Thermometers assist us in comparing or fixing the temperature of certain industrial operations. This is important, as in a great many manufacturing operations it is necessary to know when a certain temperature is reached. As a result a number of different kinds of thermometers have been invented. They are all based upon the same principle as are the Fahrenheit and Centigrade thermometers, namely, that substances expand with an increase and contract with a decrease of temperature. In the measurement of heat in stoves and furnaces where the temperature exceeds 900° F. an instrument called a pyrometer* is used.