Spark-breakdown delay times were measured for N2, H2, Ar, SF6, and CC12F2 in a uniform field gap provided with a small current (-10 ' A) of free electrons by uv illumination of the cathode. Laue plots of the delay times yielded straight lines with slope iP, where i is the photocurrent and P is the breakdown probability. The dependence of the breakdown probability on voltage for N2, H&, and Ar was in good agreement with predictions of the Townsend breakdown mechanism. In SF6 and CC12F2, a transition was observed with increasing pressure from a dependence that agreed with the Townsend theory to a more gradual rise with voltage, characteristic of a streamer mechanism. This transition was ascribed to a decrease in the secondary-ionization coefficient with increasing pressure in SF6 and CC12F2, which resulted in an average electron-avalanche size at the static breakdown voltage that approached the critical value for streamer formation. A unified breakdownprobability theory, for which the Townsend and streamer mechanisms are limiting cases, was developed to account for the data over the full pressure range. The implications of these results for measurement of the static breakdown voltage and the secondary-ionization coefficient are discussed.

Drift velocities and mobilities of ions of Kr and Xe in their respective parent gases have been measured over a wide range of values of E/po, the ratio of electric field strength to normalized gas pressure. Two ions appear in each gas identified as KLr + and Kr2 + in Kr and Xe + and Xe2 + in Xe. The relation that drift velocity varies as (E/po)* at high E/po has been found to hold for the atomic ions and has been used to determine the equivalent hard sphere cross sections at high fields. The cross sections are 157X10 -16 cm 2 for Kr and 192 X 10~1 6 cm 2 for Xe. The Langevin theory of mobilities gives excellent agreement with experimental results extrapolated to zero field strength provided that, in the theory, the hard sphere cross section is taken as large for the atomic ions and very small for the molecular ions. The range of the polarization forces is such as to render them insignificant in atomic ion collisions and of primary importance in molecular ion collisions. T HE work reported by Hornbeck 1 on drift velocities in helium, neon, and argon has been extended using the same equipment to include krypton and xenon. The technique described in the references is essentially a time-of-flight method. As reported by him, an oscilloscope pattern is obtained in which sharp steps appear at time intervals after the start of the sweep equal to the transit time for ions to cross from anode to cathode in the parallel-plate Townsend tube.In Kr and in Xe, two steps appeared in the oscilloscope pattern for values of E/p 0 below 75 volts/ (cmXmm Hg). (The symbol p 0 is used to indicate that pressure readings are adjusted to the value which would be-produced by the same gas density at 0°C.) These two steps indicate the presence of two ions which are identified as the molecule ions Kr 2 + and Xe 2 + as the faster and the atomic ions Kr + and Xe + , respectively, as the slower ions. The identification is supported by the further observation that the supposed molecular ions disappear at higher E/p 0 in accordance with the greater difficulty of producing these ions, the fact that they do have greater speed (see reference 1), and the Kr + IN Kr .^y

The drift velocities of ions of the parent gas in oxygen, nitrogen, and carbon monoxide have been measured as a function of field strength to pressure ratio by techniques previously reported. Oxygen gave results similar to those in the rare gases reported previously. A log-log plot of drift velocity against E/po in volts/ (cmXmm Hg) starts with a slope near unity which gradually decreases to one-half at high values of E/po. The mobility, extrapolated to zero field and atmospheric pressure is 2.25 cm 2 /volt-sec. Nitrogen and carbon monoxide both show a novel characteristic; the drift velocity first rises with E/po but reaches a maximum and actually decreases, then finally resumes a more normal rise with E/po as described for oxygen. It is believed that at high E/po the drift velocity is characteristic of N 2 + ions and CO + ions, respectively. At low fields the ion in nitrogen is believed to be N 4 + . In CO the ion at low fields is believed to be CO + , with (CO)2 + being formed at intermediate fields. The results are complicated by an additional ion which appears in the range of E/po from 95 to 250 and which has a higher speed than the other ion. It is suspected of being C + .

The concept of electromotive force as first presented by Volta seems to be all but forgotten. Introduction of the term often occurs without any definition at all and is often confused with electrostatic potential difference. When a formal definition is given it is usually neither in accord with Volta’s original idea nor conceptually useful or comprehensible. We emphasize Volta’s use of the term to describe nonelectrostatic action on charges. The relationship of emf’s to electrostatic potential differences is presented through the introduction of electrostatic and nonelectrostatic fields (as was described by Abraham in 1904). A failure to note a distinction between these types of electric fields mars some highly important works including J. C. Maxwell’s.

the Ta had a gas film on it. As observed in this study, flashing the Pt target removed most of the active gas films so that the work function of the Pt used was not less than the accepted work functions of clean Pt of value 5.3 volts. 26 In consequence if the value of the work function alone determines ji the y* for Ta should be greater than that for Pt. Actually the reverse is true as noted. Again Hagstrum 11 observed that for He 4 " ions on Mo after the background gas had formed a monolayer on the target yi was lowered by 30 percent while cj> had not changed by more than 0.1 volt.A guess as to the nature of this action comes from Hagstrum's observation that the energy distribution of electrons from the gas-covered surface had more slow electrons than that from the clean surface. This indicates that the probability of the excited electron occupying a low level in the range of levels available to it is greater for the gas covered surface than for the clean one. Such a condition would obviously also lower yi. The process active may be crudely envisioned as follows: with a clean surface neutralization involves direct interaction with only one electron of the Fermi band. With a covered surface it could well involve two for the electrons causing neutralization can come from a local state produced by an adsorbed gas atom.In general since such a local level does not lie in the Fermi band it will be refilled after neutralization by a metallic electron. Such a two-step process could well act to lower the energy available to the excited electron.Thus gas coatings not only lower yi by raising , 26 H. B. Michaelson, J. Appl. Phys. 21, 536 (1950).but also lower yi by reducing the available energy for excitation by a more complicated neutralization process. If the lowering of , for instance by H2 treatment, is small the value of yi can still be decreased by H2 films because of reduction of available escape energy. Likewise, clean Pt could yield a higher y* despite its 0.4-volt higher value of relatively to Ta, as Ta has a gas film at its surface.That yi increases with ion energy is not surprising since kinetic liberation superposes on potential for all surfaces. For gas treated surfaces the observed increase of yi with energy is greater than for clean surfaces. This is to be expected for aside from some "cleanup" of the film by high-energy bombardment more of the high-energy ions penetrate the gas film and interact directly with metallic electrons; i.e., the interaction time with the local gas surface states is decreased relative to that with the lattice.The initial decrease in y»-with ion energy, observed only with nitrogen ions, can be accounted for by the very high energy of association of N atoms to form N 2 molecules. Thus in addition to the neutralization energy of N + ions these can react at the surface to yield N2 at 9.6 ev and this energy may be available for electron liberation. Such a reaction may take place for slowly-moving low-energy ions and may decrease as ion energy increases.In conclusion the writ...