Electrical device that can generate a constant amount of steam until the device is switch off
If you want to BUY one then this is not the place to ask :-).

Sound like fun, but, why do you want steam in a cold lab?



Concepts. Made up on spot. Other methods may be better.


Kettle with feed tube in via hole such that it stops say 50mm above bottom.
Feed from feed bottle such that water feeds if level drops.   

Power up kettle for N minutes with timer on demand. Heats and boils. Water self maintains level.
As above but fee is into ceramic or Pyrex beaker.
Use timer to turn on DC ppwer to an eg Nichrome element in water to boil or vaporise water as required.
Water dripper is solenoid controlled.
Water drops fall onto heater element or catridege heater with surface temperature > 100 C when on.
Enable resistor power and dripper.
Pish... pish... pish ... Ah!
Carpet steamer ...
Steam iron with feed ...
If you want water vapor and not necessarily STEAM as such then an ultrasonic mister does this very well indeed.