Interesting Pull Station Switch

Recently I salvaged two IBM 4251-1 pulls from an abandoned building, and their switches are quite weird. Instead of having the switch make contact with something, it uses a bulb with some conductive liquid. Here is an exclusive video for the forums:


I haven’t seen or heard of something like this in a pull station before. Someone correct me if I am wrong, but that appears to be a mercury switch. Being liquid, it can run from one part of the tube to the other freely. But it is also metal, which makes is quite conductive. When the mercury is on the side of the contacts in the bulb, it closes the circuit. Not common anymore due to environmental and health concerns.

Yes, these early models utilized a mercury bulb switch from the early to late 50’s I believe. If you take the cover off or if there is a metal tab instead of a button it is a mercury switch. The later local alarm chevrons branded under Simplex and Couch in the later 60’s to early 70’s had electric switches in them, mine does.

Once upon a time these were quite common in mechanical thermostats. The principle is to reduce bounce — the tendency of raw switch contacts to bounce between an open and closed circuit for a short period of time when switched. With a high current, this can lead to arcing, which leaves oxidation and carbon deposits which eventually will cause the switch to fail. In a thermostat, it has the added benefit of being able to switch depending on the angle of a bimetallic coil. Especially in this case it has much less tendency to have serious bounce when it is just approaching the critical temperature, which of course would also be quite detrimental to fans and compressors. I’d imagine this pull station was designed to pass the current of a long string of mechanical horns or bells.

The mercury switch is immune to bounce, but, of course, it has its downfalls. Mercury is a dangerous neurotoxin, and these switches won’t work properly if they are not mounted level. Nowadays bounce can be taken up by digital electronics and thermostats use electronic temperature sensors. The switches in pull stations only pass a few milliamps of current, which triggers a chain of transistors and relays housed in addressable circuitry and control panels. So these are completely obsolete, but a nice curiosity, as long as you don’t break the glass! :shock: