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....many times the spark is from a loose/poor connection a the bat. terminal itself. I am sure you have all heard that little 'PoP" sometime when bat connection is starting to fail..That pop is a spark..
... and a good reason to always make the neg. the jumper cables last hook-up on the engine frame , away from the battery, so if a spark does occur , you will not ignite battery fumes that accumulate over the bat from attemps to start a car ..