H

hadron - particles that feel the strong nuclear force e.g. protons and neutrons
half life - time taken for the activity of a radioactive source to decrease by half
Hall effect - formation of potential across a current carrying conductor in a magnetic field
Hall probe - slice of semiconductor material used to measure steady magnetic fields
Hare's apparatus - apparatus used for comparing the densities of two liquids by column height
harmonic - note with a frequency an integral number of times the fundamental frequency
heat - a form of energy - radiant energy or kinetic energy of the molecules
heat sink - method of getting rid of excess heat often a piece of metal in electronic circuits
heavy water - water with two deuterium atoms, used as a moderator in some reactors
heliostat - sunlight collector in a solar furnace
helmholtz coils - pair of coils separated by a distance equal to their radius giving uniform field
helix - a three dimensional spiral as in a 'helical spring' (wrongly called a spiral spring)
henry - unit of inductance
hertz (Hz) - unit of frequency. 1 Hz = 1 vibration per second.
height - distance above a fixed point such as the ground
hoarfrost - immediate freezing of the water vapour in the air due to a rapid fall in temperature
hologram - a three dimensional image
Hooke's law - extension is directly proportional to load
Horsehead nebula - a dark nebula in Orion resembling a horse's head
horsepower - non-scientific unit of power = 746 W
HR diagram - Hertzsprung-Russell diagram – a plot of stars luminosity against their spectral type
HST - Hubble space telescope
hubble constant - a measure of the rate at which a galaxy is receding. Relates speed to distance.
Hubble's law - velocity of recession of a galaxy = Hubble constant x distance of galaxy
humidity - concentration of water vapour in the atmosphere
huygenian - type of telescope eyepiece, common in small refractors but slightly curved field
Hyades - open star cluster in Taurus
hydraulic - describes a machine that works by transmitting fluid pressure (hydraulic brakes)
hydrometer - instrument for measuring the density of a liquid
hydrostatics - the physics of fluids at rest
hygrometer - instrument for measuring the humidity of the air
hypermetropia - long sight. Long sighted people can only see distant objects clearly.
hypotenuse - the side opposite the right angle in a right angled triangle
hypothesis - an idea, suggestion, informed guess, prediction
hysteresis - energy 'loss' within a material due to a stress – mechanical, electrical, magnetic