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Determination of size of colloidal particles by means of alternating current fields
664
CURRENT TOPICS.
[J. F. L
was excited by picking up the waves originating at the d,iaphragm. So long as the periodicity of these was below the natural period of the membrane there was attraction of the latter toward the diaphragm. When the frequency was above that of the membrane, repulsion took place. When the waves had just the natural frequency of the membrane, neither attraction nor repulsion was found. The relation between force and frequency was discovered to depart from the curve predicted on the assumption that air is incompressible. A force of repulsion seems to be superimposed on the forces of attraction and repulsion. The experiments are being repeated with the substitution of a resonator for the membrane. The forces dealt with are of the order of i/IOO dyne per sq. cm. G.F.S.
Determination of Size of Colloidal Particles by Means of Alternating Current Fields. E . F . BURTON and MISS B. M. REID. ( P h i l . . M a g . , Dec., I 9 2 5 . ) - - I n a colloidal solution the particles in suspension do not settle to the bottom as they might be expected to do, when they have a density greater than that of the surrounding liquid. It is the effect of the Brownian movement that contravenes the downward pull of the earth, if it is still permitted thus to speak in these days of the Einstein theory of gravitation. For any given material forming the suspended particles there is a certain size for which the velocity due to gravity is equal to that caused by the Brownian movement. " F o r all particles smaller than this the Brownian movement masks entirely the gravitational settling, and consequently the suspensions are permanent; for example, there exists at the present time in the Royal Institution, London, samples of gold colloidal solutions in water prepared some seventy-five years ago by Faraday." The aim of this paper is to present a method by which the motion of settling can be segregated and measured. This is accomplished by applying a vertical electrical field to the colloidal solution, whose particles have electrical charges. When this field is so directed as to exert a downward force on the particles, it is augmented by the pull of gravity on them and a particle in a given time moves downward through a certain distance as a result of the joint action of the two forces against the opposition of the Brownian movement. Let the electrical field be reversed in direction and applied for the same length of time as before. The electrical force now opposes the force of gravity and a particle will move upward, provided the former is the stronger. The Brownian movement interferes just as before. " The difference between the up-and-down motion will then be due to the settling caused by gravity, and to this motion Stokes' law can be applied." From this the radius of the particle can be calculated'. It should be emphasized that the applied field must be sufficiently strong to impart a velocity to the particles much greater than that which they derive from either gravity or the Brownian movement. A series of copper sols with particles of different sizes was
May, 1926.]
CURRENT TOPICS.
66 5
prepared. One such colloidal solution was introduced into the lower part of a U-tube. Above it stood, water of the same specific conductivity as the solution. A sharp boundary separated the two liquids. The electrical field was supplied by an 8o-volt storage battery joined to electrodes dipping into the water. This field was reversed every 9 ° seconds automatically and it was applied for seven hours in an experiment. The settling of the boundary during this interval ranged from .225 to .325 cm. and the corresponding radii of the particles were computed to be 7.I4 x lO-6 and 8.60 x To" cm. When somewhat larger particles were used in the colloidal solution it was found that coagulation soon took place " showing that the size of the particles had increased to such an extent that the Brownian movement no longer masked the effect of gravity." G.F.S. Coal-dust E x p l o s i o n s . - - T h e use of powdered coal is increasing. The hazards involved in it having proved, serious, they have been extensively investigated by the U. S. Bureau of ~[ines. In a recentlv issued Bulletin (242) of that bureau a summary is given of both experiments, observations and investigations of accidents. Ic has long been recognized that combustible dusts of all types are capable of causing violent explosions, even aluminum dust having this property. The Bureau of Mines discovered some years ago that starch dust in admixture with air is a most powerful explosive. Powdered coal offers considerable advantage as fuel. It has been found especially applicable in plants that have been originally established to operate with natural gas, when the supply of this has declined, since the furnaces are not adapted to the usual form of coal. The application of powdered coal is useful when a very high temperature is required. As the supply of natural gas may be expected to decline, the use of powdered fuel may be expected to increase and the fire and explosion hazards will become more and more important. The average fineness is such that 93 per cent. will pass through a hundred-mesh screen and 71 per cent. through one of 2oo mesh. Coal which is retained on a 2o-mesh screen is not a cause of explosion. O f course, the coal even of the most extreme fineness is not explosive in bulk, but only when thoroughly mixed with air. On the other hand, such coal is liable to spontaneous combustion. The liability to explosion will depend not merely on the fineness of the grains, but upon the percentage of inert matter. The amount of incombustible matter in the material that will render it non-explosive is directly proportional to the ratio of the volatile matter to the total combustible, and is expressed by 4ividing the per cent. of volatile matter by the same datum plus the fixed carbon, but the rule will not apply to a sample in which the ash of composition vlus the inert matter equals 9° per cent. The hazards in the use of powdered fuel are by no means limited to the use in furnaces. Accumulations of unused material in bins and on ledges of buildings, dust arising in the loading, unloading and other transfer of the fuel, set up dangerous conditions and many instances
CURRENT TOPICS.
