Calorimeter is a heat measuring device. 

Principle:

The heat transfer takes place from a body of higher temperature to a body of lower temperature, provided no heat is lost to atmosphere.

Apparatus description:

A calorimeter consists of metallic vessel and a stirrer both are made of same material (copper or aluminium).  The vessel is kept in a wooden jacket so that there is no heat loss.  A mercury thermometer can be inserted through a small opening in the outer jacket.

The thermometer is used to measure the temperature of the liquid (usually water) in the inner vessel. The stirrer is used to stir the liquid and distribute heat in the vessel.

Working:

Let us consider how we can use calorimeter to measure the specific heat of iron and water.

1. Fill the inner vessel with 125 ml water. Close the lid and leave it for about 30 minutes. After 30 minutes, read and record the water temperature.  Check if the bulb of the thermometer is in the water.
2. Fill up a test tube half way with iron nails. Weigh and record the weight of the nails. Place the test tube with nails in hot-boiling water for about 10 minutes. This will allow the nails to get to 100ºC temperature without getting wet.
3. Open the lid of the calorimeter and transfer the nails to the inner vessel. Immediately close the lid. Move the stirrer up and down and read the temperature. Record the highest temperature. This will be the final temperature of the water and nails.
4. Calculate the temperature increase of water and temperature loss of iron nails.
5. Multiply the temperature increase of water by 125 ml of water to calculate the total heat transfer in calories.
6. Divide the total heat transfer by the temperature loss of iron; then divide the results by the weight of iron to calculate the specific heat of iron.