I think we need that picture.

 

My guess for now:

You may replace it with a modern 2 prong 15 amp duplex receptacle unit or 2 prong 120 volt 15 amp round single receptacle somewhat resembling it.

It has standard hot and neutral terminals (possibly both terminals or both terminal pairs gold, unpolarized, and interchangeable) One terminal or terminal pair connects to the left and top holes and the other terminal or pair connects to the right and bottom holes.

Back in the 1920's they also had (120 volt) plugs that fit just the top and bottom holes.

 

 

 

doityourself.com doesn't seem to like tinypic. replace the **** with tinypic dot com in the URL

 

Box is probably not properly grounded so replacing with a two prong duplex receptacle would be the easiest.
https://www.homedepot.com/p/Leviton-...-00W/100356969

 

 

If you question whether there is a neutral conductor you better halt all plans and do some checking before doing anything. Yes, it is entirely possible there is no neutral so do not install a 3 prong 125 volt receptacle without knowing what you are dealing with. It's also likely there is no grounding conductor so using a 3 prong receptacle is out of the question unless it is GFCI protected.

 

 

 

In the U.S. all 120 volt circuits have one hot and one neutral. If there are indeed two hots either they are from different branch circuits (okay in one junction box) or have 240 volts between them.. I am quite sure that for your existing receptacle the two wires connected to the receptacle are hot and neutral although you should measure for good measure to see 120 volts between them.

You will have to find out which one is the neutral before installing the new 2 prong receptacle unit; the neutral is connected to the "silver" screw. Usually the neutral wire is white and in modern wiring the neutral must be white.

The technical name for "the neutral" is "grounded conductor." The two wires bringing 120 volts to the outlet box would both be hot except by convention and by code one is tied (bonded) to ground the correct location for that bond is at the first whole house disconnect breaker or switch. Assuming that all the connections are solid, voltage measured between neutral and a grounded object is almost zero at all times. Note that in old systems considered "ungrounded" the outlet box itself is not expected to be grounded by virtue of being tied to ground via some combination of wires, metal conduit, and other daisy chained junction boxes.

Meanwhile a neutral wire is not supposed to be used as a ground wire. By properly grounded we mean a circuit or system with a complete separate path back to the main disconnect as the ground path (equipment grounding conductor).

Three prong receptacles connected to ungrounded circuits by mistake can be grounded using a separately run ground wire down to the panel where the breaker for that circuit is located.

 

 

You can't get 124v without a neutral or incorrectly used ground. The 120 volts is derived from one leg of the 240v supplied to your house and neutral (aka grounded conductor from the center tap of the secondary of the service transformer).

 

Next try this:
Measure voltage from one of the receptacle wires to the edge of the box. Then measure from the other receptacle wire to the edge of the box. If you get approximately 120 volts for one measurement and also get approximately zero volts for the other measurement then label the wire that gave you the near zero measurement as the neutral. Now you can install the new receptacle.

If either or both of the measurements are not as I described then more tests are needed before you install the new receptacle.

 

But that is assuming the box is grounded. When the old receptacles like the OP showed us was used grounding conductors weren't used. 2 wires of the same color in the box back then might be 110 volts or might be 220 volts. Today, 120 or 240 volts. Still see some of those old receptacles with the original old wiring in a lot of the old turn-of-the-century houses in the older parts of most big cities.

 

Instead use a 3-wire extension cord plugged into a correctly grounded receptacle as your reference point. To determine neutral measure wide slot to each wire. The Wire that shows ~120v is hot. Try narrow slot to metal box to see if it is grounded. ~120v indicates grounded. There are other ways but that should work.

 

 

Plug the drop cord in a different socket, er, receptacle, that is properly grounded.

To find the neutral, test using the wires attached or formerly attached to the old receptacle, not the holes in the receptacle. (You can test using the terminal screws of the old receptacle if the wires have not yet been unhooked.)

If the box was not grounded then you will not get about 120 volts from one old receptacle wire to the lip of the box and also about zero volts from the other old receptacle wire (the neutral) to the box as I had suggested earlier. Then you would need the drop cord.