| This is a differential instrument: the slug
of water or mercury in the horizontal section of tubing would be pushed
in one direction or the other, depending on the rate at which the air in
the two bulbs is expanded by electrical heating. Its ancestor is
Thermometer. The heating is accomplished by thermocouples in the two
bulbs; the manufacturer called this a "Peltier Effect Apparatus." The catalogue
notes that "a 2-Ampere flow of current for 10 seconds will produce a 10
to 12 mm movement of a water index in the horizontal connecting tube.
The wooden stand is cut out behind the horizontal tubing, allowing the apparatus to be used with a light source to throw an enlarged image on the screen for the entire class to see.
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