Transformer wiring hookup 110/220

I received my transformers today!!!
I went to wire them up and had to stop to think about how to wire
them.
The transformers are (http://www.mpja.com/product.asp?product=7846+TR)
I hook the blues up for 24V, no problem
The primary is what is confussing me.
There are 2 red wires and one black.
the label on top shows as follows
RED 110V| |BLUE 12V
| |
Black | |Yellow
| |
RED 220V| |BLUE 12v

I would assume if you were using a 220V you would just wire the reds,
and leave the center black disconnected.
but what about 110V? I Hooke one red and the black.
and bingo 24V across the blues.
I started to think about this and though but I don't have the other
half of the primary hooked up.
So I wired the other red to the first red.
The transformer hummed for a second, then tripped the reset on my
power strip.
How do I wire for both primarys to power up?


Jerry