# SHE Waddles in Beauty, Like the Night

I saw, via twitter, so links to a story that seems to be a repeat from a few months ago: about a bra (dubbed SHE) that delivers a jolt to would-be attackers.

The problem with a majority of them: they claim the shock is 3,800 kV (kilovolts). That’s curious jargon, to use that number and that prefix — why not just say 3.8 MV (megavolts)? I suspect from that alone that this is a transcription error somewhere, and the original was 3,800 Volts, which is 3.8 kV, and someone combined them. It’s even possibly an error of notation, because some folks use commas and periods in the opposite sense in their display of numbers. Maybe someone got confused.

But really, the true clue that this is wrong is the physics. The dielectric constant strength of a material tells you how much of a potential difference you can apply before it fails as an insulator. For things like neoprene or polyethylene, i.e. rubber or plastic, which are good insulators, the value is around 20 MV/m and we have almost 1/5 of that. In other words, you’d need about 20 cm of the material at an absolute minimum, which gives new meaning to the term “padded bra”. Since nobody with engineering sense would fail to have a safety margin — conditions will be less than ideal, materials wear and crack, etc. — it’s larger, perhaps double (or more). So, in a word, no.

But scale that down to 3.8 kV and the thickness is now of order a millimeter, which is reasonable. Within the context of the discussion.

## 2 thoughts on “SHE Waddles in Beauty, Like the Night”

1. I don’t think dielectric constant relates to insulation breakdown. I don’t think I need to provide wiki links, I suspect you will realize your mistake..

2. Yes, you’re correct. That should be dielectric strength, which was the table I was looking at for the values, just prior to my brain fart.