I found a strange and old electrical device. It seems it could be plugged into the wall and you could plug another equipment in it. Inside there is a sort of heating resistor, I'm not sure. I don't know what is it or where it comes from. Has someone already see that? Thanks for any suggestions unknown device the other side inside

EDIT I live in France so we have 220v 50Hz but before we had 110v 50Hz, so maybe it could be a adaptative device?

  • \$\begingroup\$ It seems there's something broken in one of the contacts of the receptacle. Can you open the device (to fix it) and see what is inside? \$\endgroup\$
    – Anonymous
    Commented Oct 30, 2016 at 21:16

3 Answers 3


The device is indeed a vintage resistive mains adapter plug:


enter image description here

As you suggested, it is used to drop 220V mains down to 110V for older equipment.

  • \$\begingroup\$ Why would someone design it so that live prongs stick out?! \$\endgroup\$ Commented Oct 31, 2016 at 2:03
  • 1
    \$\begingroup\$ @immibis it doesn't. That sitting on a table. The prongs go into the outlet on the wall. \$\endgroup\$
    – Passerby
    Commented Oct 31, 2016 at 2:37
  • \$\begingroup\$ Yeah I think it's that, thanks. But it only works with a 735 ohms load? \$\endgroup\$ Commented Oct 31, 2016 at 4:00
  • \$\begingroup\$ Goes great with furniture :) \$\endgroup\$
    – Thomas
    Commented Oct 31, 2016 at 5:37
  • \$\begingroup\$ @JamesMagnus In theory, yes -- In order to drop 110V it would need to have about 150mA flowing through it, which means that the load would need to be around 735 ohms. But these things were never accurate, and the voltage would fluctuate wildly depending on the current draw. There's a reason they're not used anymore. \$\endgroup\$
    – DerStrom8
    Commented Oct 31, 2016 at 12:53

Fairly wild guess here, but one possible use would be for old vacuum tube radios that used a series resistor to drop the mains voltage to a suitable level for the series string of filaments. Assuming 735R is the resistance, that would be a plausible value.

Old radios sometimes used an internal resistor, an internal "ballast tube", some of which bear a resemblance to the object in question, or a resistive line cord. The latter proved something of a fire hazard as people did not always follow directions and routed the cord under rugs or balled them up.

The device shown could have been used with such a radio that had its main cord replaced with a non-resistive type.


This seems to be a 735 Ohm load resistor. Measure the resistance! Either there's 735 Ohm between the two pins, or 735/2 Ohm between each pin and the hole on the opposite side, so you can either stack multiple of them for parallel resistances, or for serial resistances (in which case you'd have to terminate the last pair of holes with a short).


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