Hygrometers: Ways to Measure the Atmosphere's Water Content

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Author: Janice VanCleave

An instrument that measures the humidity of air is called a hygrometer. The efficiency of the instrument depends on its hygroscopic (water-attracting) nature.

In this project, you will make hygrometers and use them to measure and compare the hygroscopic quality of different papers and to indicate differences in humidity. Hygrometer scales will be designed to give more accurate humidity measurements. You will study how things in nature, such as pine cones, indicate changes in humidity. You will also construct a psychrometer and use it to measure relative humidity.

Getting Started

Purpose: To make a hygrometer.


  • scissors
  • ruler
  • lightweight aluminum foil
  • newspaper
  • transparent tape
  • two short pencils about 6 inches (15 cm) long
  • masking tape
  • marking pen
  • two 1-quart (1-liter) jars with lids
  • tap water
  • two thread spools
  • hair dryer
  • timer


  1. Cut a piece of aluminum foil about 4 × 12 inches (10 × 30 cm).
  2. Cut a strip of newspaper 1 × 10 inches (2.5 × 25 cm).
  3. Lay the newspaper strip in the center of the aluminum piece.
  4. Tape the edges of the paper to the foil with a single layer of tape.
  5. Trim the foil around the newspaper edges, leaving about 1/4 inch (0.63 cm) off oil on all sides.
  6. Tape one end of the strip to the center of one pencil.
  7. Repeat steps 1 through 6 to prepare a second strip.
  8. Use the masking tape and pen to label one jar "humid" and the other "dry."
  9. Pour about 1 inch (2.5 cm) of hot tap water into the "humid" jar, stand one thread spool in the jar, then secure the lid.
  10. Use the hair dryer to heat the inside of the "dry" jar for about 30 seconds, stand the second thread spool in the jar, then secure the lid.
  11. Use the hair dryer to thoroughly dry the newspaper on the strips.
  12. Wind the strips snugly around the pencils so that the aluminum foil is on the outside.
  13. Open each jar and stand the pencils in the holes in the thread spools, then again secure the lids on the jars (see Figure 29.1).
  14. After 1 hour, remove the pencil-spool sets from each jar and observe the strip around each.
  15. Hygrometers: Ways to Measure the Atmosphere's Water Content


The coil of paper unwinds slightly in the humid jar and the coil makes little or no change in the dry jar.


The newspaper is hygroscopic, which means it absorbs water from the air. The water molecules may be held in pores and imperfections in the paper. As the pores in the paper fill with water, the paper expands and pushes against the aluminum foil, which is not hygroscopic. The force of the expanding paper causes the coil to unwind.

Humidity is the amount of water vapor or moisture in the air. The air in the jar above the warm water has a high humidity. This is due to the increased rate of water evaporation from the warm water's surface. All or most of the water in the air in the heated jar evaporated and left the jar. Thus, the air in this jar has a low humidity. The paper coils act as hygrometers (instruments used to measure the amount of water or humidity in the air). The more humid the air, the more water the coil can absorb and the more the coil expands.

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