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Allan C.G. Mitchell 《Journal of The Franklin Institute》1930,210(3):269-286
It has been shown that electrons with 11.5 volts energy can dissociate a hydrogen molecule into its two constituent atoms, but up until recently no experiments have been performed to see whether fast positive ions are able to dissociate hydrogen.In the present experiments Li or Cs positive ions of various velocities are produced in a tube containing hydrogen molecules. The tube is immersed in liquid air, and the rate of decrease of pressure of hydrogen is measured as a function of the velocity of the positive ions. It is found that with no positive ions entering the tube there is a certain decrease in the pressure of the hydrogen due to its thermal dissociation on the hot filament and its subsequent condensation on the cold walls of the tube. With ions of energies from 15 to 320 volts flowing in the tube the rate of decrease of pressure is greater, showing a formation of some condensible product due to the action of the ions.The rate of pressure decrease with the voltage applied has been found to be proportional to the ion current flowing. The rate per unit current is proportional to the pressure; and the rate per unit current per unit pressure is practically independent of the voltage for Li and Cs ions of energies from 15 to 320 volts.The effect has been shown not to be due to secondary electrons.The process may be due to the formation of alkali hydrides in the gas phase. The number of hydrogen molecules disappearing per positive ion entering the tube varies from 0.01 to 0.5. No critical potentials have been found in this experiment, in disagreement with recent experiments of Leipunsky and Schechter. 相似文献
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This case study of an advanced physics class analyzes the implications of a practice that creates opportunities for groups of science students to share thoughts and observations, defend viewpoints, and negotiate consensus about their thinking. The activity that facilitates such sense-making processes is computerized journaling. Because these documents are more than a static mechanism for organizing notes and observations, they are called learning logs. A system of analysis drawn from the literature on writing and learning in science was developed and systematically applied to characterize the formal and stylistic elements of writing contained in learning logs and to investigate changes in the nature and pattern of science discourse over time. These scientific conversations were found to support a dynamic classroom environment that helped learners to make sense of complex science topics. Writing in learning logs provides a vehicle for students and teachers to make their knowledge public, and builds an atmosphere for valuing the conceptual understanding of others. The journals also mediate an ongoing dialogue between the instructor and students, a feature that redefines traditional classroom roles and responsibilities. Conversing about science through the medium of computerized learning logs resulted in a closely knit community of reflective learners. © 1996 John Wiley & Sons, Inc. 相似文献