Feeling electric fieldsWhy is electricity dangerous to humans? The main reason is that the human body is controlled by ‘electric wires’ itself. As a result, electricity applied to human bodies from the outside interferes with the internal signals. This has been known since 1789. In that year the Italian medical doctor Luigi Galvani (1737–1798) discovered that electrical current makes the muscles of a dead animal contract. The famous first experiment used frog legs: when electricity was applied to them, they twitched violently. Subsequent investigations confirmed that all nerves make use of electrical signals. Using electricity, one can make fresh corpses move, for example. Nerves are the «control wires» of animals. We will explore nerves in more detail below. Being electrically controlled, all mammals can sense strong electric fields. Humans сan sense fields as low as10kV /m, when hair stands on end. In contrast, several animals can sense much weaker electric (and magnetic) fields. Sharks, for example, can detect fields down to 0.5 μV/m using special sensors, the Ampullae of Lorenzini, which are found around their mouth. Sharks use them to detect the field created by prey moving in water; this allows them to catch their prey even in the dark. The elephantnose fish (Gnathonemus petersii), the salamander and the platypus (Ornithorhyncus anatinus), the famous duck-billed mammal, can also sense electric fields, but achieve only sensitivities of the order of mV/m. Like sharks, they use this ability to detect prey in water which is too muddy to see through. The muscles in living prey generate electric fields. This method is also used by the elephantnose fish. The achieved sensitivity is below 2 mV /m. Certain fish, the so-called weakly-electric fish,even generate a weak field in order to achieve better prey detection. In fact, several electric fish use time-varying electric dipole fields to communicate! They tell each other their species, their sex, their identity, and communicate about courtship, aggression, appeasement and dangers. The frequencies they use are in the range between a few and 200 Hz, and the fields are dipole fields created between the anterior and posterior sections of their bodies. No land animal has special sensors for electric fields, because any electric field in air is strongly damped when it encounters a water-filled animal body. Indeed, the usual atmosphere has a low, vertical electric field of around 100 V/m; inside the human body this field is damped to the μV/m range, which is far less than an animal’s internal electric fields. In other words, humans do not have sensors for low electric fields because they are land animals. (Do humans have the ability to sense electric fields in water? Nobody seems to know.) However, there a few exceptions. You might know that some older people can sense approaching thunderstorms in their joints. This is due the coincidence between the electromagnetic field frequency emitted by thunderclouds – around 100 kHz – and the resonant frequency of nerve cell membranes. The water content of the human body also means that the electric fields in air that are found in nature are rarely dangerous to humans. But whenever humans consciously sense electric fields, such as when high voltage makes their hair stand on end, the situation is potentially dangerous. The high impedance of air also means that, in the case of time-varying electromagnetic fields, humans are much more prone to be affected by the magnetic component than by the electric component Can humans feel magnetic fields? So far, there is no definite answer. Magnetic material seems to be present in the human brain, but whether humans can feel magnetic fields is still an open issue. Maybe you can devise a way to check this? (from Christoph Schiller “MOTION MOUNTAIN”)
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