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)
Knowing the values of V, I, A and a, Q can be calculated.