sphere gaps under impulse voltages may be explained by the comparative scarcity of ions in the gap. Methods are described by which this deficiency of ions may be remedied and greater consistency of breakdown be obtained. Suggestions are made for the use of the 0.5-inch diameter sphere gap as a standard for impulse-voltage measurement when irradiated by light from a spark discharge.

On the Accuracy of Impulse Flashover Values of Sphere Gap Published by the IEG

On the Accuracy of Impulse Flashover Values of Sphere Gap Published by the IEG

High-pressure gas as a dielectric

Impulse and 60-cycle breakdown strength of sphere gaps, rod gaps, and of samples built up of solid and fluid dielectrics are reported. The tests were made in nitrogen up to 200 pounds per square inch gauge pressure and in Freon up to 70 pounds per square inch gauge pressure. Dielectric strength values were compared with the breakdown of transformer oil between the same electrodes. Gap conditions were found for which the 60 cycle breakdown was twice its impulse breakdown.

High-Pressure Gas as a Dielectric

Impulse and 60-cycle breakdown strength of sphere gaps, rod gaps, and of samples built up of solid and fluid dielectrics are reported. The tests were made in nitrogen up to 200 pounds per square inch gauge pressure and in Freon up to 70 pounds per square inch gauge pressure. Dielectric strength values were compared with the breakdown of transformer oil between the same electrodes. Gap conditions were found for which the 60 cycle breakdown was twice its impulse breakdown.

The electric strength of air at high pressure — II

Results are given of an investigation of the electric strength of air at pressures up to 21 atmospheres. Sparking voltages are presented for air under pressure as measured with 60-cycle alternating voltage applied to sphere gaps. Deviation from the linear pressure-voltage relation that exists at low pressures is discussed, and certain characteristics of sparking in compressed air are noted. A study of the electric strength of air under pressure was reported three years ago. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> Because of increased interest in the subject this study has been extended, and parts of the work have been repeated with a higher degree of accuracy. Data thus obtained are of interest in the design of air-blast circuit-breakers and other types of compressed-air switches, gas-insulated transformers, power cables operating under pressure, gas-filled radio transmitting condensers, etc. The present paper presents data relating to sparking between spherical electrodes in air, the air pressure ranging from 1 to 21 atmospheres (0 to 300 pounds per square inch gauge pressure). Results obtained with spherical electrodes may be applied with reasonable accuracy to any spark gap in which the electric field is substantially uniform. The results of this investigation are therefore applicable to sparking between the parallel plates of condensers, electrostatic shields of high-voltage apparatus, and other smooth surfaces. Breakdown between points, and at sharp edges, and between small wires, is discussed in the earlier paper. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup>

The electric strength of air at high pressure - II

Results are given of an investigation of the electric strength of air at pressures up to 21 atmospheres. Sparking voltages are presented for air under pressure as measured with 60-cycle alternating voltage applied to sphere gaps. Deviation from the linear pressure-voltage relation that exists at low pressures is discussed, and certain characteristics of sparking in compressed air are noted.

Enclosed Spark Gaps

Ionizing radiation has been discovered to be present in spark gaps of the type commonly used in commercial lightning arresters for the last 15 years. This radiation is emitted from discharges near the insulator-electrode contacts, when impulse voltage is applied to the gap. Ionization is produced by the absorption of this radiation in the spark gap. This effectively reduces the time lag of breakdown. The presence of this radiation explains why higher impulse breakdown voltages are obtained with a sphere gap in the dark than with a lightning-arrester spark gap with porcelain spacer. The radiation effect observed here is similar to that investigated by Wynn-Williams in the three-point gap. This paper tells of experiments performed to determine the reason for the low impulse ratios observed with rutile-spacer spark gaps. The results of this study explain the reason for the low impulse ratio of the ``quench gap'' and the ``ionization gap'' utilized in lightning arresters. This new information aids in understanding why low impulse ratios are obtained in high-voltage lightning-arrester spark gaps.

Enclosed spark gaps

Ionizing radiation has been discovered to be present in spark gaps of the type commonly used in commercial lightning arresters for the last 15 years. This radiation is emitted from discharges near the insulator-electrode contacts, when impulse voltage is applied to the gap. Ionization is produced by the absorption of this radiation in the spark gap. This effectively reduces the time lag of breakdown. The presence of this radiation explains why higher impulse breakdown voltages are obtained with a sphere gap in the dark than with a lightning-arrester spark gap with porcelain spacer. The radiation effect observed here is similar to that investigated by Wynn-Williams in the three-point gap. This paper tells of experiments performed to determine the reason for the low impulse ratios observed with rutile-spacer spark gaps. The results of this study explain the reason for the low impulse ratio of the “quench gap” and the “ionization gap” utilized in lightning arresters. This new information aids in understanding why low impulse ratios are obtained in high-voltage lightning-arrester spark gaps.

