4. 4. 16
Victorian Telegraph Office Commutator.
U is the terminal for the Up line
D is the terminal for the Down Line
I and I are the terminals for the internal office wires
E and E are wired to Earth and act as lightning Arresters.
Every line in an office is fitted with such a commutator and changes between lines are made by cords with plug ends, the plugs being placed in the block holes.