I just came upon these photos of a 120/240V travel transformer and they reminded me of one I acquired back in the late 1980s which had the exact same arrangement. It seems there might still be quite a few of these around, so I'm posting this as a warning.
Here's the item in question:
In case you can't figure out the deal, here's how it works. The sockets on top of the transformer housing are the type which will accept both flat-pin American plugs and round-pin European plugs. The "special power cord" (as I recall the advertising called it on the one I had) is fitted with one plug of each type.
So if you are in the U.S. and want to use European equipment, you put the European plug from that cord into the 120V socket, plug the other end into your wall outlet, and then plug your European appliance into the 240V socket.
If you are in Europe and want to use American equipment, you put the American plug on that "special" cord into the 240V socket, plug the other end into the European wall outlet, then you get 120V at the other socket for your U.S. appliance.
It's a very ingenious arrangement to allow both-way use with one power cord, but one which is a potential death-trap. The risk of electric shock with the two-plugged cord is obvious and it would be all too easy for the transformer-end to get pulled out accidentally leaving energized prongs exposed. Especially dangerous if there are children or pets around.
Even more scary is the prospect that it's easy to accidentally plug the cord into the wrong side of the transformer. Transformers alter the voltage based upon the ratio of the number of turns in the windings, so if you accidentally connected the 120V socket to your 240V wall outlet, you'd end up with 480 volts on the other socket!
Please, if you ever come across one of these units, for safety's sake give it a miss.
(In case you're wondering what I did with the one I had, I only needed it for 240-to-120V conversion so I rewired it properly to make it safe.
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