rort] LAND—CONSTANT TEMPERATURE APPARATUS 307 
It is realized that most botanists are not electricians, and therefore 
the description of the construction and wiring of the instrument 
is not written for experts. 
The action of the apparatus is as follows: When the platinum 
points of the thermostat are in contact, the current flows through 
Fic. 4.—Photograph of thermostat, heater, ana switch, with connections. 
the electromagnet, the armature is drawn down, closing the switch 
and making a circuit through the heating coil. The circuit will 
remain closed only so long as the platinum points of the thermostat 
are in contact. As the temperature of the oven rises, the platinum 
points are separated, the circuit through the electromagnet is 
broken, and the switch is opened by the release of the armature. 
