Quote:

That's what I always thought. Turns out that there is some evidence that three were used.

The two main engine telegraphs on each end of the bridge, and the emergency telegraph as well.

I learned this information on marconigraph.com, where testimony is examined and shown that the engine room heard more than two telegraphs sounded.

The emergency telegraph was used usually when both main units either broke a chain or stopped working. But in this case, it seems like Murdoch rang it as well to let the engine room know it was a real emergency.

I think James Cameron had Murdoch do this as well on the new movie, because the second telegraph he rings does not look like it was on the other side of the bridge, but more like the one in the middle which would be the emergency unit.

http://titanic.marconigraph.com/telegraph22.jpg