If you ask most people what temperature is, they will probably say something along the lines of "temperature is a measure of how hot or cold something is." This definition works in informal contexts.
A step up from this definition would be to say that temperature is a measure of the average kinetic energy of all the particles in an object. This is also a functional definition, but CSEC requires that you relate temperature slightly differently.
Temperature is a measure of the tendency of an object to spontaneously give up (lose) thermal energy to its surroundings. When two objects are in thermal contact, the one that tends to spontaneously lose thermal energy is at the higher temperature. So, we can measure temperature based on the net direction of the movement of thermal energy.
However, in order to measure temperature accurately, we must be able to observe a physical change that correlates to an increase or decrease in temperature. Any physical property that depends on temperature (known as a thermometric variable), and whose response to temperature is reproducible, can be used as the basis of a thermometer.
For example, for most substances, an increase in temperature causes and increase in volume. This is the basis of alcohol thermometers, mercury thermometers and bimetallic strips. Other thermometric variables include electrical resistance and electromotive force.
In non-digital thermometers that you are familiar with, a liquid is usually enclosed in a tall narrow glass (or plastic) column. So, the volume change would result in a change in the height of the liquid in the column. The markings on the thermometer are equally spaced and represent 1 degree graduations.
Thermometers are calibrated according to either the Celsius scale or the Fahrenheit scale. This calibration is done by placing the thermometer in two substances of known temperatures, an upper fixed point and a lower fixed point. In the Celsius scale, the range between the lower and upper fixed points is divided into 100 divisions, each called a degree Celsius, written as . The lower fixed point is taken as 0 and the upper fixed point is taken as 100 .
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