To measure temperature accurately, a thermometer is used which works on the principle of liquid expansion with temperature change. There are two main types of thermometers - mercury thermometers and alcohol thermometers. Mercury thermometers can measure up to 350°C but mercury is poisonous, expensive and difficult to obtain. Alcohol thermometers are cheaper and easier to obtain but can only measure up to 78°C. To construct a temperature scale, fixed points of melting ice and boiling water are chosen and the temperature range between them is divided into equal parts to establish the scale.

1. MeasuringTemperature
To masure the temperature accurately,we needtouse aninstrumentcalleda thermometer.
The work ing principle ofthe thermometer isbasedon the nature of liquidexpansionversus
temperature change.
Basedon the typesof liquidused,there are twotypesof thermometer, the mercury thermometer
and alcohol thermometer.
The advantages of usingmercury:
Inside the tube,the mercuryisclearlyvisible due toitssilveryshiningappearance anditdoesnot
wetthe tube.
Mercury temperature canreach350 C.
Disadvantages of usingmercury:
Mercury ishard to aobtainand isexpensive.
The lowesttemperature thatcanbe reachedby mercuryis -40 C.
Mercury ispoisonus.
The advantages of usingalcohol:
Alcohol is easytoobtainis cheaperthanmercury.
The lowesttemperature of alcohol is112 C.
Disadvagantes of usingalcohol:
Alcohol westthe thermometertube.
Alcohol canonlyreadup to a temperature of about78 C.
To construct a temperature scale, youneedto take four steps:
1.Chose the lowerfixedpoint.
To choose the lowerfixedpointwe use the temperature of pure meltingice.
If you do notmeltingpointwill change.
2. Choose the upper fixedpoint.
To choose the upperfixedpointwe use the temperature of pure boilingice.
3. Divide the temperature range betweenthe two fixedpoint intoa numberof equal parts.

2. For example. Inthe centigrade scale,there are 100 equal marks betweenthe lowerand upper
fixedpoints.Each mark on the thermometerisameasure of 1 C.
4. Extendthe scale belowthe lowerfixedpoint and above the upper fixedpoint.
Make marksbelowthe meltingpointandabove the boilingpointwithanequal range asinthird
step.