Mechanical equivalent of heat

We know that heat is a form of energy and, since energy is the capacity to do work, heat is potentially capable of doing work. This was first demonstrated by James Prescott Joule who, in the early nineteenth century, proved that there was a definite mathematical relationship between heat and work. Joule rigged up a contrivance similar to the one. 

Two heavy known weights were lowered over two wheels and, while being lowered, they turned a paddle that was submerged in water. The turning of the paddle warmed the water and the distances the weights dropped were measured on yardsticks. 

The amount of water in the jar was exactly one pound, and as soon as the temperature of the water had risen one degree Fahrenheit, the work done by the weights was computed and found to be 778 foot-pounds.

Joule correctly concluded that 778 foot-pounds of work is necessary to raise one pound of water one degree Fahrenheit. This amount of work is called a British thermal unit and is designated by B.T.U. now a B.T.U. is also defined as the heat necessary to raise 1 pound of water 1 pound of water 1 degree Fahrenheit.

Joule expressed in the English system. We are using the SI unit for all scientific measurements. If we want to convert it to the metric system we use grams and centimeters. In the metric system, the unit is a calorie. A calorie is the amount of heat necessary to raise 1 gram of water 1^o Centigrade.

Specific heat

Just as specific gravity refers to the ratio of the weight of a given volume of a substance compared to the weight of an equal volume of water, so the specific heat of a substance is the amount of heat necessary to raise a given mass of the substance one-degree centigrade compared to the amount of heat required to raise the same mass of water the same amount in temperature. In short, the specific heat of water is taken as unity, just as the specific gravity of water equals unity. For this reason, the specific gravity of water equals unity.