ET-162 -:.-.- ... September 1940
United States Department of Agriculture
Bureau of Entomology and Plant Quarantine
CONSTRUCTION OF A HAIR-HYGROSTAT WITH MAGNETIC SWITCH
FOR HUMIDITY CONTROL IN AN INCUBATOR ROOM
By K. D. Arbuthnot,
Division of Cereal and Forage Insect Investigations
A hair-hygrostat capable of operating for long periods of
time without attention either for adjustment or maintenance is
described herein. The construction is simple and inexpensive and
can be done with a few common tools usually found in a laboratory
or work shop. Detailed descriptions and instructions for fabrica-
tion of the instrument are presented in the following pages,
Hair-hygrostats which employ exposed electrical contact
points are uncertain in operation, owing to oxidation and sticking
of the contact points. In addition to this uncertainty of opera-
tion there is a hazard to humidifying equipment, such as bearings,
valves, and motors, due to excessive operation when the hygrostat
fails to operate properly.
This hair-hygrostat has a hermetically sealed mercury contact
switch which is not affected by dust, dirt, or corrosion. The
switch is operated by a magnet mounted outside the glass tube con-
taining the switch.
This hair-hygrostat has been in constant operation in an
Incubator room at the New Haven, Conn., field station for a period
of 5 months, during which time its reliability has been demonstrated.
Humidity has been maintained within a range of 2 percent, from 92
to 94 percent relative humidity (hygrograph record), in a room
13 feet long, 8 feet wide, and 7 feet high, utilized in conducting
genetic studies of the European corn borer. Drops in humidity of
greater magnitude (below 92 percent relative humidity) sometimes
occur when the incubator-room entrance door is opened at too fre-
quent intervals. However, the humidity conditions are again regu-
lated without attention to the instrument when the room is closed.
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Material Required for Construction
The following materials were used in constructing the hygrostat:
Specific essential parts:
Hermetically sealed mercury
contact switch with supporting
clamp, having electrode and
arirature actuated by a per-
A permanent magnet and mount-
ing arm designed to operate
the mercury switch
Stabilizer and guide with
stop head for magnet mounting
"8 x 11" x 18"
Small brass cabinet hinge
Brass post with threaded
hole in basal end and
slotted horizontal hole
Music-wire spiral spring
screws 1/16" x 3/4"
1/16" x 11"
Wire nails i"
Strand of human hair
Finish for base and
Hinge for magnet arm
for hair and spring
Fastening parts to
base and back
Hair and spring
together to form
base and back
unit to operate
Description of S.Bncific Essential Parts
The hermetically sealed mercury .:.tact switch is one which
reirains in a fixed position and operates by ircj.ae'tic attraction
through the glass wall of _:e switch tube. Specifications are:
Electrical capacity 9/10 amperes, 24 volts; riaxnriuui capacity 1/60
The perna'.,e.t magnet should be one designed to operate the
switch which is used. The magnet is mounted on an arm to be at-
tached to the actuating mecbai .iT of a thermostat and can be easily
attached to a hinge for use in this hLiostat.
The stabilizer and guide with stop head for magnet are made
to fit on the upper end of the magnet arm and maintain the magnet
in its proper position in relations to the switch.
Method of Construction
The construction will be clarified by reference to figures
1 and 2, wherein the various parts discussed are labeled.
The base of the hygrusLat, which is 2' inches by 13'I inches,
and the back, 31 inches by 3- inches, are constructed by nailing
together 3 layers of presswood board ai.d nailing the back to the
base as illustrated. After the necessary holes for irachine bolts
are male, as indicated below, the base and back are painted with
several coats of black enamel. This provides an inexpensive, rigid,
insulated frame for supporting the operating parts.
The mercury switch is a type commonly used with a bimetallic
coil in commercial heating plants for dwellings. The essential
features for the present purpose are the hermetically sealed mercury
contact points and the mag',etic actuating device. A clamp for sup-
porting the switch (glass tube) is attached by machine screws to
the back so that the s',itch is supported at the proper angle for
A wire, 1/32 inc'.. inch, with ends curv-, to serve as
hooks for attachL-.- the ,:.ir arnd ,. ., is soldered into a hole
Sinch from the lower end of the c.,net arm. The i,ac.gi.et arm is
then soldered to one side of a hiige. The proper position of the
magnet and its supporting arm for operating the switch is determined
by holding the hinge on the base with the hand. When the proper
position has been determined, the hinge is fastened to the base
with a machine screw. The stabilizer, guide, and stop head consist
of a single unit as used in a thermostat employing this type of
switch. This unit is placed in the proper position in relation to
the magnet and attached to the back by a machine screw. The con-
necting wires from the switch are fastened with machine screws
to the face of the back. These screws project through the back and
serve as binding posts for wires to the relay transformer which
actuates the humidifying unit.
The two brass posts are 1- inches long and I inch in diameter,
with a hole in the :2-al end threaded to receive a 1/16 inch by
inch machine bolt and with a slotted hole, i.e., straight on one
side, to receive a 1/16 inch bolt. They are attached with machine
screv,'- near the ends of -he base and in line with the hinge attach-
ment. Ti.o two long machine screws are flattened so that they can
slide through but not rotate in the slots at the tops of these
posts. They are provided with hooked ends for attachment of the
hair and spring. Tw.*' nuts on each of these screws hold the screws
at desired distances th,,:g," the slots and serve as spring and hair
adjusters. The spring arid hair are attached to their respective
adjusters and to the hooks near the base of the magnet arm.
The strand of hair consists of 50 human hairs, treated with
ether to remove all fats and dirt, looped at the ends, and fastened
with ho.i-E-hold cncni-it to form eyes for attachment.
Operation and Adjustment
As humidity in the room decreases, the strand of hair dries.
This causes the hair to contract, pulling the arm and u'agnet toward
the moveable electrode (armature) of the switch. The magn.etic field
of the magnet pulls the moveable electrode toward the magnet, and
the mercury on this electrode contacts with the stationary electrode,
thus completing the circuit to the relay. The relay then closes
the circuit which operates the humidifier.
As the humidity in the room increases, the strand of hair
becomes moist. This causes the hair to expand, and the spring
pjlls the arm and aEar.et away from the moveable electrode. When
the magnet has moved away from this electrode until its magnetic
field is weaker than the pull of gravity on the electrode, the
electrode drops back to its original position and breaks the circuit
to the relay, thus stopping the humidifier.
The hb'grostat is set by simultaneously adjusting the spring
tension, which controls the differential within which the instru-
ment operates, and the hair adjustment, which controls the set of
the instrunert until the desired percentage of humidity and dif-
ferential are obtained.
Utility of the Instrument
The hern'etically sealed switch has a long life under the
most strenuous Usage.
The instrument has a wide range of possible adjustments,
since the spring tension and hair balance for various ranges and
settings are practically unlimited. By reversing the relay switch
and connectii,, a drier (any dehumidifier in which air is circulated
through an apparatus designed to remove moisture from the air, with
the air-circulating, device actuated by the hygrostat, can be
adapted for this purpose), the same hygrostat can be used for de-
humidification where this type of control is desired.
HAl R HYGROSTAT WITH MAGNETIC
TO RELAY AND
FIG. 2 %
UNIVERSITY OF FLORIDA
3 1262 09239 5218