Speedometer issues

Thanks everyone, I knew there was a magnet but unsure of zackly how it worked, thanks for the explanation RR, and for the "alt" as well Ben.

Cosgig, if you order a new cable be careful we checked several websites and also spoke on the phone to rockauto....95% of the listings are wrong! We spent $11 on the cable and $26 in shipping to get the wrong cable, they wont refund shipping...so it looks like we have one that is nfg to us. I will try to get the right # for you. The speedo doesnt return to zero on its own so I think the return spring might be pooched, but I think we will wait until winter, to R&R the dash bezel and get everything done and inspected. Wonder what the chances of finding a 67 150 speedo with trip if need be?? LOL

Again, thanks everyone for their input