Here are some more positive and negative numbers using sign-magnitude form, using several different bit lengths:

number     4 bits     8 bits     12 bits
------     ------     ------     -------
0          0000       00000000   000000000000
1          0001       00000001   000000000001
-1         1001       10000001   100000000001
-15        can't do   10001111   100000001111
7          0111       00000111   000000000111
-7         1111       10000111   100000000111
8          can't do   00001000   000000001000
-8         can't do   10001000   100000001000
576        can't do   can't do   001001000000
-576       can't do   can't do   101001000000