An Experimental Study of the Effects of Roentgen Rays on the Gonads of the Sexually Mature Domestic Fowl

WITH the aid of standardized x-ray equipment we have under taken a systematic study of the effects of roentgen rays on the gonads of the domestic fowl. In this study, we have attempted to correlate the age of the animal, the dosage given, and the results obtained. In the first part of the study (1) we were dealing with the embryo and the young chick. The present paper deals with the study of the adult male and female bird. Materials and Methods The x-ray machine2 used was mechanically rectified and provided with a Landauer roentgenometer. A universal Coolidge therapy tube was used. The setup of the machine for the entire experiment was as follows: The kilowatt meter was set at 96, which delivered 112 kv.p. as measured by the sphere gap; the milliammeter was set at 6 ma.; the focal distance was 10 in.; the filter was equivalent to 4 mm. aluminum; the roentgenometer was kept at 3.2 micro-amperes, which, by calculation, gave 0.6 r per second. The desired r, or dosage, was obtained by varying the time of expo...

Institution of Electrical Engineers: Faraday Medal

DR. ALEXANDER RUSSELL has been awarded the eighteenth Faraday Medal by the Council of the Institution of Electrical Engineers. The medal is awarded for notable achievement in, or help to, electrical science or industry, and Dr. Russell is qualified in both respects. His works on alternating currents and the theory of cables have long been classical; not less well known are his researches on the sphere gap, and many other mathematical researches and papers communicated to the learned societies. He recently published the "Life of Lord Kelvin", whom he knew in the early days. Dr. Russell has been connected with Faraday House from its inception in 1889, and became principal in 1909. During fifty years he has helped in, or been responsible for, the training of thousands of electrical engineers, now to be found all over the world. The outstanding feature of this education has been the encouragement of men to bear their own responsibilities; many of his students, both in the early days and recently, have achieved important work even during their student days. He has been for many years a valued contributor to NATURE.

First Measurements with the Spanish Standard for the Absolute Determination of the International Roentgen Unit

THE Laboratory for Radiological Measurements annexed to the Service of Physical Therapy, directed by the author, began operation in 1931. In 1932, there was installed a Piccard compensation electrometer, with which we began to make determinations of the charge of radium containers. Finally, in 1934, we found it possible to adapt to this installation the standard chambers for the absolute measurement of the roentgen unit. Varied circumstances until now have hindered our comparing this standard with the standards of other countries, but it will be done as soon as possible. The arrangement we have employed has been adjusted to the agreements of 1934 between the National Laboratories (1). I.—X-ray Generator We have employed one of the constant high tension machines—“Stabilivolt” S.R.V.—in our Department of Roentgen Therapy. The feeder line presents fluctuations of voltage which, up to this time, we have not found the means of compensating for, by the use of a voltage stabilizer, but we hope to do so in the future, when our equipment will operate as a public service of standards. Notwithstanding, we have practically eliminated these unpleasant oscillations by making our measurements during the hours in which the industrial engines are stopped. In every case, the filament current has been furnished by a storage battery having a capacity of 100 ampere-hours. We have employed two x-ray tubes: one of the Multix model, for voltages up to 110 kv., cooled by thermosiphon, and entirely protected against loss of radiation. The other, of the Metwa Matalix type, up to 200 kv., having the same cooling and protecting system as the former, and a glass window for the passage of the rays. These models are not shockproof, but our new tubes will be so protected in the future, on account of the very efficient screening that the shockproof tubes and conductors possess, which renders unnecessary every other protection against interference from magnetic fields. The milliammeter for measuring the intensity of the current passing through the tube has been calibrated against a precision instrument. We have marked only a fixed point, corresponding to the reading of 4 ma. A second milliammeter, in series with the first, checks the constancy of its readings. The margin of error in the current measurements is of the order of 2 per cent. The high voltage of the Stabilivolt generator has been calibrated by means of a Muller sphere gap, with spheres of 125 mm. diameter. The connection of the spheres with the high tension line has been made through high ohmic resistances, formed by 6 meters of glass tubing of 1 cm. diameter, rolled in a spiral and filled with 95° alcohol. The spark obtained in the sphere gap is very fine, and is spontaneously interrupted, affording periodical discharges (relaxation oscillations).

