18WuD2x

ohmic contact • oil diffusion pump 491
bilateral linear conduction. It exhibits none of the
properties of a rectifying junction or a nonlinear
resistance.
ohmic heating 1. Heating caused by current passing
through a resistive material (i.e., I 2R losses in
the material). 2. In an electric field, heat generated
by charged particles when they collide with
other particles.
ohmic loss Loss resulting from the direct-current
resistance in a circuit or transmission line.
ohmic region The portion of the response curve of
a negative-resistance device that exhibits positive
(ohmic) resistance. The E-I curve of a tunnel
diode, for example, has two such positiveslope
regions with a negative-slope (negativeresistance)
region between them.
Zero
adjust
Voltmeter
mA
Range
switch
+ −
Unknown
resistance
ohmmeter
Current
0
Ohmic
region
Nonohmic
(negative-resistance)
region
Ohmic
region
Voltage
ohmic region
ohmic resistance A resistance exhibiting OHMIC
RESPONSE.
ohmic response Response that follows OHM’S
LAW: I = E/R. In strictly ohmic devices, neither
resistance nor reactance changes with current or
voltage. Compare NONOHMIC RESPONSE.
ohmic value Electrical resistance expressed in
ohms, or in multiples or fractions of ohms (kilohms,
megohms, milliohms, etc.)
ohmmeter An instrument for the direct measurement
of electrical resistance. It usually consists
of a milliammeter or microammeter, a battery,
and several switchable resistors having very close
tolerances. The scale is calibrated in ohms; the
switch selects multiplier factors (e.g., ×100,
×10,000, ×1,000,000). The scale is usually reversed
(i.e., 0 is at the extreme right and “infinity”
is at the extreme left).
ohmmeter zero 1. The condition of proper adjustment
of an ohmmeter, indicating zero resistance
for a direct short circuit. 2. The potentiometer, or
other control, used for adjusting an ohmmeter to
obtain a reading of zero with a short circuit.
ohm-mile A rating meaning 1 mile of wire having a
resistance of 1 ohm.
ohms adjust The rheostat or potentiometer used to
set the pointer of an ohmmeter before it is used to
take resistance readings.
Ohm’s law The statement of the relationship
among current, voltage, and resistance. In a
direct-current circuit, current varies directly
with voltage and inversely with resistance: I = E/R,
where I is the current in amperes, E is the voltage
in volts, and R is the resistance in ohms. For
alternating current, Ohm’s law states that I =
E/X = E/Z, where X is reactance and Z is
impedance.
ohms per square The resistance (in ohms) between
two parallel edges of a square of thin-film resistance
material.
ohms-per-volt A specification that indicates the
sensitivity and impedance of a voltmeter. In general,
the higher the rating, the better. When measuring
voltage in high-impedance circuits, the
rating should be as high as possible. Field-effecttransistor
(FET) voltmeters and vacuum-tube
voltmeters have the highest ratings.
oil-burner control An electronic system for starting
and stopping the operation of an oil burner to
prevent puffback and to interrupt the supply
when the flame becomes erratic.
oil calorimeter A calorimeter used to measure
power in terms of the rise in temperature of oil
heated by the electrical energy of interest.
oil capacitor A capacitor impregnated or filled
with oil, such as high-grade castor or mineral oil.
Also see OIL DIELECTRIC.
oil circuit breaker A circuit breaker filled with a
high-grade insulating oil for cooling and arc elimination.
oil-cooled transformer A heavy-duty transformer,
through which oil is circulated for heat removal
and arc prevention.
oil dielectric A highly refined oil used as an electrical
insulating material (e.g., between the plates
of a capacitor). Familiar examples are castor oil,
mineral oil, and the synthetic oil chlorinated
diphenyl.
oil diffusion pump See OIL PUMP.