10 mm
Sources and simplifications
  • The heart is modelled as one current dipole that swings through the P wave, the QRS complex and the T wave. Its magnetic field lines are circles about the dipole axis (the Biot–Savart law), with the return currents in the torso left out.
  • The skin potential map assumes the same dipole sitting in a uniform conductor. Real torsos have lungs and bone, so measured maps are less symmetric.
  • Fall-off with distance uses a dipole curve fitted to the KRISS measurements (an effective source depth of 139 mm). It matches 70%, 51% and 38% at 40, 60 and 80 mm.
  • The brain is one alpha-rhythm dipole in the visual cortex oscillating at 10 Hz. Real MEG sees many sources.
  • Line brightness is on a display scale, not to scale between sources. The log chart gives the true ratios: the Earth's field is about a million times the heart's.
  • The 1.75 m body is a smooth implicit surface (not an anatomical scan). The nerve path is schematic. Time is slowed so the beat, the alpha rhythm and the nerve impulse can be followed.
  • No 'aura': everything drawn here has been measured with electrodes or magnetometers. The fields are far too weak to sense without instruments in a shielded room.

Sources:

The sensor face: the field is read hereSkin coloured by the heart's surface potential, red + / blue −Field lines: heart (orange), brain (blue), Earth (green)The body's real electromagnetic fields, as a three.js scene. Each heartbeat is an electrical wave through the heart muscle. Its currents raise a potential of about 1 mV on the skin, which the ECG records [8]; the skin here is coloured by that potential as it swings through the beat. The same currents make a magnetic field, the strongest in the body, about 10–100 pT near the chest [1]: 100 pT with a sensor 10 mm away [2]. It falls off fast: at 80 mm it is 38% of its value at 20 mm [3]. Move the SQUID dewar with the slider to see it fall. The brain's field is about a hundred to ten thousand times weaker: ~10 fT for cortical activity and ~1 pT for the alpha rhythm [4], read by the helmet of MEG sensors. Nerves carry impulses at 50–60 m/s [10]. When one reaches a muscle, its fibres fire, and electrodes on the skin read the EMG, up to about 10 mV [9]. For scale, the Earth's field is 25–65 µT [5], over a hundred thousand times the heart's. Even city magnetic noise, about 0.1 µT, is a thousand times stronger [4], which is why MCG and MEG need shielded rooms and sensors with a noise of a few fT [7]. An MRI magnet is 1.5–3 T [6]. There is no 'aura': everything here has been measured. The body is a smooth implicit surface meshed at 4 mm, with about 300,000 triangles. Field lines use a glowing shader, the skin potential is painted as vertex colours, and the log chart shows the true ratios.