The Effects of Irradiation, Humidity and Sphere Material on the Sparkover Voltage of the Two-Centimeter Sphere Gap

Data have been obtained on the sparkover voltage of the two-centimeter sphere gap set at a gap of 4 mm (10.3-kv peak) showing that: (1) The effect of irradiation is to reduce the sparkover voltage by approximately 2.5 percent from its unirradiated value and to decrease the scattering of individual observations by a factor of about four. This irradiation effect is readily saturated by an open, coredcarbon, arc at 50 cm from the gap. (2) The effect of humidity, which is apparently independent of the sphere material for the five metals used here, is to increase the sparkover voltage by +0.13 percent per mm (of mercury) increase in vapor pressure of the water in the atmosphere. (3) There seems to be no choice between the metals used for spheres (aluminum, brass, chromium, nickel and steel) so far as repeatability of results is concerned, the probable error of a day's results averaging ±0.28 percent. This probable error can be largely if not wholly accounted for in terms of known sources of uncertainty. (4) The final sparkover voltages for the various metals, even when corrected to the same humidity, differ from each other by far more than can be accounted for by any definitely recognized source of uncertainty.

The Dielectric Strength of Benzene and Heptane

The dielectric strengths at 25°C of benzene and of heptane in a uniform electric field produced with a sphere gap and of heptane in a non-uniform field between concentric cylinders, were measured on the pure filtered liquids and with rigorous control of the electrode surface. With any one experimental procedure, the breakdown gradients were reproducible to within 5 percent provided all variables were carefully controlled. The most important of these variables, the character of the electrode surface, is influenced by such factors as the degree of polish or etching, adsorbed gas or ions, and the gases dissolved in the electrodes. Adsorbed oxygen or hydrogen, for example, induces incipient discharges in either liquid with molybdenum electrodes, but not in heptane with nickel electrodes; in the latter case, the gas adsorbed on the electrodes slightly increases the breakdown gradient. Below one atmosphere, the dielectric strengths of benzene and heptane are independent of the pressure. In a non-uniform field, the dielectric strength of heptane is the same for a.c. and d.c. of either polarity. Although reproducible dielectric strengths were obtained by any one experimental procedure, the results from different procedures varied from 100 to 1700 kv (Peak)/cm. It appears, therefore, that the dielectric strengths of pure liquids depend primarily on the nature of the electrode surface which varies with the experimental procedure.

The Electric Strength of Air at High Pressure

Results are given of an investigation of the electric strength of air at pressures up to 21 atmospheres. Sparking voltages are presented for air under pressure as measured with 60-cycle alternating voltage applied to sphere gaps. Deviation from the linear pressure-voltage relation that exists at low pressures is discussed, and certain characteristics of sparking in compressed air are noted. A study of the electric strength of air under pressure was reported three years ago.1 Because of increased interest in the subject this study has been extended, and parts of the work have been repeated with a higher degree of accuracy. Data thus obtained are of interest in the design of air-blast circuit-breakers and other types of compressed-air switches, gas-insulated transformers, power cables operating under pressure, gas-filled radio transmitting condensers, etc. The present paper presents data relating to sparking between spherical electrodes in air, the air pressure ranging from 1 to 21 atmospheres (0 to 300 pounds per square inch gauge pressure). Results obtained with spherical electrodes may be applied with reasonable accuracy to any spark gap in which the electric field is substantially uniform. The results of this investigation are therefore applicable to sparking between the parallel plates of condensers, electrostatic shields of high-voltage apparatus, and other smooth surfaces. Breakdown between points, and at sharp edges, and between small wires, is discussed in the earlier paper.1

Polarity Limits of the Sphere Gap

It is found that (10-cycle polarity characteristics correlate with the positive and negative impulse calibration curves for grounded sphere gaps sufficiently well to use them for determining the polarity limits for sphere gaps. The available data show that the maximum spacing at which a grounded sphere gap has equal positive and negative spark-over is a linear function of the size of the spheres. From this relation, it follows that the maximum sparking distance for equal positive and negative sparkover, expressed as a per cent of the diameter of the spheres, is not constant for all sphere diameters. Therefore, arbitrary polarity limits expressed in terms of the diameter of the sphere cannot apply to all sizes of sphere gaps.

Polarity limits of the sphere gap

Polarity limits of the sphere gap

Proximity Effect of the Conducting Objects on the Flashover Voltages of the Sphere Gap

Proximity Effect of the Conducting Objects on the Flashover Voltages of the Sphere Gap