J. K. Bragg

Electric Strength and Molecular Structure of Saturated Hydrocarbon Liquids

A number of investigators have reported that the apparent electric strengths of straight‐chain hydrocarbon liquids increase in a regular manner with increasing molecular chain length. In a recent publication we have presented evidence that this phenomenon is an illustration of a kind of Paschens law for liquids. An observed linear dependence of the electric strengths of such hydrocarbons upon density suggested that chain length variation merely provided a means of changing the electron mean free path in the liquid.In the present paper we describe the development of an improved technique for measuring electric strengths of liquids, and the application of this technique to a series of pure straight‐chain and branched‐chain liquid alkanes. As had been observed previously, the strengths of the straight‐chain members of the series exhibit a linear relationship with density. The introduction of branches into the hydrocarbon chain, however, results in a definite decrease in strength. Extension of the measuremen...

A Semiempirical Expression for the First Townsend Coefficient of Molecular Gases

The general utility of an approximate expression for the first Townsend coefficient of a gas, derived several years ago by Druyvesteyn and Penning, has been tested by introducing into the theoretical treatment somewhat more realistic assumptions regarding the relevant scattering cross sections of the gas molecule. It has been demonstrated by numerical calculations that the functional dependence of the Townsend α upon the electric field and the gas pressure is, within certain limits, insensitive to the assumed energy dependence of the elastic and inelastic cross sections. The functional form of the inelastic cross section does, however, influence the way in which the ionization energy of the gas molecule enters in the exponential part of the expression.

Cathode effects in the dielectric breakdown study of liquids

The electron emission characteristics of the surface of the cathode used in the study of the electric breakdown of insulators are believed to have a fundamental bearing on electric strengths observed. It has been suggested that the formation of a negative space charge is the mechanism through which this influence is exerted1.

On the Electric Strengths of Aliphatic Hydrocarbons

Paschens similarity law, which expresses a certain relationship between the breakdown voltage of a gas, the gas pressure, and the distance between electrodes, is known to fail at high pressures. This is owing, at least in part, to a change in the way in which electrons dissipate energy to the gas molecules. At high enough pressure, the electron transfers energy principally to molecular vibrations. In condensed phases this vibrational barrier is dominant, and one should expect a new kind of similarity law to hold under certain controlled conditions. The liquid aliphatic hydrocarbons provide an illustration of such a law. The electric strengths of nine hydrocarbons of various densities were measured and found to depend linearly upon density. It has also been found that the temperature dependence of the electric strength of a liquid aliphatic hydrocarbon can be accounted for by the change in density of the liquid due to thermal expansion.

Dependence of the Electric Strengths of Liquids on Electrode Spacing

Using a reproducible pulsed voltage technique, the electric strengths of liquid hexane, heptane, and tetradecane have been measured with small electrode spacings (approximately 2 to 200 microns). The electric strengths are found to change inversely with the logarithm of the separation in accord with theoretical expectation. On the basis of an approximate model of the breakdown process, the measurements on heptane yield values of 9×10−7 cm for the electron mean free path in the liquid and 9 for the number of multiplications per electron required for breakdown.

Electric strength and molecular structure of liquid aliphatic hydrocarbons

In 1951 it was reported by Salvage (1) that the electric strengths of liquid straight-chain aliphatic hydrocarbons, as measured under d.c. conditions between hemispherical electrodes, increased with increase in the chain length of the hydrocarbon molecules. Until that time, a correlation of electrical breakdown of liquids with known molecular properties had been virtually impossible because of inadequate experimental techniques. The measured electric strength of a liquid is well known to be extremely sensitive to electrode shape and surface condition as well as to the presence of suspended foreign bodies in the liquid itself. Unless a concentrated effort is made to eliminate such complicating factors, the results obtained are usually highly scattered and unreliable.

The dependence of the measured electric strengths of liquids on electrode spacing

For some twenty-five years the decrease in the measured electric strength of liquids with increasing electrode spacing has been an important subject of study because of its practical as well as theoretical significance. Early investigators were handicapped by the lack of reproducibility of electric strength values as well as their inability to measure accurately small electrode spacings. With the recent development of a reproducible pulsed-voltage technique (1) and a means of determining small electrode separations (1–10μ) by capacitance measurement, we have re-examined this phenomenon using several carefully purified hydrocarbons as the liquid dielectric. Our measurements extend and, in some cases, supplant those of Edwards (1) and Goodwin and MacFadyen (2).

The measurement and interpretation of the electric strengths of aliphatic hydrocarbons

It has been reported-recently by Salvage (1) that the electric strengths of liquid, straight-chain hydrocarbons increase with increase in molecular weight from n-pentane through n-nonane. The purpose of the present work is to verify and investigate more carefully this behavior, and to interpret the results, if possible, in terms of the physical properties of the hydrocarbons.

On the temprature dependence of the electrical breakdown process

A crucial test of any theory of electrical breakdown is the comparison of observed and predicted temperature dependence of the electric strength. The theories of von Hippel, Seitz, and Callen give the proper temperature dependence at low temperatures (for ionic crystals) but the agreement is lost at high temperature. In order to explain the high temperature region, Frohlich has adduced another breakdown mechanism which is supposed to cover this region.