Hygrometers are commonly used to measure humidity, but can also calculate the dew point or the water vapor temperature at which droplets will begin to condense. Hygrometers can also measure heat index.
Cut the 3-by-3-inch flat plastic into an isosceles triangle (a triangle with two angles that are the same) using scissors. This will serve as the pointer of your homemade hydrometer.
Stick a dime to the triangle's apex and secure the coin with tape.
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Insert a nail through the pointer found at the base of your triangle. Make sure the pointer can move freely around the nail by wiggling it.
Use glue to stick the hair strands onto the plastic between the nail hole and the dime. The hair strands should be parallel to the triangle’s base.
Place the pointer on your Styrofoam base around three-quarters from the top edge down.
Attach the nail from the pointer running to the Styrofoam base. Drive the nail just far enough to keep the pointer attached to the Styrofoam base. Make sure the pointer can still move freely after driving the nail.
Drive your other nail into the Styrofoam base about an inch from the top while remaining in line with the nail holding onto the pointer.
Pull out the hair strands tightly so that the pointer becomes parallel to the Styrofoam base’s bottom edge. This should make the hair strands perpendicular to the position of the base.
Apply glue to the hair strands to attach them to the upper nail. Use your scissors to trim the hair strands if necessary.
Place your homemade hygrometer outside your house. The hair strands will lengthen due to the moist air, allowing the pointer to point down; dry air will cause the strands to contract, driving the pointer to point up.