As β is too small therefore we can take
Rt = R0( 1 + αt)
where, α = temperature coefficient of resistance and
R0 and Rt, are electrical resistances at 0 ° C and t ° C.
where R1 and R2 are electrical resistances at temperatures t1 and t2.
where R100 is the resistance at 100 ° C.
Platinum resistance thermometer can measure temperature from— 200 ° C to 1200 ° C.
( iii) Length of Mercury Column in a Capillary Tube
lt = l0( 1 + αt)
where α = coefficient of linear expansion and l0, lt are lengths of mercury column at 0 ° C and t ° C.
Thermo Electro Motive Force
When two junctions of a thermocouple are kept at different temperatures, then a thermo-emf is produced between the junctions, which changes with temperature difference between the junctions. Thermo-emf
E = at + bt 2 where a and b are constants for the pair of metals. Unknown temperature of hot junction when cold junction is at 0 ° C.
Where E100 is the thermo-emf when hot junction is at 100 ° C.
A thermo-couple thermometer can measure temperature from— 200 ° C to 1600 ° C.