Why is by battery cable sparking?

After having the alternator on my MF255 diesel tracor repaired I was still unable to maintain a battery charge. The positive cable was well worn and taped in several places (probably by my grandpa who bought it new in 1977). I replaced the cable and when attempting to reconnect the battery experienced a great deal of sparking. It makes nodifference which cable I attach first the second one sparks.

Marsha
August 2010