Thermometers and Thermostats

299. Mercury Thermometers. There are two kinds of thermometers in use—the Fahrenheit and the Centigrade. Since the thermometer is used now in cooking, the housewife often has to meet the problem of translating temperatures from one to the other.

The centigrade thermometer is marked on the assumption that the temperatures of boiling water and freezing water are constantly the same. The boiling point is marked 100, and the freezing point 0. The space in between is marked into even divisions and numbered 1 to 99.

The Fahrenheit thermometer was made on the assumption that a mixture of ice and salt was the coldest temperature that could be reached, so this temperature of a certain proportion of ice and salt was marked zero.

The hundred point was given to what was supposed to be the normal body temperature. The intervening spaces were marked into equal divisions, and these divisions were carried below 0 degree and above 100 degrees. The boiling temperature of water came at 212 degrees Fahrenheit, and the freezing point at 32 degrees. This makes 180 degrees difference between thawing and freezing and boiling. So 100 degrees Centigrade equal 180 degrees Fahrenheit. Therefore, 1 degree Centigrade equals 9/5 degrees Fahrenheit, and 1 degree Fahrenheit equals 5/9 degree Centigrade.

For example, if 40 degrees Centigrade is to be translated into Fahrenheit degrees, first multiply 40 by 9 = 360, then divide by 5 = 72, and add 32, because 0 degree Centigrade is the same as 32 degrees Fahrenheit, and the result is 104 degrees Fahrenheit equal 40 degrees Centigrade. If 41 degrees Fahrenheit is to be translated into Centigrade degrees, first subtract 32 from 41 = 9, then multiply by 5 = 45, and divide by 9, and the result is 5 degrees Centigrade = 41 degrees Fahrenheit. Fig. 161 is a diagram showing relative readings of Fahrenheit and Centigrade thermometers.

300. Oven Thermometer. Some oven thermometers depend on the expansion of metal to indicate the temperature. A hand on the clock-like face of these indicators shows the degree of heat. Few of these give the actual temperature, but they do indicate a slow, a moderate and a hot oven.

301. Maximum Thermometers. A maximum thermometer is one in which the mercury rises to register the maximum amount of heat to which it has been subjected. It stays at this height when the temperature falls, until it is shaken back.

It is sometimes used in ovens to ascertain the temperature they have reached before the oven door is opened.

TABLE OF TEMPERATURES USEFUL TO HOUSEKEEPERS

302. Thermostats. Thermostats are devices which open or close valves or dampers in order to keep rooms, boilers, ovens, incubators, etc., at an even temperature. All metals expand on being heated, and contract on being cooled. Some expand more than others. Two materials which expand at different rates are frequently used in making thermostats. Any certain temperature causes a given piece of metal to expand to a certain size, or to contract on cooling to a different size. Some thermostats are made of a straight rod of metal like copper which expands more than iron when heated. The rod is so placed that when cool it will allow fuel like gas or oil to pass thru a pipe, and when heated, it will expand enough to close the pipe, shutting off the fuel. They are placed so that they close the pipe at the temperature desired for an oven or supply of hot water.

Other thermostats are more complicated, as the expanding metal moves a series of levers. These thermostats are used to regulate dampers on coal and wood furnaces, when they are placed in the rooms to be heated. They are often used on other devices, such as incubators.

Still others control an electric current. When the metal expands, it closes the circuit, causing the electricity to do the work desired. When it contracts, it opens the circuit. Thermostats can be set to do work at different temperatures.

These are sometimes attached to clocks which, with a device similar to the alarm, will change the indicator of the thermostat so as to set it from one temperature to another at a stated time for which the clock is set and turn it back at another hour.