The Bendix G-15 at System Source, the one repaired by Usagi Electric over the past few years. Operational, and on display, and demoed every so often.

via history in the kitchen

The Bendix G-15 from 1956, the world's first "personal computer." One of these cabinets weighed a literal 1000 lbs. and cost half a million dollars in today's money. Or you could rent it for the equivalent of $17,000 a month.

It was numeric only, you had to "talk" to it via a typewriter, and when you turned it off it 'forgot' everything. It used 'drum memory,' which used the same rust-covered-spinning-magnetic-thing technology all hard drives would continue to use until flash memory became a thing. But in the Bendix it looked something like this (one from a slightly earlier computer):

This computer is so old no one seems to have figured out what 'bits' it is. All I know is that drum thing "holds 2,160 words of twenty-nine bits." It can also do basic math problems in 270 microseconds. ...Which sounds fast, but that means it can do 2+2=4 in 27 thousandths of a second. Which is probably exactly how long it took your stupid brain to do that. For slightly more complex math, hiring a human mathematician at this time would be both cheaper and easier than dealing with this computer.

This is a "vacuum tube / diode analog architecture" computer, and it already had some kind of OS that meant you didn't have to know machine code? But that slowed it down even more...?

I have absolutely no idea how analog computers worked, or how anyone used one. There are tape decks on this thing, and it is plugged into a typewriter. Like I get the digital 1 and 0 thing, but I have no concept how you make light bulbs and blobs of germanium do that electronically.

But someone did, and this eventually led to you streaming Now That's What I Call Music! No. 86 on a Samsung phone.