An EX-NOR gate can only have two inputs. The o on the output means not showing that it is a Not EX-OR gate. The output of an EX-NOR gate is true when its inputs are the same (both true or both false).
Galvanometer A very sensitive meter used to measure tiny currents, usually 1mA or less.
Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the page on logic gates. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).