In an issue devoted to sensory phenomena, it may seem odd to include an article on sensing something that we cannot consciously perceive: electric force. Of course, we can sense the dramatic power of a lightning bolt: we see the flash, hear the boom, feel the rumble, and, if we're close enough, smell and taste the ozone produced. Lightning is caused by an enormous electric field that develops under a thundercloud due to the separation of electrical charges between the cloud and the earth. Almost 250 years after Benjamin Franklin's kite-in-the-storm experiment, we still don't fully understand how this charge separation is generated. But it's nevertheless a fact that if you are standing under a thundercloud, you are immersed in a large vertically directed electric field, typically a few hundred volts between your head and your feet. Just before a lightning strike, this electric field becomes large enough to literally rip gas molecules in the air apart by pulling negatively charged electrons in one direction – towards the positively charged ground – and positive nuclei towards the negative cloud. Once a column of air becomes ionized in this way, the charged particles zoom to their respective ‘electrodes’, discharging the cloud–ground capacitor in a bright, hot flash of enormous electrical current. Lightning is a manifestation of a phenomenon called ‘dielectric breakdown’, something we've all seen in more controlled contexts, such as cheesy horror movies, Ask-Dr-Science demos at science museums and aluminium foil mistakenly placed in the microwave oven. Dielectric breakdown of most materials occurs at electric fields on the order of a few million volts per metre. But there is a great irony here: whereas we sense all of the secondary consequences of lightning, we are utterly blind to its most fundamental element – the electric field that gets the whole thing going. Humans just never evolved electric-force sensors. (But fish did – and migratory birds and turtles navigate by ‘feeling’ the earth's weak magnetic field.

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