"The object of the invention is an electrical door contact which makes it possible to distinguish the opening or closing of different doors from each other by hearing." 
A door contact, characterized by the arrangement of a tooth comb a2 of conductive material, over the teeth of which, which have equal or different distances, a metallic conductive tooth a1 slides when the door is opened and closed and comes into contact with the teeth a2 one after the other, whereby bell signals are evoked, the interruption duration and number of which depend on the distance and number of the teeth a2, so that different signals can be evoked with one bell when opening or closing different doors." 
Even though this invention had nothing to do with furnace construction, it already shows the inventive spirit of the Ruhstrat brothers.
We can vividly imagine the fun the imperial servants had when opening doors equipped in this way particularly slowly or particularly quickly....
 Imperial Patent Office, PATENT WRITING - No. 49816, Berlin December 4, 1889