How to Make a Simple Hygrometer

A hygrometer is an instrument used to measure humidity, the amount of water vapor in the air. The earliest form, the mechanical hygrometer, was invented in 1783 by Horace Bénédict de Saussure, using human hair to determine the moisture content of the air. Modern forms include the dual-thermometer psychrometer, used to measure relative humidity, and the chilled mirror hygrometer, used to measure dewpoint. These instructions are for a mechanical hygrometer.

Difficulty:

Moderate

Instructions

Things You'll Need

• Scrap piece of wood or flat Styrofoam, 9 inches long by 4 inches wide
• Flat piece of plastic, 3 inches long by 3 inches wide
• Two small nails
• Three 8-inch long strands of human hair
• Dime
• Glue
• Tape
• Hammer
• Scissors sturdy enough to cut plastic

Making the Pointer

1. Cut the plastic into the shape of an isosceles triangle to form a pointer.

2. Tape a dime to the apex of the triangle.

3. Poke a nail through the pointer near the base of the triangle and wiggle it so that the pointer moves freely around the nail.

4. Glue the hair strands to the plastic between the dime and the nail hole so they run parallel to the base of the triangle.

Attaching the Pointer to the Base

1. Position the pointer on the wood or Styrofoam base three-quarters down from the top edge.

2. Attach the nail running through the pointer to the base. Drive the nail far enough in to keep the pointer attached to the base, but not so far that the pointer can't move freely.

3. Drive the other nail into the base an inch from the top and in line with the nail affixing the pointer.

4. Draw the hair strands tight so that the pointer points parallel to the bottom edge of the base and is perpendicular with the hair strands.

5. Glue the hair strands to the upper nail, trimming the hair if necessary.

6. Place the hygrometer outside. Moist air will cause the hair strands to lengthen, making the pointer point down, while dry air will contract the strands, making the pointer point up.

Resources

• Buy an indoor hygrometer from the Allergy Buyers Club.
• Buy a digital hygrometer from Jon-Don.