12 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
cold weather especially, and particularly whenever high humidities 
are used, a ceiling that lacks such local heating is likely to accumulate 
a great deal of condensation which, dripping down upon the lumber, 
interferes seriously with humidity control. 
CONTROL OF KILN TEMPERATURE 
Correct determination of the temperature in the kiln is essential 
to proper control of the drying process and consequently deserves 
much more time and attention than it usually receives. Thermom- 
eters, the only temperature-determining instruments in kiln prac- 
tice, may be grouped in two classes, indicators and recorders. 
INDICATING THERMOMETERS 
Many kinds of indicating thermometers are available, and care 
must be exercised to select reliable instruments. The very cheap ones, 
with separate scales stamped on metal strips attached to the case, 
are not accurate enough for kiln work and should be avoided. A 
number of better grades also have separate scales, but the highest- 
gerade thermometers have their graduations etched on the glass stem; 
these can be obtained with or without a metal protecting sleeve. 
Plate 9, C illustrates such a, thermometer in a sleeve; Plate 9, B, a 
shows sufficiently its appearance without one. Further information 
on the subject is given on page 84. 
Indicating glass-stem thermometers for kiln work are almost 
invariably of the mercury-filled type, though sometimes alcohol- 
filled ones are selected. 
Most kiln operators find it very desirable to determine both tem- 
perature and humidity at the same time; this is commonly done by 
means of the wet and dry bulb hygrometer, which will be discussed 
later. Since such a hygrometer indicates temperature, there is littie 
need, where it is used, for a separate thermometer in the kiln. Each 
operator, however, should have at least one dependable etched-stem 
thermometer available for purposes of comparison and calibration. 
RECORDING THERMOMETERS 
Recording thermometers for kiln work are almost without excep- 
tion of the extension-tube type. In such recorders the sensitive ele- 
ment, called a bulb, is connected to the instrument by a capillary tube 
of suitable length. The tube, which is usually protected by flexible 
armor, ends in a hollow spring or other pressure-sensitive element in 
the case. This spring, which may be any one of several different 
types, is so constructed that changes in internal pressure cause in it 
a movement that, transmitted from its free end by a lever system to 
a pen arm playing over a chart, is recorded graphically. The chart 
receiving the record, either a 1-day or a 7-day form, is rotated by a 
clock movement, which is wound whenever the chart is changed. 
Plate 3 shows the case of an extension-tube recording thermometer, 
cut away to display the internal mechanism, and Plate. 4 illustrates 
complete double-pen instruments. The construction appearing in 
Plates 10, 11, and 12 is similar: 
Three types of extension-tube recording thermometers are in com- 
mon use. The principal difference among them is in the material 
