Demonstrating The Peltier Current Effect
The materials required for this experiment are a few good primary cells -
or, better, a storage cell - a flat piece of copper sheet, two binding
posts, a few steel needles and a small piece of wood. Thus equipped we
are about to demonstrate the "Peltier effect."
First let us state what that effect is, says R. E. N. in Model Engineer.
Referring to Fig. 1 if we pass a current through iron and copper wires
twisted together at X to form a series circuit the junction will be
heated when current passes from iron to copper and cooled when current
flows in the opposite direction.
This effect is not the ordinary heating effect of an electric current. As
the temperature increases the Peltier effect decreases till it becomes
zero at 280° C. (535° F.); at higher temperatures it reverses, so that
heat is developed at the junction when current is sent from copper to
iron and cooling is produced when current flows from iron to copper.
It must be understood clearly that ohmic resistance at the junction
produces the same heating whichever the direction of current flow. The
Peltier effect simply adds to or subtracts from this heating.
Suppose now that we fix binding posts R S in one end of a piece of wood
D, Fig. 2, and clamp leading-in wires and steel needles in these
terminals, so that when the other end of D rests on the table the points
A B rest evenly on the sheet of copper C.
Current can now be passed from R to S through a steel needle, the copper
plate and another steel needle in series. On starting the experiment the
needle tips are cool and the junction A (iron to copper) is heated and
the junction B (copper to iron) is cooled by the Peltier effect.
If, however, 4 or 5 amps, be passed through the circuit the needle tips
soon get warm, because their section is very small and their resistance
therefore relatively high. The junction A reaches the critical
temperature (280° C.) before B; but soon both needle tips are hotter than
this, and then the Peltier effect is reversed and the tip B becomes
hotter than A, so that ultimately B becomes red hot, while A is still
invisible in the dark (the lowest red heat visible in the dark is about
335° C. or 635° F, while "dull red'' heat is about 700°_ C. or 1,290°
Owing to the small weight of metal in the needle points, the small amount
of heat produced or absorbed by the Peltier effect produces quite
considerable temperature rise or fall, and thus permits one tip to become
red hot appreciably sooner than the other. If the current be great enough
both needle tips will soon be red hot, but that one becomes red first
where current leaves the copper plate. If the weight on the two needles
is distributed evenly the needle tip A will become red hot first when
current is reversed, thus proving that unequal resistance is not the
cause of the phenomenon.
A fresh pair of needles should be used often. The effect is best seen in
a subdued light.
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