Arc Welding with 9V Batteries

I used to work intermittently as a theatre technician, doing a mix of carpentry and electrical work in order to build sets and lighting systems.

Once, for a production of “West Side Story,” our crew rented an old Hobart arc welder in order to build portions of the set using chain link fencing (subtlety was not included in the production budget). We were fascinated to discover that the welder is literally just an extremely powerful soldering iron, using a high-amperage DC current to simultaneously melt steel and forcefully eject molten debris.

I became interested in seeing whether the same effect could be accomplished on a much smaller scale using a lower-power DC source. In a stroke of good luck, I discovered that another theatre at which I was working was disposing large numbers of half-spent 9V batteries from its wireless microphones. These batteries are generally changed very often, since no theatre wants to risk Maria’s mic cutting out halfway through “I Feel Pretty.”

Since many of the used batteries still had plenty of energy, I started stockpiling them. A convenient feature of 9V batteries is the metal “claspers” that allow them to be easily connected together, which allowed me to connect all of my connected batteries in series to build a power bank of ~100 V DC.

Batteries are rated in ampere-hours, which is a unit related to energy content. The idea is that, if I have a 10 ampere-hour battery, I can draw one amp for ten hours, or 10 amps for one hour, or 2 amps for 5 hours, and so on. The only limit to the amount of current that can be drawn is the battery’s internal resistance, which will steadily heat up as the current increases.

In the video below, it is apparent what happens when my 81V C circuit of ridiculous amperage is discharged through a graphite rod (a pencil lead) onto a thin piece of steel wire. As the steel wire heats up, it gradually begins to melt, resulting in the orange glow you see at the end of the video. The strange cylinder I’m holding was the closest thing I could find at the time to a UV filter, since staring directly into the arc is inadvisable. Pardon some of the poor directorial decisions involved in making the video, but definitely watch the last ten seconds or so:

I’m guessing that the shattering at the very end is the result of the interior of the graphite heating up relative to the exterior, resulting in differential thermal expansion across the lead and eventual mechanical failure (this is also why your windshield shatters if you pour boiling water onto it in the winter). Fortunately, I was wearing goggles and gloves at the time.

I eventually had to discontinue my experiments, as the now-overheated batteries began to hiss at me. But what I like about this project is that it shows directly the connection between the low-energy electrical devices we encounter every day and the dramatic electric arcs that we encounter in science classrooms.


One thought on “Arc Welding with 9V Batteries

  1. Edison Carbon Light | gammacephei

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