Voltmeter Measures voltage. The proper name for voltage is potential difference but voltage is more widely used.
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).
An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.