The thermodynamic temperature scale One of the difficulties in the accurate measurement of temperature is that the various types of thermometer, e.g., mercury-in-glass, platinum resistance, therm electric, and so on give different readings when used to measure the same temperature, because the value obtained depends on the properties of the substance used in the thermometer. It was not […]
Choice of liquid for thermometers Whether mercury or alcohol is used in a thermometer depends on the range over which temperature is to be measured. Mercury freezes at – 39°C and boils at 357°C, while alcohol freezes at – l l S “C. It is therefore essential to use alcohol thermometers in places such as northern Canada and […]
Six’s maximum and minimum thermometer This thermometer is popular among gardeners for use in greenhouses. Its purpose is to record the maximum and minimum temperatures reached since the thermometer was last read. Generally speaking, a minimum temperature occurs during the night and a maximum during the day. Invented by James Six towards the end of the eighteenth century, […]
The clinical thermometer This is a thermometer specially designed for measuring the temperature of the human body, and so it is only necessary for it to have a range of a few degrees on either side of the normal body temperature (Fig. 14.4). The thermometer is generally placed beneath the patient’s tongue and left there for at least […]
To measure temperatures with the thermometer When a Celsius thermometer is graduated in a factory the stem between the two fixed points is divided into 100 equal parts by means of a special dividing engine. This is generally beyond the resources of the ordinary laboratory, but the thermometer may still be used to measure an unknown temperature […]
Determination of the lower fixed point The thermometer is placed in a glass funnel kept full of pure ice shavings, having a beaker underneath to catch the water. The mercury thread is allowed to show just above the top of the ice. When the level of the thread has remained steady for some time its position is […]
Determination of the upper fixed point This is done by pushing the thermometer through a hole in a cork and placing inside a double-walled copper vessel called a hygrometer (Fig. 14.2). Water is steal boiled in the lower part of the hygrometer, thus keeping the bulb surrounded pure water vapor at atmospheric pressure. For reasons already mentioned, important […]
Celsius temperature scale The difference in temperature between the two fixed points is called the fundament interval (Fig. 14.1). This is divided into 100 equal degrees, the ice point being call o °C and the steam point \00 °C. This method of subdividing the interval w suggested by a Swedish astronomer named Celsius, and is now called the […]
The fixed temperature points The principle underlying the graduation of all types of thermometer is to choose two fixed and easily obtainable temperatures called the upper and lower fixed points and then to divide the interval between them into a number of equal parts or degrees. The upper fixed point is the temperature of steam from water boiling […]
To construct a mercury thermometer One end of a length of clean capillary tubing is heated in a bunsen flame until the glass softens and seals the end of the tube. The tube is then withdrawn from the flame and a small bulb is blown at the end. By repeating this operation the size of the bulb may […]