Absolute partial and total cross sections for electron-impact ionization of methanol, ethanol, and 1-propanol are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in most of the absolute cross sections is ±6%–8%. Significant discrepancies are seen between the only prior methanol partial cross section determination and this study but the majority of published total cross section measurements and calculations are in good agreement with this work.

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THE potentiometer incorporated in the Unicam SP500 spectrophotometer consists of a slidewire bearing a linear transmission scale and a logarithmic density scale. Many workers, finding that the use of the logarithmic scale is apt to lead to confusion and error, and that inadequate discrimination is obtained when the density of the solution under test exceeds 0.2, prefer to make use of the linear transmission scale ; it is then necessary to calculate the density d from the transmission reading t by means of the relationship t = 10-d. The linear optical density potentiometer now described was designed with the object of avoiding this inconvenient procedure.If a resistance rl is connected in series with a slidewire of ohmic value r,, the range of the slidewire will extend from zero to an increment p in density, the value of which is given by the relationshipProvided that the increment p is sufficiently small, the calibration of the slidewire in terms of density is then approximately linear, i.e. equal increments in the resistance of the slidewire correspond to approximately equal increments in density. In order to extend the maximum range of the potentiometer from p to 2p, provision may be made, by means of a selector switch, for connecting a resistance equal in ohmic value to r, in series with r, and r2, and simultaneously connecting across the series combination of r, and r, a shunt R,, the value of which is given by the relationship R, = rl(rl -t r2)lr2 . . .. .. . . (2) If n series resistances, each equal to r,, and n shunts, each equal to R,, are provided and controlled by the selector switch, the latter may be used to extend the range of the potentiometer in n successive steps, each of which corresponds to an increment p in optical density.The practical circuit diagram of an optical density potentiometer which was constructed in accordance with these principles is given in Figure 1.It was decided to restrict the range of the slidewire to a density increment of 0.1, and to make the total resistance of the potentiometer equal to that of the slidewire incorporated in the spectrophotometer, viz., 109 ohms. As r, -I-r, = 109 and p = 0.1, it follows from equations (1) and (2) that the appropriate values of r,, r,, and R, are 86-56, 22.44, and 420.5 ohms respectively. The main potentiometer arm consists of a selector switch S2 controlling a series of 9 resistance coils, each of 22.44 ohms, and a slidewire Sly also having a resistance of 22.44 ohms. Each coil thus 148