Mystery Box

I love flea markets, especially when I come across interesting-looking pieces of technology that I’ve never seen (or maybe just read about) before. Even better if I can’t tell what they are or what they do. Those always have something interesting to teach you.

So when I saw a large, vaguely telco-looking, Bakelite box for four bucks at a flea market in Maine, I had to get it. I’ve seen a lot of tech in half a century, but this was not only something I’d never seen before — it was heavy, obviously contained mechanics, and looked like some kind of telco device. (If you’ve seen the underside of mid-1900s rotary phones, you know what I mean.)

The bottom side was not much more help in determining what it might be. Normally if it’s some kind of telephone ringer, you can hear the bells resonate when you shake it — but not so, in this case. Just a heavy, solid piece of Bakelite, metal, and whatever else was inside.

The two screws on the front were the obvious place to start opening it up (and I had tried this at my folks’ place when I first bought it), but the crank on the side was preventing the cover from opening. Turning it in either direction seemed to be turning a generator / dynamo mechanism inside the box, so it looked like the only way in was to punch out a retaining pin. I decided to wait until I came back with a car, because whatever this thing was, it was too big and too heavy to fit into my luggage. And I could just imagine the conversation with TSA, trying to explain why I’m flying with something that I didn’t know what it was.

So the Mystery Box got to go home the slow way — via The Great Kia Niro EV Adventure.
(The station wagon, in all its forms, remains one of humanity’s best inventions ever.)

Back home, I punched out the retaining pin, only to discover that the crank would have simply unscrewed — it was just stuck.) With the crank removed, the cover came right off, revealing a generator, mechanical crank-activated switch, terminal block — and two large dry cells. (Note to self: Carrying this back home by car and not plane was a good idea. It’s not a bomb — but I bet it would look like one if x-rayed.)

…So, what is it, (other than old enough to have batteries I’ve never seen in use — and I’m 52)?

A few minutes’ Googling found the answer: it’s apparently the power unit of a “monophone,” a predecessor of the two-element telephone. It was made by Automatic Electric, and is roughly 100 years old. (I’m guessing the batteries might be newer, but who knows?)

Near as I can tell, the batteries (which seem to be non-rechargeable) power the monophone for speech, while the generator creates higher voltage for the ring functionality. (There’s a switch that’s activated when the crank is turned, changing the output terminals from battery to generator. When the crank is released, it springs back to its center position, releasing the switch.)

Neat.

But the coolest part is, somehow the dry cells still produce voltage! (They can only source microamps, but the voltage is still there.)

…I wonder if it will power some 1970s-era telephones I have?

If nothing else, those Hercules batteries will make great Thévenin source demonstrators.