28 
BULLETIN 8&4, U. S. DEPARTMENT OF AGRICULTURE. 
consists of'both an expansive element, which expands and contracts 
under slight temperature changes and is placed in the kiln where the 
temperatures are to be controlled, and a special valve outside the kiln 
on the steam-supply pipe. The effect of expansion or contraction in 
the expansive element is transmitted either by a system of levers or 
by fluid pressure through a fine tube to the special valve, which is 
operated either directly or by an auxiliary s^/stem of compressed air. 
fluid, or electricity. 
Fig. 9.— Simplified humidity control. Complete connections for one kiln of a series. (See page 40.) 
The method of actuating the valves divides the thermostats into 
two main types — the direct-acting, self-controlled type and the type 
actuated by an auxiliary or relay system of air, water, or electricity. 
In the direct-acting class the pressure of the expansive element, which 
may be saturated vapor, gas, or liquid, operates the valve directly 
through a system of levers or an accordion bellows. In the relay 
system the expansive element controls the auxiliary pressure system, 
which actuates the steam valve. 
