Abvolt to Microvolt

The CGS electromagnetic system uses centimeters, grams, and seconds as base units instead of meters, kilograms, and seconds. When you derive the unit of voltage from these smaller base units, the resulting "natural" voltage unit comes out absurdly small — 10⁻⁸ V. This is not a flaw but a consequence of the choice of base units: the CGS system was designed to make electromagnetic equations simpler (no factors of 4π or μ₀ in certain formulas), and the price was impractical unit sizes. The abvolt is to the volt what a grain of sand is to a boulder.

Rarely in isolation. Physicists working in the CGS-EMU system in the late 19th and early 20th centuries used abvolts in theoretical derivations and internal calculations, but they almost always converted results to "practical" units (volts, amperes, ohms) for publication and laboratory records. The practical units were specifically designed by the British Association for the Advancement of Science in the 1860s–1870s as convenient multiples of the CGS units. The volt was defined as exactly 10⁸ abvolts precisely so that real-world voltages would have sensible numerical values.

They come from two different CGS subsystems. The abvolt belongs to CGS-EMU (electromagnetic units), where the unit of current (abampere = 10 A) is defined by magnetic force. The statvolt belongs to CGS-ESU (electrostatic units), where the unit of charge (statcoulomb) is defined by Coulomb's law. The ratio between them is the speed of light: 1 statvolt = c × 10⁻⁶ volts ≈ 299.8 V, while 1 abvolt = 10⁻⁸ V. So one statvolt equals about 29.98 billion abvolts. The two systems produce wildly different unit sizes because one is optimized for magnetism and the other for electrostatics.

Because electricity and magnetism were studied as separate phenomena before Maxwell unified them in the 1860s. Electrostatics researchers defined units based on Coulomb's force law (ESU system), while magnetism researchers defined units based on Ampère's force law (EMU system). Each system made its own equations clean but produced incompatible units for shared quantities like voltage and charge. Gaussian units tried to merge both by using ESU for electric quantities and EMU for magnetic ones, with the speed of light as the bridge. SI finally resolved the mess by treating the ampere as a base unit independent of mechanical units.

In 1861, a committee led by William Thomson (Lord Kelvin) and James Clerk Maxwell chose centimeter, gram, and second as base units because they were already standard in laboratory physics. They then derived "absolute" electromagnetic units — the abvolt, abampere, abohm — from mechanical force equations. The resulting unit sizes were wildly impractical (the abvolt is 10⁻⁸ V), so the same committee created "practical" multiples: the volt (10⁸ abvolts), ampere (0.1 abampere), and ohm (10⁹ abohms). These practical units eventually became SI, while the absolute units faded into textbook footnotes.

By the time electrical activity from neurons reaches your scalp, it has been attenuated enormously. Each neuron fires at roughly 70 millivolts internally, but the skull and cerebrospinal fluid act like a lossy, low-pass filter. Billions of neurons fire asynchronously, and their fields mostly cancel. Only when large populations synchronise — as in alpha waves during relaxed wakefulness — does a coherent signal of 20–100 μV emerge at the scalp. Intracranial electrodes placed directly on the brain surface (electrocorticography) pick up signals 10–100 times larger, in the millivolt range.

The Seebeck effect: when two different metals are joined and the junction is heated, electrons in each metal diffuse at different rates, creating a net voltage. A type-K thermocouple (chromel–alumel) generates about 41 μV per degree Celsius. This means measuring a 0.01°C change requires resolving 0.41 μV — well within the microvolt regime. The effect works because the electron energy distribution in each metal responds differently to temperature, and the voltage is the integral of these differences along the wire.

Not directly, but a good moving-coil phono cartridge outputs about 3–5 mV at its hottest, and the quietest grooves on a vinyl record may produce only 5–20 μV. A phono preamp with 40–60 dB of gain boosts this to line level. The signal-to-noise challenge is real: the thermal noise of the cartridge's coil resistance at room temperature is itself in the microvolt range, which is why audiophiles obsess over low-noise preamp designs. Below about 1 μV, you are essentially trying to hear the random jiggling of electrons.

Electrooculography (EOG) picks up eye-movement potentials of 15–200 μV. Electroretinography (ERG) captures retinal responses as low as 5 μV. But the subtlest commonly measured biosignal is the auditory brainstem response (ABR), used in newborn hearing screening — it is about 0.1–0.5 μV, requiring hundreds of averaged recordings to pull the signal out of background EEG noise. Foetal ECG detected through the mother's abdomen sits at roughly 1–10 μV. Below that, you need implanted electrodes.

Because the noise you are trying to reject is millions of times larger than the signal. Mains hum from power lines induces about 1–10 mV of 50/60 Hz interference on the human body — up to 10,000 times bigger than a 1 μV biosignal. A differential amplifier subtracts the signal at two nearby electrodes, cancelling the common interference while preserving the local signal difference. Common-mode rejection ratios above 100 dB (100,000:1) are standard in medical instrumentation. Without this, every EEG recording would just be a picture of your wall socket's frequency.