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 .