Mercury Cell:

Mercury cells come in two categories; one is a flat cell which is shaped such as a button, although the other is  a  cylindrical  cell  which is  looks  like  a  regular  flashlight  battery. Every cell produces about 1.35 volts.  These cells are extremely rugged and have an associatively long shelf life. The mercury cell has the benefits of maintaining a fairly constant output under varying load conditions.  By this reason, they are used in products like as cameras, hearing aids, electric watches and test instruments.