Chapter: 11th 12th std standard Class Physics sciense Higher secondary school College Notes
Angstrom pyrheliometer
Angstrom pyrheliometer
Pyrheliometer is an instrument
used to measure the quantity of heat radiation and solar constant.
Pyrheliometer designed by
Angstrom is the simplest and most accurate.
Angstrom?s pyrheliometer consists
of two identical strips S1 and S2 of area A. One junction
of a thermocouple is connected to S1 and the other junction is
connected to S2. A sensitive galvanometer is connected to the thermo
couple.
Strip S2 is connected to an
external electrical circuit as shown in Fig.. When both the strips S1 and S2
are shielded from the solar radiation, galvanometer shows no deflection as both
the junctions are at the same temperature. Now strip S1 is exposed to the solar
radiation and S2 is shielded with a cover M. As strip S1 receives heat
radiations from the sun, its temperature rises and hence the galvanometer shows
deflection. Now current is allowed to pass through the strip S2 and
it is adjusted so that galvanometer shows no deflection. Now, the strips S1
and S2 are again at the same temperature.
If the quantity of heat radiation
that is incident on unit area in unit time on strip S1 is Q and a its absorption co-efficient, then the
amount of heat radiations absorbed by the strip S1 in unit time is
QAa.
Also, heat produced in unit time
in the strip S2 is given by VI,
where V is the potential difference
and I is the current flowing through it.
As heat absorbed = heat produced
QAa = VI (or)
Q=VI/Aa
Knowing the values of V, I, A and a, Q can be calculated.
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