Bell transformers are typically an UNRECTIFIED transformer in the range of 10 to 16 volts.
This is by its very nature AC not DC.
It could be tapped and then rectified for a better signal using a voltage divider or regulator. (regulator preferred).
Either way it needs to be both isolated and changed to DC before it is easily useful to an Arduino to keep the cost down or prevent damage..
There are small CURRENT SENSORS also available for Arduino's that may be more useful too.