Unless this Wardian box is being kept someplace really cold, I'd agree with
Rick's suggestion that a little light would do the trick. If this was my
project, I'd buy a thermostat (a $30 Dayton one would be fine), hook it up
to a light bulb, and be done with it. Maybe I'd rig a small light-tight
enclosure for the light bulb if I were afraid the light emitted would
adversely affect the plant because of some photoperiod requirements...
40W is a potent power source, especially for a small enclosed area.
>Many may consider what I am about to write a nit, but as a physicist,
Glenn, maybe it is a nit, but an important one. Let's just not lapse into
discussion of local thermodynamic equilibrium. We can think it though.
Barry