Suggest you read the following:
Catherine Sease.  1990.  A new means of controlling relative humidity
in exhibit cases.  Collection Forum 6(1):12-20
 
The system described in this article regulates humdity in several cases
from one central unit.  Catherine Sease is at the Field Museum, which
has a most excellent Egyptian exhibit, so you should contact her directly
there.
 
John Simmons
Museum of Natural History
University of Kansas