Laboratory thermometer

Structure:

• The thermometer is made relatively small so that it is portable and cheap.
• The liquid is contained in a thin-walled glass bulb. The bulb is made relatively larger than its bore to contain more of the liquid, so as to improve sensitivity.
• The narrow bore of the capillary tube is uniform. The round glass stem around the capillary tube is made thick. It acts as a magnifying glass.

Sensitivity:

• Small expansion of the liquid in the liquid bulb will cause a big change in the length of the liquid thread in the capillary tube as it is made narrow. The narrower the bore, the higher the sensitivity.

Range:

• The range is limited by the freezing and boiling points of liquid.
  For mercury thermometer: -39 to 357°C
  For alcohol-in-glass thermometer: -115 to 78°C
• In colder countries, most of its liquid-in-glass thermometers use alcohol and not mercury.
• The range can be increased by lengthening the bore.
• Range is the converse of sensitivity, i.e., the longer the range, the lower is its sensitivity. Factors that increase range would at the same time reduce its sensitivity.

Linearity:

• Mercury expands quite uniformly over a good range of temperatures.
• Alcohol expands non-linearly over different range of temperatures.

Responsiveness:

• As the liquid is contained in a thin-walled, small glass bulb, it will be more responsive (faster response) to heat.
• Mercury reacts quickly to the temperature changes whereas alcohol reacts slowly.
• It takes several tens of seconds to record one reading.