10 BULLETIN 1136, II. S. DEPARTMENT OF AGRICULTURE. 
CONTROL OF KILN TEMPERATURE. 
The proper measurement of the temperature in the kiln is essential 
to proper control and deserves much more time and attention than it 
usually receives. Temperature-measuring instruments or thermom- 
eters may be grouped in two classes, indicators and recorders. Indi- 
cating glass-stem thermometers for kiln work are almost invariably 
of the mercury-filled type, though sometimes alcohol-filled ones are 
used. 
INDICATING THERMOMETERS. 
There are many kinds of mercury thermometers available, and 
care must be used to select reliable instruments. The very cheap 
ones, with separate scales stamped on metal and 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- 
grade thermometers have the graduations etched on the glass stem. 
These can be obtained with or without a metal protecting case. 
Occasionally it is desirable to insert the thermometer through the 
kiln wall, with the bulb inside and the scale outside. Industrial- 
type thermometers are well adapted for this purpose. These have 
a brass extension tube surrounding the bulb and part of the stem, 
and a weatherproof brass casing with a' glass face protecting the 
scale. The extension tubes can be made 3 feet or longer, and the 
stem fitted on at almost any desired angle. A right-angle stem is 
desirable where the extension tube projects horizontally into the kiln, 
because it permits the scale to be vertical and therefore most easily 
read. 
An electrical-resistance thermometer has recently been developed 
for dry-kiln use. This thermometer has a special panel and con- 
necting wires, so that the temperatures at a number of places can 
be read from the one instrument. The temperature is indicated by a 
pointer moving over a graduated dial. 
RECORDING THERMOMETERS. 
Recording thermometers used in kiln work are almost invariably 
of the extension-tube type provided with 1-day or 7-day charts. In 
recorders of this type the sensitive element or bulb is connected to 
the instrument by a capillary tube of suitable length. (See PL II, 
fig. 2.) This tube is usually protected by a flexible armor and ends 
in a spring capsule in the case. This capsule may be any one of 
several different types, all of which are flexibly constructed, so that 
changes in internal pressure produce a movement of the capsule 
which is usually transmitted through a series of levers to a pen arm, 
which moves across a slowly revolving chart and produces a graphic 
record of the temperature in the kiln. The chart is rotated by a 
clock movement which is wound whenever the chart is changed. 
There are three types of recording thermometers, the difference 
being in the material used for filling the bulbs. These three types 
are commonly known as liquid-filled, gas-filled, and vapor-filled. 
The choice of type depends upon the accuracy desired and the con- 
ditions under which the thermometer is to be used. 
In dry-kiln work the tube and the case of the thermometer are 
liable to be subjected to wide variations in temperature, which influ- 
