Hi there!
I've had the priviledge of visiting The Temple Institute in Israel under the instruction of a Rabbi. The Temple Institute is the place in Israel where the Jewish people are accummulating the furnishings for the third temple.
In the Temple Institute there is a large painting of the High Priest going into the Holy of Holies and it displays how the urim and the thummin worked, and the Rabbi explained it to me further in more detail.
It seems that the urim and thummin that Joseph Smith claimed to have received, were not used in the same manner, but rather that he looked into a "hat" for the revelation. Interestingly, the urim and thummin of the Bible required light to "reflect" the message from God. The urim and thummin of the bom required darkness. Why?
~serapha~