## equinoctial Quick reference

### A Dictionary of Weights, Measures, and Units

...Relating to an equinox, i.e. one of the two points in the year when Earth's axis is perpendicular to the direction of the Sun, making day and night equal in length. For equinoctial year see year...

## foot⋅pound Quick reference

### A Dictionary of Weights, Measures, and Units

...form is the foot⋅poundal .) The composite name punctuated by a hyphen is misleading. The pound⋅foot , although having the same units and thereby interchangeable with foot⋅pound, is preferably used distinctively for torque. With torque, the applicable foot is measured perpendicular to the line of the force; with work it is along that...

## candela Quick reference

### A Dictionary of Weights, Measures, and Units

...the temperature of solidification of platinum is 60 new candles per square centimetre’. 1948 9th CGPM ‘candela’ adopted in place of (new) candle. 1967–68 13th CGPM: ‘ decides to express the definition of the candela as follows: The candela is the luminous intensity, in the perpendicular direction, of a surface of 1/600000 square metre of a black body at the temperature of freezing platinum under a pressure of 101 325 newtons per square metre’. 1979 16th CGPM: ‘ decides 1. The candela is the luminous intensity, in a given direction, of a source that emits...

## Nernst effect Quick reference

### A Dictionary of Physics (8 ed.)

...Nernst effect An effect in which a temperature gradient along an electric conductor or semiconductor placed in a perpendicular magnetic field, causes a potential difference to develop in the third perpendicular direction between opposite edges of the conductor. This effect, an analogue of the Hall effect , was discovered in 1886 by the German physicist Walter Nernst ( 1864–1941...

## proper motion Quick reference

### A Dictionary of Physics (8 ed.)

...proper motion The apparent angular motion of a star on the celestial sphere . This is motion in a direction that is perpendicular to the line of...

## gimbal Quick reference

### A Dictionary of Physics (8 ed.)

...gimbal A type of mount for an instrument (such as a gyroscope or compass) in which the instrument is free to rotate about two perpendicular...

## principal plane Quick reference

### A Dictionary of Physics (8 ed.)

...principal plane The plane that is perpendicular to the optical axis of a lens and that passes through the optical centre. A thick lens has two principal planes, each passing through a principal point...

## angle of reflection Quick reference

### A Dictionary of Physics (8 ed.)

...angle of reflection 1. The angle between a ray leaving a reflecting surface and the perpendicular (normal) to the surface at the point at which the ray leaves the surface. 2. The angle between a wavefront and a surface that it leaves....

## angle of incidence Quick reference

### A Dictionary of Physics (8 ed.)

...angle of incidence 1. The angle between a ray falling on a surface and the perpendicular (normal) to the surface at the point at which the ray strikes the surface. 2. The angle between a wavefront and a surface that it strikes....

## maxwell Quick reference

### A Dictionary of Physics (8 ed.)

...maxwell A c.g.s. unit of magnetic flux, equal to the flux through 1 square centimetre perpendicular to a magnetic field of 1 gauss. 1 maxwell is equal to 10 −8 weber. It is named after James Clerk Maxwell...

## parabola Quick reference

### A Dictionary of Physics (8 ed.)

...parabola A conic with eccentricity e =1. It is the locus of a point that moves so that its distance from the focus is equal to its perpendicular distance from the directrix . A chord through the focus, perpendicular to the axis, is called the latus rectum . For a parabola with its vertex at the origin, lying symmetrically about the x -axis, the equation is y 2 =4 ax , where a is the distance from the vertex to the focus. The directrix is the line x =− a , and the latus rectum is 4 a . Parabola. ...

## lines of force Quick reference

### A Dictionary of Physics (8 ed.)

...of the field to be visualized. They are used primarily in electric and magnetic fields; in electric fields they are sometimes called tubes of force , to express their characteristic of being perpendicular to a conducting surface. The tangent to a line of force at any point gives the direction of the field at that point and the number of lines per unit area perpendicular to the force represents the intensity of the...

## couple Quick reference

### A Dictionary of Physics (8 ed.)

...couple Two equal and opposite parallel forces applied to the same body that do not act in the same line. The forces create a torque, the moment of which is equal to the product of the force and the perpendicular distance between them. See moment of a force...

## angle of refraction Quick reference

### A Dictionary of Physics (8 ed.)

...angle of refraction 1. The angle between a ray that is refracted at a surface between two different media and the perpendicular (normal) to the surface at the point of refraction. 2. The angle between a wavefront and a surface at which it has been refracted....

## anisotropic Quick reference

### A Dictionary of Physics (8 ed.)

...anisotropic Denoting a medium in which certain physical properties are different in different directions. Wood, for instance, is an anisotropic material: its strength along the grain differs from that perpendicular to the grain. Single crystals that are not cubic are anisotropic with respect to some physical properties, such as the transmission of electromagnetic radiation. Compare isotropic...

## magnetic flux Quick reference

### A Dictionary of Physics (8 ed.)

...magnetic flux Symbol Φ . A measure of quantity of magnetism, taking account of the strength and the extent of a magnetic field . The flux d Φ through an element of area d A perpendicular to B is given by d Φ = B d A . The SI unit of magnetic flux is the...

## graph Quick reference

### A Dictionary of Physics (8 ed.)

...graph A diagram that illustrates the relationship between two variables. It usually consists of two perpendicular axes, calibrated in the units of the variables and crossing at a point called the origin . Points are plotted in the spaces between the axes and the points are joined to form a curve. See also Cartesian coordinates ; polar coordinates...

## quantum wire Quick reference

### A Dictionary of Physics (8 ed.)

...quantum wire A linear conductor that is narrow enough for quantum effects to affect the resistance. If the wire diameter is small then the electrons have quantized energy levels for motion perpendicular to the axis (as for a particle in a box ). Consequently the resistance of the wire is also quantized. Carbon nanotubes have been investigated as practical implementations of quantum...

## dihedral Quick reference

### A Dictionary of Physics (8 ed.)

...dihedral ( dihedron ) An angle formed by the intersection of two planes (e.g. two faces of a polyhedron). The dihedral angle is the angle formed by taking a point on the line of intersection and drawing two lines from this point, one in each plane, perpendicular to the line of...

## lever Quick reference

### A Dictionary of Physics (8 ed.)

...lever A simple machine consisting of a rigid bar pivoted about a fulcrum. The mechanical advantage or force ratio of a lever (the ratio of load to effort) is equal to the ratio of the perpendicular distance of the line of action of the effort from the fulcrum to the perpendicular distance of the line of action of the load from the fulcrum. In a first-order lever the fulcrum comes between load and effort. In a second-order lever the load comes between the fulcrum and the effort. In a third-order lever the effort comes between the fulcrum and the load. Levers...