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Electrical Engineers Learn The Hard Way

The Puzzler


RAY: Here's the answer. You're using a wrench to disconnect the battery cables. The wrench is made out of metal, of course. Now, if your wrench contacts a piece of metal that's connected to the frame, you've got a short circuit, because all the ground circuits are connected to the frame, and the negative terminal of the battery is connected to the frame.

So if you should, in disconnecting the positive terminal first, touch the wrench to a metal ground on the car—

TOM: It's a short circuit between positive and negative and badda bing, badda bang!

RAY: It's tantamount to laying your wrench across the two terminals of the battery.

TOM: If you do that, run like hell!

RAY: It's too late to run!

TOM: Actually, this is quite dangerous because the battery can explode.

RAY: So the reason that you disconnect the negative first is if in the process of loosening the bolt to remove the negative terminal, the wrench should contact the frame, it doesn't make any difference.

TOM: Because the negative terminal is already connected to the frame.

RAY: And that's why when you go to put the battery back in, you reverse the order. You connect the positive first and then you connect the negative last.

Do we have, do we have a winner?

TOM: The winner is Monica Hartley from Houston, Texas. And for having her answer selected at random from among all the good answers that we got, Monica will win a $26 dollar gift certificate to the Shameless Commerce Division of Cartalk.com where she can get her father or someone else's father a Car Talk necktie, which features (as you know) in very small print the names of the people on the Car Talk staff like Picov Andropov, and Huey, Louie, Dewey.

[ Car Talk Puzzler ]

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