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There are two basic ways to build an instrument that can
measure the amount of moisture in the air.

a. Open two paper clips part way. Press them into a block
of wood, about halfway down one side, keeping the paper
clips about 1/2 inch apart. The block of wood should be
about 10 inches high and sturdy enough that it will not
tip over easily.

Glue a fine wire or piece of straw in the eye of a
needle. Set the needle on the two paperclips so that the
straw or wire sticks out past the edge of the block.

Wash a long strand of hair in hot soapy water to remove
the natural oils. When it is dry, tie the hair to a pin
stuck into the side of the block at the top.

Wrap the free end of the hair once around the needle in a
counterclockwise loop, then tie a paperclip onto the end
of the hair so that it is pulled tightly around the

Make a measuring scale on a card, and mount it on the
side of the block. Set the straw so that it is at
the center of the card. The scale can be calibrated by
draping the hygrometer with a cloth soaked in very hot

Hair stretches when wet and contracts when dry. This
action takes place at an exact rate and is proportional
to the amount of water in the air.

The humidity under the cloth will cause the hair to
stretch, and will cause the pointer to rise. When it 
stops rising, the humidity will be very close to 100 
percent. This position can be marked on the card.

Other positions can be calibrated by comparing this
hygrometer with a commercial one.

b. Mount two identical thermometers side by side on a milk
carton or on a board. Cut the tips from a clean white
cotton shoestring, and slip the loose fibers of one end
over the bulb of one thermometer. Tie the shoelace end
in place with a piece of thread, then put the other end
in a small container of water.

In operation, the two thermometers will produce two
different readings.

The wet bulb thermometer will give a reading based upon
the evaporation of the water from the shoestring.
Evaporation has a cooling effect -- the reading will be
lower than that of the dry bulb thermometer. The rate of
evaporation depends on the amount of water already in the
air. When there is a great deal of water in the air, the
rate of evaporation slows down; thus the evaporation
reading of the wet bulb thermometer is higher and closer
to that of the dry bulb thermometer.

By comparing the readings of the two thermometers, you
can determine the relative humidity of the air.

Generally, a difference in temperature of 15 degrees F (9
degrees C) or more is an indication of low humidity,
while a difference of less than 15 degrees F (9 degrees
C) indicates high humidity.

A table can be used to determine the relative humidity more
precisely. For example, if the dry bulb thermometer reading
is 65 degrees F and the wet bulb thermometer reading is 56
degrees F, the difference in temperature is 9 degrees F.

Reading down the left side of the table to 65 degrees F and
across the row to the entry under the heading for 9 degrees
F. gives a value of 56. The numeral in the cell at the intersection of this column and row indicates that the relative humidity is 56 percent.

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