[J. F. L
was excited by picking up the waves originating at the d,iaphragm. So long as the periodicity of these was below the natural period of the membrane there was attraction of the latter toward the diaphragm. When the frequency was above that of the membrane, repulsion took place. When the waves had just the natural frequency of the membrane, neither attraction nor repulsion was found. The relation between force and frequency was discovered to depart from the curve predicted on the assumption that air is incompressible. A force of repulsion seems to be superimposed on the forces of attraction and repulsion. The experiments are being repeated with the substitution of a resonator for the membrane. The forces dealt with are of the order of i/IOO dyne per sq. cm. G.F.S.
Determination of Size of Colloidal Particles by Means of Alternating Current Fields. E . F . BURTON and MISS B. M. REID. ( P h i l . . M a g . , Dec., I 9 2 5 . ) - - I n a colloidal solution the particles in suspension do not settle to the bottom as they might be expected to do, when they have a density greater than that of the surrounding liquid. It is the effect of the Brownian movement that contravenes the downward pull of the earth, if it is still permitted thus to speak in these days of the Einstein theory of gravitation. For any given material forming the suspended particles there is a certain size for which the velocity due to gravity is equal to that caused by the Brownian movement. " F o r all particles smaller than this the Brownian movement masks entirely the gravitational settling, and consequently the suspensions are permanent; for example, there exists at the present time in the Royal Institution, London, samples of gold colloidal solutions in water prepared some seventy-five years ago by Faraday." The aim of this paper is to present a method by which the motion of settling can be segregated and measured. This is accomplished by applying a vertical electrical field to the colloidal solution, whose particles have electrical charges. When this field is so directed as to exert a downward force on the particles, it is augmented by the pull of gravity on them and a particle in a given time moves downward through a certain distance as a result of the joint action of the two forces against the opposition of the Brownian movement. Let the electrical field be reversed in direction and applied for the same length of time as before. The electrical force now opposes the force of gravity and a particle will move upward, provided the former is the stronger. The Brownian movement interferes just as before. " The difference between the up-and-down motion will then be due to the settling caused by gravity, and to this motion Stokes' law can be applied." From this the radius of the particle can be calculated'. It should be emphasized that the applied field must be sufficiently strong to impart a velocity to the particles much greater than that which they derive from either gravity or the Brownian movement. A series of copper sols with particles of different sizes was
May, 1926.]
CURRENT TOPICS.
66 5
prepared. One such colloidal solution was introduced into the lower part of a U-tube. Above it stood, water of the same specific conductivity as the solution. A sharp boundary separated the two liquids. The electrical field was supplied by an 8o-volt storage battery joined to electrodes dipping into the water. This field was reversed every 9 ° seconds automatically and it was applied for seven hours in an experiment. The settling of the boundary during this interval ranged from .225 to .325 cm. and the corresponding radii of the particles were computed to be 7.I4 x lO-6 and 8.60 x To" cm. When somewhat larger particles were used in the colloidal solution it was found that coagulation soon took place " showing that the size of the particles had increased to such an extent that the Brownian movement no longer masked the effect of gravity." G.F.S. Coal-dust E x p l o s i o n s . - - T h e use of powdered coal is increasing. The hazards involved in it having proved, serious, they have been extensively investigated by the U. S. Bureau of ~[ines. In a recentlv issued Bulletin (242) of that bureau a summary is given of both experiments, observations and investigations of accidents. Ic has long been recognized that combustible dusts of all types are capable of causing violent explosions, even aluminum dust having this property. The Bureau of Mines discovered some years ago that starch dust in admixture with air is a most powerful explosive. Powdered coal offers considerable advantage as fuel. It has been found especially applicable in plants that have been originally established to operate with natural gas, when the supply of this has declined, since the furnaces are not adapted to the usual form of coal. The application of powdered coal is useful when a very high temperature is required. As the supply of natural gas may be expected to decline, the use of powdered fuel may be expected to increase and the fire and explosion hazards will become more and more important. The average fineness is such that 93 per cent. will pass through a hundred-mesh screen and 71 per cent. through one of 2oo mesh. Coal which is retained on a 2o-mesh screen is not a cause of explosion. O f course, the coal even of the most extreme fineness is not explosive in bulk, but only when thoroughly mixed with air. On the other hand, such coal is liable to spontaneous combustion. The liability to explosion will depend not merely on the fineness of the grains, but upon the percentage of inert matter. The amount of incombustible matter in the material that will render it non-explosive is directly proportional to the ratio of the volatile matter to the total combustible, and is expressed by 4ividing the per cent. of volatile matter by the same datum plus the fixed carbon, but the rule will not apply to a sample in which the ash of composition vlus the inert matter equals 9° per cent. The hazards in the use of powdered fuel are by no means limited to the use in furnaces. Accumulations of unused material in bins and on ledges of buildings, dust arising in the loading, unloading and other transfer of the fuel, set up dangerous conditions and many instances