79 mustang keeps cranking after start

so i hop in my mustang like any other day go to start it and when it turns over i keep hearing the starter trying to crank. i imediatly turn the key off and it is still cranking. it stops after about 20 seconds. i think wtf this has never happened before so i go for attempt number 2. no luck same thing happens. so i pop the hood and cant see anything out of the ordinary. turn the key here it again so i look. its this little black cylinder about 3 inches long with the positive end of the battery connected to it smoking and the car keeps trying to turn over. what does this mean and what has gone wrong. i couldnt get any pictures attached so in the mean time maybe someone that know what im talking bout can comment back.