Spiral Filaments Research Articles

The distribution of thin films condensed on surfaces by the vacuum evaporation method

The problem of determining the laws of distribution of thin film deposits evaporated in vacuum is discussed in detail. From various types of practical sources, idealised sources are evolved for which the emission of vapour can be calculated. As a fundamental law, the ‘cosine law’ of Knudsen is recognised. The calculation is extended to point sources, long strip sources, wire sources and ring sources. Evaporation on to plane and spherical receiving surfaces is considered. The practical application of these results to the evaporation of zinc sulphide and magnesium fluoride from strip heaters, and aluminium from spiral filaments is considered together with experimental confirmation. The range of validity of the results is discussed.

Problems in Radiation Therapy

Part I.—Production of X-Rays BEGINNING with the terminals of the switch-box and ending with the x-ray target, an attempt will be made to give important intervening steps in the production of x-rays. X-ray Tubes.—An x-ray tube is a glass envelope, or bulb, containing a filament and anode sealed into the glass and so placed that the filament is opposite the anode. In order to produce x-ray tubes which will withstand the bombardment of a target with fast moving electrons, all metallic parts before being sealed into the x-ray tube must be treated with heat in a vacuum furnace. The x-ray tube, containing all parts sealed into the bulb, is then placed in a furnace and the bulb sealed to a vacuum pumping system. The temperature of the furnace is increased and maintained at a point just below the temperature at which glass begins to soften. The temperature is maintained for a matter of hours in order to liberate the occluded gases of the glass walls. The terminals of the x-ray tube are connected to a high voltage energizing unit, and as the voltage is increased on the target, the gases liberated by the hot filament, the target and any occluded gases from the walls of the x-ray tube are pumped out of the bulb. This process is kept up, gradually increasing the voltage on the target, until there is no more gas liberated after prolonged operation at voltages and milliampere currents greater than the ratings of the x-ray tube. Cathode.—The size of the focal spot is governed by the diameter and shape of the spiral filament and also the relative position of these with respect to the target. In a fine focus tube, the filament is placed farther back in the focussing cylinder. The focus of x-ray tubes designed for therapy have been made divergent by Coolidge in projecting a molybdenum pin through the center of the filament. The line focus with an anode face at 20 degrees instead of the conventional 45 degrees, makes possible an x-ray tube of greater capacity with the retention of the fine definition of a smaller round focus tube. Metal disks about the filament are intended to intercept stray electrons from striking the glass. Choice of Anode.—Less than 1 per cent of the kinetic energy (½ mv2) is consumed in producing x-rays by the impact of electrons at the face of the target of an x-ray tube, and more than 99 per cent of the kinetic energy goes into heat. The capacity of an x-ray tube depends upon the dissipation of heat from the target. A fine focus x-ray tube will not have as high rating as to voltage and milliampere current as a broad focus tube. In a fine focus tube, the heat is concentrated over a smaller area of the target than in a broad focus tube. An increase of voltage will cause a greater quantity of heat to be developed in the target because the velocity of the electrons is increased. As the milliampere current is increased, a greater number of electrons strike the target, hence more heat is developed.

Bombardment of Metal Surfaces by Slow-Moving Electrons

Secondary Electrons Produced by Electronic Bombardment of Nickel.---The main features of the apparatus used are: an equi-potential nitrate-coated Pt cathode heated by radiation from a tungsten spiral filament; a series of insulated diaphragms to limit the beam of primary rays and for use in determining their velocity; and a long Faraday cylinder in front of which the nickel target could be alternately interposed and withdrawn. By baking out the tube and using liquid air traps an extremely low vacuum corresponding to less than ${10}^{\ensuremath{-}7}$ mm. was attained. The secondary current was determined from the difference between the current to the Faraday cylinder and that to the interposed target. (I) Ratio of the secondary (emergent) to the primary (incident) electron current was found to be independent of the roughness of the surface, but to vary with the treatment. After heating the target red-hot by high frequency induction for some minutes, a limiting curve was reached which probably represents the characteristics of nickel itself, free from surface contamination. In this case the secondary electrons begin when the primary velocity is as low as 0.2 volt; the ratio to primary current then increases rapidly with primary velocity to about 4 volts, remains constant to about 9 volts, then again increases reaching a value of unity for a primary velocity corresponding to about 260 volts. The effect of exposure to air or hydrogen is to increase very considerably the secondary current and to round out the flat part of the curve between 4 and 9 volts. (2) Velocity distribution curves of the secondary electrons indicate that for primary electrons of less than 9 volts velocity, most of the secondary electrons have velocities nearly equal to the primary velocity, while for primary velocities above 9 volts, the percentage of secondary electrons having small velocities increases with the primary velocity, although a small proportion have velocities nearly equal to the primary velocity up to at least 110 volts.Reflection and Emission of Electrons from a Nickel Surface Bombarded with Electrons of Velocity 0 to 260 Volts.---It is suggested by the above results, that reflection occurs for all the primary velocities investigated, and that emission or ionization begins at about 9 volts and increases with the primary velocity.Contact difference of potential between a gas-free nickel surface and an ordinary baked nickel surface was found to be 0.8 volt.Method of making a gas-tight glass joint for high vacuum work by soldering with Wood's metal, the platinized and copper-coated surfaces of a ground joint, is described.

1. On animal and vegetable fibre as originally composed of twin spiral filaments, in which every other structure has its origin; a note showing the confirmation by Agardh, in 1852, of observations recorded in the philosophical transactions for 1842

After referring to the drawings to his paper on Fibre, published in the Philosophical Transactions for 1842, and the opinions entertained by physiologists regarding the peculiar views lie advanced in that paper with, reference to the original composition of organic fibre, the author states that, after the lapse of eleven years, these views have been fully confirmed, and in proof of this refers to a paper— “De cellula vegetabili fibrillis tenuissimis contexta” (Lundæ, 1852), by Agardh.

Mikboskofische bt.tckf. in den inneren bad nvn das leben dee gewächse, &C. (Microscopical Glance into the Intimate Structure and Life of Plants. In the form of Popular Lectures.)

ABSTRACT The whole is contained in four discourses. In the first the author compares the different modes in which natural history, or the idea contained in it, is comprehended by different classes of minds. To the one it is simply a chamber of mystery—to another a study—to a third a means of climbing to ease and renown—to a fourth a picture-book. To all, he says, it ought to be a “beautiful maternal home, to be a stranger in which should be regarded, in any one, as a loss and shame.” He touches upon the question whether natural history is a real or merely a humanistic science, and decides in favour of the latter, seeing that the homo is still a part of the kingdom of nature—a convenient, but surely a dangerous argument ! He lauds the value of the microscope in the natural sciences ; and then proceeds, in more immediate application to his subject, to show, that plants consist of cells., explaining what is meant by the latter. In the second discourse the theme is continued ; the peculiar forms of cells are described, and then the cell-contents : colouring matter, crystals, nucleus, starch, and the spiral filaments. The ensuing chapter discourses about the vessels and their modifications, and then adverts to the tissues formed immediately from the elementary organs, such as the cuticle with its stomata, hairs, setæ, and scales ; to the structure of the leaf and ligneous stem in the mono- and dicotyledonous plants. The fourth chapter embraces the distinction between animals and plants, with respect to individuality and their various vital actions ; the nutrition of plants and their importance with respect to the habitability of the earth ; and lastly, the alternating relation between animals and plants. The last discourse treats of the root, the changes of matter effected in the interior of plants, the ascent and descent of the nutritive sap, and finally, of the parts belonging to fructification, and of fructification itself.