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74 ⏐⏐⏐ RESISTANCE
ρ
Resistivity
0
Tc Superconductor
Conventional conductor
FIG. 3.16
Defining the critical temperature T c.
T (K)
Even though ceramic compounds have established higher transition
temperatures, there is concern about their brittleness and current density
limitations. In the area of integrated circuit manufacturing, current density
levels must equal or exceed 1 MA/cm 2 , or 1 million amperes
through a cross-sectional area about one-half the size of a dime.
Recently IBM attained a level of 4 MA/cm 2 at 77 K, permitting the use
of superconductors in the design of some new-generation, high-speed
computers.
Although room-temperature success has not been attained, there are
numerous applications for some of the superconductors developed thus
far. It is simply a matter of balancing the additional cost against the
results obtained or deciding whether any results at all can be obtained
without the use of this zero-resistance state. Some research efforts
require high-energy accelerators or strong magnets attainable only with
superconductive materials. Superconductivity is currently applied in the
design of 300-mi/h Meglev trains (trains that ride on a cushion of air
established by opposite magnetic poles), in powerful motors and generators,
in nuclear magnetic resonance imaging systems to obtain crosssectional
images of the brain (and other parts of the body), in the design
of computers with operating speeds four times that of conventional systems,
and in improved power distribution systems.
The range of future uses for superconductors is a function of how
much success physicists have in raising the operating temperature and
how well they can utilize the successes obtained thus far. However, it
would appear that it is only a matter of time (the eternal optimist)
before magnetically levitated trains increase in number, improved medical
diagnostic equipment is available, computers operate at much
higher speeds, high-efficiency power and storage systems are available,
and transmission systems operate at very high efficiency levels due to
this area of developing interest. Only time will reveal the impact that
this new direction will have on the quality of life.
3.7 TYPES OF RESISTORS
Fixed Resistors
Resistors are made in many forms, but all belong in either of two
groups: fixed or variable. The most common of the low-wattage, fixed-
R
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