Finding transformer's wire wrapping direction without taking it apart.

[FortranMan]

So I have the following transformer

Is their a particular name for the kind of transformer I have above? I've learned the right side middle is called a center tap, and is used for some weird signallying things in engineering and I don't plan to use it. It's the left part that concerns me. Now I've learned that if I was in Europe, I should wire the primary as follows to connect it to a wall socket.

But I am in America, and from what I learned I should wire them this way to attach to an American wall socket.

This will allow me to generate the same voltage in the secondary as if I had plugged it into a European wall socket. Now if the vendor was nice they would have indicated the polarity of the primary's output wiring, but they did not, so I am worried about wiring them up like this:

Now I suppose trial and error would be the way to go, or if I had a fancy backemf sensor or a $1k oscilloscope I could determine the correct polarity, but is there a way to determine the polarity using a simple digital multimeter? One method I saw was to actually connect the primary and secondary together, then connect a DC battery over the primary and another DC battery over the secondary in something like a Wheatstone bridge, then determine whether the polarity is right based on the voltage sign and total. Is this correct or is there a better way?

 

[berkeman]

The left side is called a "split center tap". The pins should have numbers, and/or the datasheet should have a mechanical drawing with the pin numbers and polarity dots. Can you post a link to the datasheet?

And what are you doing wiring up an AC Mains transformer if you don't have this basic understanding? Are you aware of the safety regulations for what-all needs to be included on the AC Mains side connection of this transformer?

EDIT/ADD -- Since you are teaching a science class, what do you suppose would happen if one of your students was shocked and hurt by this? And if you started a fire with this in your classroom, how do you think that would be viewed by the school and the parents?

 

[Averagesupernova]

If you are thinking you can use DC to sort things out then you are in over your head already.

 

[Dullard]

Assuming no marking, no documentation, and an absolute need to determine the primary wiring, I would:

1. Use an ohmmeter to determine the 'pairing' of the primary windings.
2. Tie 1 wire from each winding together - leave the remaining 2 wires 'open' (like the "Europe" picture)
3. Apply a small AC test voltage across the secondary. (from an AC wall-wart, another transformer...)

The voltage across the mystery windings will either "sum" or "cancel" depending on how you guessed (in step 2, above). Try both arrangements (and compare) if doubt exists.

-Do not use a test voltage higher than the nominal secondary voltage of the transformer
-Treat all conductors as lethal (while the test voltage is connected)