David L. Johnson

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The essential element, manganese, can produce chronic neuromotor impairment related to basal ganglia (BG) damage when it is presented in excessive quantities. The uptake and elimination patterns of manganese following ingestion have been well studied and, under normal conditions, excretion appears to keep manganese levels under tight control. Less is known(More)
Experiments on isotropic compression of a granular assembly of spheres show that the shear and bulk moduli vary with the confining pressure faster than the 1/3 power law predicted by Hertz-Mindlin effective medium theories of contact elasticity. Moreover, the ratio between the moduli is found to be larger than the prediction of the elastic theory by a(More)
Global positioning system (GPS) data recorders were worn by subjects in the Oklahoma Urban Air Toxics Study (OUATS) for automatic logging of their location as they went about their normal daily activities. The location information obtained by the GPS units had an uncertainty of about 10-20 m, which was sufficiently precise to track subjects' movements on(More)
3D computer simulations and experiments are employed to study random packings of compressible spherical grains under external confining stress. In the rigid ball limit, we find a continuous transition in which the stress vanishes as f 2 fc b , where f is the (solid phase) volume density. The value of fc depends on whether the grains interact via only normal(More)
Little research has focused on treatment of cows with subclinical mastitis during lactation. Ceftiofur is a new broad-spectrum, third-generation cephalosporin antibiotic for veterinary use that inhibits bacterial cell wall synthesis by interfering with enzymes essential for peptidoglycan synthesis. Ceftiofur should be effective against a wide range of(More)
While Internet connectivity has reached a significant part of the world's population, those living in rural areas of the developing world are still largely disconnected. Recent efforts have provided Internet connectivity to a growing number of remote locations, yet Internet traffic demands cause many of these networks to fail to deliver basic quality of(More)
Experimentally it is known that the bulk modulus K and shear modulus m of a granular assembly of elastic spheres increase with pressure p faster than the p1 3 law predicted by effective medium theory (EMT) based on Hertz-Mindlin contact forces. To understand the origin of these discrepancies, we perform numerical simulations of granular aggregates under(More)
The theory of the dynamic bulk modulus, K̃(v), of a porous rock, whose saturation occurs in patches of 100% saturation each of two different fluids, is developed within the context of the quasi-static Biot theory. The theory describes the crossover from the Biot–Gassmann–Woods result at low frequencies to the Biot–Gassmann–Hill result at high. Exact results(More)
A teat disinfectant containing a phenolic combination was evaluated in a natural exposure study in two dairy research herds. Premilking teat disinfection was compared with a negative control using a split-udder experimental design. In both herds, premilking and postmilking teat disinfections with the phenolic combination were significantly more effective in(More)