British Thermal Unit. It has alreadybeen established that a thermometer measuresonly the intensity of heat and not the quantity.However, in working with heat it is oftennecessary to determine heat quantities. Obviously, some unit of heat measure is required.Heat is a form of energy, and as such is intangible and cannot be measured directly.Heat can be measured only by measuring theeffects it has on a material, such as the change intemperature, state, color, size, etc. The most universally used unit of heat measureis the British thermal unit, abbreviated Btu. ABTU is denned as the quantity of heat required tochange the temperature of 1 lb of water 1° F.This quantity of heat, if added to 1 lb of water,will raise the temperature of the water 1°F.Likewise, if 1 Btu is removed from 1 lb of water,the temperature of the water will be lowered1°F.