Point discharge in atmospheric electricity

Investigations of the current from an earth-connected point 0.002 cm in diameter, supported by masts at heights of 20m, 27m end 34m are described. Simultaneous measurements of the potential gradient at the ground to windward of the point, and wind speed at the point, indicate that the current can be represented by the equation I = K(W + C) (F - M) where I is the point discharge current in mioroamps W the wind speed in metres per second F the potential gradient in volts per metre M the onset value of the potential gradient (200 v/m at 20m, 135 v/m at 27m, l00 v/m at 34m) C and K are constants (C = 4 m/s and K = 2.56 x 10(^-4)µa per v/m per m/s at 27m) The general equation still holds when the potential gradient is measured 7m below the 34m mast but at 2m below the 20m point the equation is I = A(W + D) (F - M)(^n) where A and D are constants is dependent on wind speed. Measurements of point discharge currents down the trunk of a tree indicate that these are somewhat lower than those through a single point of corresponding height in similar conditions. The results are compared with those of previous workers and discrepancies are attributed to wind speed and wind direction effects, whilst good agreement is found between the present findings and the theoretical work of Chalmers and Mapleson (1955) and Chapman (1956). A reassessment of the Alti-Electrograph results of Simpson and scrase (1937) is made and suggests that the potential gradients measured by this means immediately below thunderclouds are of the earns order of magnitude as those measured by observers in aircraft (Gunnl953)

Mammalian Sperm Motility: Observation and Theory

Mammalian spermatozoa motility is a subject of growing importance because of rising human infertility and the possibility of improving animal breeding. We highlight opportunities for fluid and continuum dynamics to provide novel insights concerning the mechanics of these specialized cells, especially during their remarkable journey to the egg. The biological structure of the motile sperm appendage, the flagellum, is described and placed in the context of the mechanics underlying the migration of mammalian sperm through the numerous environments of the female reproductive tract. This process demands certain specific changes to flagellar movement and motility for which further mechanical insight would be valuable, although this requires improved modeling capabilities, particularly to increase our understanding of sperm progression in vivo. We summarize current theoretical studies, highlighting the synergistic combination of imaging and theory in exploring sperm motility, and discuss the challenges for future observational and theoretical studies in understanding the underlying mechanics.\ud Acronyms and Definitions\ud Acrosome: the cap of the sperm head containing enzymes allowing penetration of the zona pellucida via the acrosome reaction\ud Adenosine triphosphate (ATP): the currency unit of chemical energy transfer in living cells\ud Axoneme: a phylogenetically conserved structure within the eukaryotic flagellum consisting of a ring of nine microtubule doublets and a central pair, frequently referred to as 9 + 2\ud Bending moment density: the moment per unit length associated with flagellar bending; it can be divided into a hydrodynamic moment, an elastic moment (from the flagellar bending stiffness), an active moment (generated by dyneins exerting forces between adjacent microtubule doublets), and a passive moment resisting shear\ud Capacitation: the physiological state of a sperm required for fertilization, which is accompanied by the motility patterns associated with hyperactivation, characterized in saline by high-amplitude asymmetric beating\ud Central pair: a pair of microtubules along the length of the axoneme, symmetrically and slightly offset from the axoneme centerline\ud Cumulus oophorus: the outer vestment of the mammalian egg consisting of hundreds of cells radiating out from the egg embedded within a non-Newtonian hyaluronic acid gel\ud Dynein: a molecular motor within the axoneme, attached between adjacent microtubule doublets, that exerts a shearing force to induce axonemal bending\ud Flagellum: a motile cellular appendage that drives the swimming of sperm and other cells; this article focuses on the eukaryotic flagellum\ud Microtubule doublet: a pair of proteinaceous filament structures running the length of the axoneme; dyneins drive their bending, which induces flagellar motion\ud Mid-piece: the region of a sperm flagellum with a mitochondrial sheath, where ATP is generated\ud Oocyte: the egg\ud Outer dense fibers and fibrous sheath: accessory structures reinforcing the mammalian sperm flagellum; the combined axoneme and accessory structures are referred to as 9+9+2\ud Resistive-force theory: an approximation for the local drag of a slender filament element in Stokes flow (or a viscoelastic generalization thereof)\ud Rheotaxis: directed motility in response to the influence of fluid flow\ud Shear: in the context of the flagellum, the relative movement of adjacent microtubule doublets\ud Slender-body theory: an improved approximation for the local drag on a slender filament element in Stokes flow (or a viscoelastic generalization thereof)\ud Zona pellucida: a tough glycoprotein coat between the human egg and the cumulus oophorus, which a sperm must penetrate for successful fertilizatio