Committee on science and the arts

In contradiction to results previously reported, it has been found that DNA prepared by a method which utilizes strong NaOH is acted on by the specific nuclease, although at a rate only 20 per cent. of that of DNA prepared by a milder method. The nature of the changes responsible for the low rate of hydrolysis of the alkali-treated DNA are not known.     

Secondary electron emission from oxide-coated cathodes

The secondary electron emission from alkaline-earth oxide-coated cathodes has been investigated under both continuous and pulsed bombardment. Various factors affecting the yield, such as dependence upon primary voltage, collecting voltage, temperature, time, and angle of incidence, are noted, and the present state of the theory is discussed.Experiments have been performed with three types of apparatus. Yield vs. Energy data reveal values of δ of 4–7 at room temperature, with a more or less flat maximum at approximately 1,000 volts primary energy.The yield increases with temperature in an exponential manner, and plots of log Δδ (i.e. δ_K° ? δ_300°K) vs. 1/T give straight lines. Values of Q₁ between 0.9 – 1.5 eV. are generally indicated, and from extrapolation of these curves, yields exceeding 100 at 850° C. are deduced. The secondary emission depends upon the degree of activation, and increases with enhancement of the thermionic emission characteristics. Short-time effects such as growth or decay of secondary current after the onset of primary bombardment or persistence after the cessation of bombardment have not been observed, and values of yield obtained by pulsed methods are in accord with those obtained under D.C. conditions. Tail phenomena reported by J. B. Johnson and interpreted as “enhanced thermionic emission” from oxidecoated cathodes become manifest only under experimental conditions characterized by certain space-charge effects, and have been effectively simulated by bombarding a tantalum target adjacent to an electron-emitting tungsten filament. Various measurements of the energy distribution of secondary electrons as a function of primary voltage and temperature have been obtained. It was observed that the average energy of the secondary electrons decreases with temperature at a rate which more than compensates for the increase in the number of secondaries emitted per incident primary. The mechanism of the observed dependence of yield upon temperature is not well understood. Various alternative explanations are discussed and, in the light of the present state of our knowledge, regarded as untenable.     

Some effects of variable excitation on synchronous motor oscillation

In this paper are presented the results of a study of synchronous motor oscillations resulting from sudden application of mechanical load. By means of the Differential Analyzer at the Moore School of Electrical Engineering of the University of Pennsylvania, theoretical synchronous machine equations were solved for conditions of suddenly applied load, and the effect on the resulting oscillations of variations in field excitation was determined for the most practical cases. The analyzer results were checked by a number of experimental observations, and it was found that the magnitude and duration of the oscillations could be greatly decreased by using the proper type of excitation variation.     

Electron diffusion,electron attachment,and the aging of negative ions in commercial nitrogen at atmospheric pressure

A criticism by Loeb and Bradbury of my interpretation of the experimental results of that part of a previous paper which dealt with the aging of negative ions in commercial nitrogen is shown to be invalidated by a mistake in their analysis and by assumptions which do not represent facts with sufficient accuracy. A more extended analysis of the previous work and new evidence which is given confirm the correctness of the former contention that the negative ions in question changed in size with age. The lateral spread, owing to diffusion, of a stream of electrons in this nitrogen at atmospheric pressure, when moving in an electric field of about 100 volts/cm., was found to be very nearly the same as that for positive ions under like conditions. The average energy losses of the electrons at collision with the molecules must therefore have nearly balanced the gains from the electric field and hence were greater than would result from purely elastic collisions. The experimental results indicate that in fields down to at least 20 volts/cm. nearly all of the attachment of electrons to molecules in the gas used occurred close to the place of their liberation.