8 BULLETIN 951, U. S. DEPARTMENT OE AGRICULTURE. 
the relay circuit to four volts, and thus prevent arcking and burning 
the thermostat and relay contacts. Condensers are placed in the 
circuits where shown (fig. 6) for the purpose of absorbing the induc- 
tive discharge from the coils when the circuits are broken. 
Fig. 5. — Temperature-control board of incubating room in Patbological Division. 
The relay is connected for reverse action ; that is, when the electric 
circuit is completed through the thermostat, owing to rising tempera- 
ture in the room, the relay operates to break the main heating-coil 
circuit and thus cut off the supply of heat. On the other hand, when 
the relay circuit is broken in the thermostat, owing to a lowering 
