I found a beautiful electromechanical calculator at Goodwill in phenomenal condition for once (usually they're filthy and broken beyond reasonable repair). Watch this video to see me fumble my way through using a simple-ass adding machine!

00:35: Description of the device
04:45: Operating mechanism by hand
07:50: Electrical modification
19:30: Usage demonstration
23:30: Mechanics demonstration
23:45: Slow-motion footage

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