Lake Shore 372, Руководство пользователя, Страница: 39 / 214

2.5.1 Why is the Model 372 Called a Bridge? 27
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4
He reservoir: a reservoir of liquid 4 He at atmospheric pressure that provides
bulk cooling down to 4.2 K. The reservoir precools leads entering the vacuum
space, and it acts as a radiation shield for refrigeration components.
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Precooling stage: the precooling stage brings the system to a low enough tem-
perature for magnetic cooling to become practical. It also acts as a heat sink for
the first magnetic cooling stage. This can be a cryogenic refrigerator,
4 He bath,
pumped 4 He cryostat or pumped 3 He cryostat. This is a good location for a tem-
perature sensor for cool down monitoring and troubleshooting.
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Vacuum space: the vacuum space insulates the refrigeration components and
provides a clear workspace.
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Heat switch: this allows thermal contact between the precooling stage and salt
pill during magnetization then provides thermal isolation during demagnetiza-
tion (cooling). The construction of the switch varies depending on temperature
but could be superconducting, exchange gas (gas-gap) or mechanical. Several
types of heat switches can be controlled by the Model 372 still heater or analog
output.
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Salt pill: this is a mass of paramagnetic salt that acts as the refrigerant. It is mag-
netized while in thermal contact with the precooling stage so the heat of magne-
tization is removed and entropy is reduced. Then it is demagnetized while it is
thermally isolated and as field is decreased, the salt must decrease in tempera-
ture to maintain constant entropy. This is the best location for a temperature
control sensor. The Model 372 sample heater output can control temperature by
trimming current in the magnet with its heater output current source. Resistive
heating is not desirable.
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Superconducting magnet: the superconducting magnet magnetizes the salt pill
while in thermal contact with the precooling stage then demagnetizes it when
thermally isolated. The magnet power supply can be programmed by the
Model 372 analog output and controlled via computer interface.
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Load (sample holder): this is located at the end of the 3 He evaporator where
experiments can take place. This will require a separate temperature sensor if its
temperature is different from the
3 He evaporator.
2.5 Model 372
Theory of
Operation This section describes the theory for operation of the Model 372.
2.5.1 Why is the Model
372 Called a Bridge? Historically precision resistance measurements were made using one of several
bridge configurations that are a combination of known reference resistors and the
unknown being measured. Normally in a bridge, one of the known resistors is
trimmed until a null appears at the measurement point of the circuit and at that time
the unknown resistance can be calculated. The technique is preferred because it
relies only on resistance ratios and a null measurement, which eliminates many of
the errors normally associated with amplifiers, voltage references and current
sources. As electronic circuits improve, fewer and fewer applications require the com-
plexity of a bridge measurement.
Some applications benefit so much from the bridge configuration that they may
never be replaced. One example is strain gauge measurements that integrate both
measurement and temperature compensation into the bridge. Another application,
often seen in physics laboratories, is low-noise resistance measurement where the
reference resistors and unknown are all cooled to reduce thermal noise.