Cold junction compensation in a thermocouple measurement is needed because the emf depends on
Answer: C
A reading is meaningless unless the reference junction temperature is known. Either it is held in melting ice or its temperature is measured by a separate sensor and an equivalent emf added electronically, which is how a modern instrument does it.
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A thermocouple of the type used for the highest temperatures is made of
Answer: D
The noble metal pair withstands over sixteen hundred degrees and is very stable, which is why it defines part of the temperature scale. Its output per degree is small and it is expensive, so base metal pairs are used wherever they will serve.
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The output of a thermocouple is of the order of
Answer: A
Around forty microvolts per degree is typical of a base metal pair, so a hundred degrees gives four millivolts. That is small enough to demand a low-drift amplifier and careful attention to hum and thermal offsets in the leads.
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A photoconductive cell such as one of cadmium sulphide responds to light by
Answer: B
Photons free carriers within the material, so its conductivity rises with illumination. It is a passive device requiring a supply, and it is slow to respond compared with a photodiode, which limits it to lighting control and similar undemanding work.
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A photovoltaic cell is an active transducer because it
Answer: C
Light absorbed at a junction separates carriers and drives a current through an external circuit, so the device supplies power rather than consuming it. The solar cell is the same device used for generation instead of for measurement.
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A photoemissive device differs from a photoconductive one in that electrons are
Answer: D
The photoelectric effect proper requires a photon energetic enough to give an electron the work function of the surface, so there is a threshold wavelength. The photomultiplier tube built on this principle remains the most sensitive detector of very weak light.
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A Hall effect transducer produces a voltage proportional to
Answer: A
Carriers moving along the element are deflected sideways by the field and pile up on one face, giving a transverse voltage proportional to both quantities. It measures magnetic field, and with a known field it measures current instead.
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A Hall effect current sensor has the advantage over a shunt that it
Answer: B
Nothing is inserted in the circuit and there is no galvanic connection, yet unlike a current transformer it responds to a steady current. Both a supply and temperature compensation are needed, which is the price of those advantages.
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The sensitivity of a transducer is
Answer: C
Correct answer: the output per unit change of the input quantity
Sensitivity is the slope of the calibration curve - millivolts per millimetre, ohms per degree, picocoulombs per g. The other three options define three different specifications: the linear range, the maximum permissible input, and the resolution or threshold. All four are quoted on a data sheet and none substitutes for another.
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The threshold of a transducer is
Answer: D
It is the resolution measured at the zero of the scale, and it is quoted separately because friction and stiction often make the first movement harder to detect than subsequent ones. Resolution proper is the same quantity anywhere else on the scale.
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