There are two ways in which calorimetry can be used:
Measuring enthalpy
To measure heats of reactions, etc the reaction is performed in a calorimeter with a known heat capacity. As the heat capacity of the calorimeter is known, then the energy change of the reaction can be deduced by calculation:
ΔH=ΔT*c
Where:
To measure heat capacity, we must simply alter the knowns and unknowns in the above equation. In this case we do not know the heat capacity, and thus must use a known quantity of energy