The BGA Velectrik is a relatively basic device. The overall system included a giant rigid lever close to the down tube and pivoted near the bottom of the down tube. This lever controlled a rod running along below the chainstay. The derailleur was firmly bolted to this rod by two eyebolts (sporting brass nuts in the photo). Pulling the lever back physically dragged the derailleur forwards. The derailleur was mounted on a bracket that was bolted to the chainstay and had a slanting slot in it. This slot controlled the movement of the derailleur, so that, as the derailleur was pulled forward, it also moved inwards, in an early attempt to maintain chain gap.
Crucially all this was achieved without using any cables. In the 1930s there was a significant population of cyclists who believed that cables were too fragile and unreliable to use for anything important on a bicycle. In the UK, Raleigh was still selling significant quantities of their Raleigh Superb rod brake bicycles well into the 1970s - because cables were somehow too dodgy to trust with your life, at least on heavily used utility bicycles.
The wire with the hook at the end, in the photo, attached to a spring running along the under-chainstay rod, providing chain tension.
This particluar example has a pleasantly prehistoric look, with splendidly worn teeth on the guide pulley - showing that this simple derailleur has been happily ridden for many, many miles.
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