No abstract

SummaryA new analysis of the current voltage characteristic of a floating double probe system is given, showing that the electron temperature must be estimated from measurements at the point of inflection of the characteristic if positive ion movement contributes to the probe circuit current. Temperature measurements will still be in error if the rates of variation of positive ion current to the two probes are not closely similar as the potential difference between them is varied. An alternative experimental technique is described which will yield correct temperature estimates in such circumstances, and is simpler and less perturbing to the plasma than the floating triple probe method.

No abstract

No abstract

It is well known that the absorption spectra of some crystals may be permanently changed if the crystals are exposed to photons with energies of tens or hundreds of electron-volts. It is therefore desirable, when studying absorption of far ultraviolet radiation in an ionic crystal, to expose it only to radiation of the wavelength for which its absorption is to be determined, rather than to a wide range of wavelengths. The usual techniques do, in fact, expose the crystal to the whole spectrum over which absorption measurements are required -either simultaneously (before dispersion) or sequentially (as with a scanning monochromator).Absorption spectra may, however, be recorded in a 'spectrograph by coating part of the photographic plate with the material concerned. The spectrum of a suitable light' source is then recorded so that each spectrum line spans the boundary between coated and uncoated photographic emulsion. The reduction in the blackening produced by anyone line due to absorption in the coating can then be measured. If the (polycrystalline) coating thickness is known, the absorption coefficient of the coating material can thus be deduced for each wavelength recorded.This technique avoids exposing the absorber to all wavelengths, but fails if the absorption of primary radiation by the coating leads to fluorescence or photoelectric emission, to which the adjacent emulsion may respond as though to transmitted primary photons. The coating then becomes a sensitizer and this scheme has been used widely to extend the useful wavelength range of plates and photoelectric detectors into the far ultraviolet.We have attempted to use partly coated Ilford "Q" plates (sensitive throughout the extreme ultraviolet wavelength range) to study the absorption spectrum of sodium chloride. While absorption measurements are possible at the longer wavelengths, we have found that sodium chloride acts as a sensitizer over wide ranges of wavelength. Our primary purpose here is to report this as a result of technical interest.In our exploratory experiments, Analar grade sodium chloride -was dissolved in distilled water at concentrations of the order of 100 gil. Ilford Q2 plates were dipped in this solution for 5 s and dried, so that the plate consisted of adjacent 10 in. by 1 in. zones, one coated with sodium chloride and the other untouched. The line spectrum from a capillary spark discharge was recorded between 150 and 1800 A in a Hilger grazing-incidence, 2 m grating spectrograph. Limiting stops were removed to give lines some 15 mm in length, half incident on the coated zone and half on the *