AI-5 SKIN EFFECT—The tendency for alternating current to concentrate in the surface layer of a conductor. The effect increases with frequency and serves to increase the effective resistance of the conductor. SLOT—Narrow opening in a waveguide wall used to couple energy in or out of the waveguide. Also called an aperture or a window. SPROCKET TUNER—Mechanical tuning device for magnetron tubes that changes the frequency of the cavities by changing the inductance. Also called a crown-of-thorns tuner. STAGGER TUNING—A method of klystron tuning in which the resonant cavities are tuned to slightly different frequencies to increase the bandwidth of the amplifier. STANDING WAVE RATIO—The ratio of the maximum to the minimum amplitudes of corresponding components of a field, voltage, or current along a transmission line or waveguide in the direction of propagation measured at a given frequency. SYNCHRONOUS TUNING—In a klystron amplifier, a method of tuning which tunes all the resonant cavities to the same frequency. High gain is achieved, but the bandwidth is narrow. TRANSIT TIME—The time an electron takes to cross the distance between the cathode and anode. TRANSVERSE ELECTRIC MODE—The entire electric field in a waveguide is perpendicular to the wide dimension and the magnetic field is parallel to the length. Also called the TE mode. TRANSVERSE MAGNETIC MODE—The entire magnetic field in a waveguide is per- pendicular to the wide dimension ("a" wall) and some portion of the electric field is parallel to the length. Also called the TM mode. TUNNELING—The piercing of a potential barrier in a semiconductor by a particle (current carrier) that does not have sufficient energy to go over the barrier. TUNNEL DIODE—A heavily doped junction diode that has negative resistance in the forward direction over a portion of its operating range. See NEGATIVE-RESISTANCE ELEMENT. VARACTOR—A pn-junction semiconductor designed for microwave frequencies in which the capacitance varies with the applied bias voltage. VARIABLE ATTENUATOR—An attenuator for reducing the strength of an ac signal either continuously or in steps, without causing signal distortion. VELOCITY MODULATION—Modification of the velocity of an electron beam by the alternate acceleration and deceleration of electrons. VERTICAL PLANE—An imaginary plane that is perpendicular to the horizontal plane. WAVEGUIDE—A rectangular, circular, or elliptical metal pipe designed to transport electromagnetic waves through its interior. WAVEGUIDE MODE OF OPERATION—Particular field configuration in a waveguide that satisfies the boundary conditions. Usually divided into two broad types: the transverse electric (TE) and the transverse magnetic (TM).