An ellipsoid; a figure resembling a sphere. Also called ELLIPSOID or ELLIPSOID OF REVOLUTION, from the fact that it can be formed by revolving an ellipse about one of its axes. If the shorter axis is used as the axis of revolution, an oblate spheroid results, and if the longer axis is used, a prolate spheroid results. The earth is approximately an oblate spheroid.
Related Terms 
DIRECTIVITY FACTOR

POLARIZEDVANE AMMETER
An ammeter of only moderate accuracy in which the current to be measured passes through a small coil, distorting the field of a circular permanent magnet, and an iron vane aligns itself with the axis of the distorted field, the deflection being roughly proportional to the current.

PLANE OF YAW
The plane determined by the tangent to the trajectory of a projectile in flight and the axis of the projectile.

PITCH AXIS
A lateral axis through an aircraft, missile, or similar body, about which the body pitches. Also known as pitching axis.

PITCH ATTITUDE
The attitude of an aircraft, rocket, or other flying vehicle, referred to the relationship between the longitudinal body axis and a chosen reference line or plane as seen from the side.

PIPEWORK SEE PIPING. PIPE WRENCH
A tool designed to grip and turn a pipe or rod about its axis in one direction only. piping

PERPENDICULAR AXIS THEOREM
Atheorem which states that the sum of the moments of inertia of a plane lamina about any two perpendicular axes in the plane of the lamina is equal to the moment of inertia about an axis through their intersection perpendicular to the lamina.

PENDULUM ANEMOMETER
A pressureplate anemometer consisting of a plate which is free to swing about a horizontal axis in its own plane above its center of gravity; the angular deflection of the plate is a function of the wind speed; this instrument is not used for station measurements because of the false reading which results when the frequency of the wind gusts and the natural frequency of the swinging plate coincide.

PARALLEL AXIS THEOREM
A theorem which states that the moment of inertia of a body about any given axis is the moment of inertia about a parallel axis through the center of mass, plus the moment of inertia that the body would have about the given axis if all the mass of the body were located at the center of mass.

NYQUIST CONTOUR
A directed closed path in the complex frequency plane used in constructing a Nyquist diagram, which runs upward, parallel to the whole length of the imaginary axis at an infinitesimal distance to the right of it, and returns from jn to jn along a semicircle of infinite radius in the right halfplane.
