Limiting stable currents in bounded electron and ion streams

The classical static analysis of the infinite planar diode has been extended to include the effects of finite transverse beam size. Simple expressions have been found for the increase in maximum stable current density over that of an infinite stream for finite cylindrical and strip streams flowing between plates of infinite diodes. The results are also given in terms of stream perveance. The effect of a nonuniform distribution of current across the stream is shown to be relatively small. Experimental values of maximum stable current agree with those obtained from the analysis. A further extension of the static analysis has been made to include the effects of additional conducting plane boundaries parallel to the stream motion. For length-to-width ratios L/D less than 0.25 the tube is adequately described by the results for the infinite planar diode and for L/D greater than 4, the infinitely-long drift tube theory suffices. At intermediate values of L/D, the maximum amount of current that can be stably passed through the tube is greater than that predicted by either asymptotic theory

Effect of Gibberellic Acid on Oats

Oat plants were sprayed with gibberellic acid in concentrations of 10 ppm, 100 ppm, 1000 ppm, and 2,500 ppm at 24 hours before and 24 and 144 hours after anthesis. The application 24 hours before or after anthesis caused 10 percent to 50 percent elongation of the upper internodes. Weight of the primary kernels was depressed approximately 10 percent and that of the secondary 15 percent. The effect upon seed size was independent of time of application or concentration

Inheritance of Seed Weight and its Relation to Grain Yield of Oats

In 3 oat crosses seed weight appeared to be inherited as a quantitative character. Each of the crosses showed transgressive segregation. In 6 crosses, the correlations between yield and seed weight were all positive, but only 4 were significant. In simulated selection experiments, the efficiency of selection for yield was compared when only yield or when yield plus seed weight were used as the selection criterion. In general, the addition of seed weight as a selection criterion decreased the gain in yield

Electron-Hole Crossover in Graphene Quantum Dots

We investigate the addition spectrum of a graphene quantum dot in the vicinity of the electron-hole crossover as a function of perpendicular magnetic field. Coulomb blockade resonances of the 50 nm wide dot are visible at all gate voltages across the transport gap ranging from hole to electron transport. The magnetic field dependence of more than 50 states displays the unique complex evolution of the diamagnetic spectrum of a graphene dot from the low-field regime to the Landau regime with the n=0 Landau level situated in the center of the transport gap marking the electron-hole crossover. The average peak spacing in the energy region around the crossover decreases with increasing magnetic field. In the vicinity of the charge neutrality point we observe a well resolved and rich excited state spectrum.Comment: 4 pages, 3 figure

Spin States in Graphene Quantum Dots

We investigate ground and excited state transport through small (d = 70 nm) graphene quantum dots. The successive spin filling of orbital states is detected by measuring the ground state energy as a function of a magnetic field. For a magnetic field in-plane of the quantum dot the Zemann splitting of spin states is measured. The results are compatible with a g-factor of 2 and we detect a spin-filling sequence for a series of states which is reasonable given the strength of exchange interaction effects expected for graphene

Yield Losses from Atypical Oat Blast in Central Iowa in 1957

An epiphytotic of blast occurred in the Iowa oat crop in 1957. The blasting was atypical in that the panicles were stratified with all of the spikelets in a section being blasted. Normally blasting occurs in the spikelets near the base of the panicle and the central axis. The environmental condition which appeared to be the most likely cause of the blasting was a cold, cloudy period between May 10 and 27. This was during early spikelet development and panicle elongation. Severity of atypical blasting was associated with earliness and lateness in the oat varieties. The highest percentage of panicles with atypical blast and the highest percentage of blast on these panicles occurred in varieties which beaded on June 13 and 14. Seeds from heads with atypical blast were from 10 to 25 per cent heavier than those from normal heads. The yield losses caused by atypical blast ranged from 7 to 15 per cent depending upon the maturity of the varieties

Effect of Heat Treatment on Oat Seeds

One method of handling oat grain with a moisture content too high for safe storage is artificial drying. This practice involves forcing heated air through the grain until the moisture content is reduced to the 13.5 percent considered safe. Problems inherent with this practice are the maximum grain moisture content at which artificial heat can be applied safely, and the maximum temperatures that can be used for drying

The Relationship Between Moisture Content and X-Ray Sensitivity of Oat Seeds, Avena sativa

About a decade ago, Gustafsson (3) reported that the sensitivity of barley seeds to x-ray treatment was positively related to their moisture content. Sensitivity was measured by germination percentage and vigor of seedlings produced from treated seeds. However, Caldecott (1) recently has shown that barley seeds with a moisture percentage between 8 and 16 were more resistant to x-ray damage than those with either a lower or higher water content. These conflicting results probably are related to differences in methods used to temper the seeds by the two investigators. Gustafsson (3) soaked the seeds in water while Caldecott (1) placed them in desicators with different relative humidities. Metabolic activity and x-ray sensitivity of seed would be expected to be enhanced more by the first method. For a discussion of the physical significance of moisture content of seeds on x-ray sensitivity the reader is referred to a paper by Caldecott (2). This paper reports experiments conducted to determine the relationships between moisture content of seeds and x-ray sensitivity in hexaploid oats

Effect of Recurrent X-Radiation on Germination and Seedling Vigor in Oats

The immediate effects of exposing oat seeds to high dosages of X-ray are generally twofold: (a) many of the seeds fail to germinate, and (b) the seedlings produced are stunted. It is generally accepted that these physiologic effects react the same as other environmental variation in biological materials. However, it is possible that even though the physiologic effects disappear in one generation that certain weaknesses may persist but remain hidden. Weakened linkages of some type within chromosomes could conceivably be a case in point. If this is the case, one might expect the effects to be cumulative with repeated x-ray treatment in successive generations, since the basic units affected are probably chromosomes and genes. The study reported herein was conducted to determine to what extent the X-ray damage to oat seeds, measured as germination percentage and seedling vigor, was cumulative with recurrent radiation in one, two, and three seed generations. The measures of seedling vigor were plant height and weight per 100 seedlings