Source : Free On-Line Dictionary of Computing

gray code
     
         A {binary} sequence with the property that only one
        {bit} changes between any two consecutive elements (the two
        codes have a {Hamming distance} of one).
     
        The gray code originated when {digital logic} circuits were
        built from {vacuum tubes} and electromechanical {relays}.
        Counters generated tremendous power demands and noise spikes
        when many bits changed at once.  E.g. when incrementing a
        register containing 11111111, the {back-EMF} from the relays'
        collapsing magnetic fields required copious noise suppression.
        Using gray code counters, any increment or decrement changed
        only one bit, regardless of the size of the number.
     
        Gray code can also be used to convert the angular position of
        a disk to digital form.  A radial line of sensors reads the
        code off the surface of the disk and if the disk is half-way
        between two positions each sensor might read its bit from both
        positions at once but since only one bit differs between the
        two, the value read is guaranteed to be one of the two valid
        values rather than some third (invalid) combination (a
        {glitch}).
     
        One possible {algorithm} for generating a gray code sequence
        is to toggle the lowest numbered bit that results in a new
        code each time.  Here is a four bit gray code sequence
        generated in this way:
     
        	0 0 0 0
        	0 0 0 1
        	0 0 1 1
        	0 0 1 0
        	0 1 1 0
        	0 1 1 1
        	0 1 0 1
        	0 1 0 0
        	1 1 0 0
        	1 1 0 1
        	1 1 1 1
        	1 1 1 0
        	1 0 1 0
        	1 0 1 1
        	1 0 0 1
        	1 0 0 0
     
        The codes were patented in 1953 by Frank Gray, a {Bell Labs}
        researcher.
     
        {(http://www.nist.gov/dads/HTML/graycode.html)}.
     
        (2002-08-29)