Fine Tunes #10 letters

A letter in response appeared in June 1999:

It's the amps what kills you

Editor: Please tell Jonathan Scull and Nagra ("Fine Tunes," April '99, p.63) that it is current that kills you, not voltage. Shame on them for not knowing their physics.—Tim O'Connor,

We also received the following two letters on the subject, neither of which was published at the time.

It's current and voltage that kills you

I read in "Letters" a criticism of Jonathan Scull for saying that high voltages can kill you. This guy said that it was the current that kills you. Folks, you were both right and wrong at the same time:

A car battery can deliver hundreds of amperes from a voltage of 12 volts and you can touch the terminals without feeling anything.

When you pull your wool sweatshirt off, you can generate tens of thousands of volts—with near-zero current—without feeling anything but a small sensation.

Why? The human body has a high resistance. It behaves like a 30k-200k ohm resistor, depending on the individual and humidity, etc. But what is undeniable is that a current of just 10mA going through your heart can kill you. That is equivalent of about 300 volts for the "low-resistance" guy, and 2000 volts (yes, 2kV) for the "high resistance guy." And as the current often doesn't run through your heart, you can even support currents up to 30mA—equivalent to 900V and 6kV volts—without necessarily dying.

But please don't experiment for yourself! These values are very unpleasant to handle and can be definitely lethal if you "stick" to the wires.

In the end, therefore, both Jonathan Scull and Tim O'Connor were right. You need both at least 10mA current source and a high voltage supply. So reaching inside a tube component when it is turned on is very dangerous. In this regard, you were completely right, J-10.—Hervé Delétraz, Switzerland,