18WuD2x

HSM • Huygens’ principle 347
HSM Abbreviation of high-speed memory.
HTL Abbreviation of HIGH-THRESHOLD LOGIC.
hub The hole in the center of a magnetic tape reel.
hue The quality of having a particular visible
color; an attribute that depends on the wavelength(s)
of light emitted or reflected. Thus, electromagnetic
energy at a wavelength of 700
nanometers (nm) has a red hue; energy at a
wavelength of 400 nm has a violet hue. Compare
SATURATION, 2.
hue control Also called tint control. In a color television
receiver, a control that allows adjustment
of the color wavelength, but does not affect the
saturation (intensity).
hum 1. Alternating current having a frequency of 60
Hz. 2. Residual ripple in the output of a power supply,
having a frequency of 60 Hz or 120 Hz, depending
on the type of rectifier circuit used. 3. An
electromagnetic field of long wavelength, usually
originating from utility lines and having a primary
frequency of 60 Hz. 4. The effects of low-frequency
electromagnetic fields or currents, such as moving
horizontal bars on a television screen. 5. An acoustic
disturbance of long wavelength (low pitch).
human engineering 1. Also called user-friendliness.
The extent to which an electronic or electromechanical
device or system is easy to
operate. 2. The branch of engineering devoted to
interfacing human beings with the machines and
instruments they operate. Both a science and an
art, the discipline is concerned with the safest
and most efficient design, arrangement, and operation
of equipment.
human interface The interface between a sophisticated
electronic device and a human operator.
human-made interference Also called artificial interference.
Electromagnetic interference to radio
and television receiving systems or to data terminals,
originating from artificial sources (such as
radio transmitters, certain electrical appliances,
and internal-combustion engines). Of the numerous
different forms, some affect only a narrow
band of frequencies or set of frequencies, and
others affect a wide band of frequencies.
humanoid robot A robot that bears structural resemblance
to a human being (e.g., has arms, a
head, and perhaps legs). In its most advanced
form, such a robot is an ANDROID.
hum-balance potentiometer A potentiometer
connected across an alternating-current power
supply, with its slider grounded. At a certain setting,
hum interference is nulled.
hum bars See HORIZONTAL HUM BARS.
hum bucking The reduction of hum interference
by introducing an alternating-current voltage of
the same frequency and amplitude as the hum,
but opposite in phase.
hum-bucking coil An auxiliary coil used in conjunction
with the field and voice coils of an electrodynamic
speaker. Reduces hum interference
via HUM BUCKING.
hum field The magnetic field surrounding a conductor
carrying hum-frequency alternating current.
humidity The amount of moisture in the air. Also
see ABSOLUTE HUMIDITY and RELATIVE HU-
MIDITY.
humidity meter See ELECTRIC HYGROMETER
and ELECTRONIC HYGROMETER.
humidity sensor A pickup whose resistance or capacitance
varies proportionally with ambient humidity.
hum interference Electrical interference resulting
from HUM in any of its various electrical forms
(see HUM, 1–4).
hum loop A ground loop that results in undesired
hum in the output of an amplifier.
hummer A nonelectronic audio oscillator similar to
the fork oscillator, but using a thick, metal reed,
instead of a tuning fork. A carbon microphone
button attached to the reed provides the feedback
path necessary for sustained oscillation.
Reed
Carbon
microphone
button
hummer
AF output
hum modulation Undesirable modulation of a radio
signal or audio amplifier output signal by
HUM interference.
hump 1. Either the positive or the negative halfcycle
of a sine wave; that is, either the portion
from 0 degrees to 180 degrees, or the portion from
180 degrees to 360 degrees. 2. A curve whose
graph is a multiple of the sine function, and
whose domain is restricted to a half-cycle (0 to
180 degrees or 180 to 360 degrees). See, for example,
DOUBLE-HUMP RESONANCE CURVE
and DOUBLE-HUMP WAVE.
hunting An undesirable condition in which an
electrical or electromechanical system oscillates
back and forth, relative to some mean mode of
operation (“hunts” for the mode), sometimes
eventually settling down at the mode. It can occur
in a servo system because of overcompensation,
lack of hysteresis, or both.
Huygens’ principle The observation that an advancing
wave is the resultant of secondary waves