CGS e.m. unit to Microampere

The CGS e.m. unit of current (10 A) was inconveniently large for everyday lab work, while the CGS e.m. unit of resistance (the abohm, 10⁻⁹ Ω) was absurdly small. Physicists created "practical" units — the ampere, volt, and ohm — as decimal multiples that gave human-scale numbers. The ampere was set at 0.1 abampere. These practical units eventually became SI, while the "absolute" CGS units became historical footnotes.

In the 19th century, electricity and magnetism were treated as partially separate phenomena, leading to separate "natural" unit choices. The EMU system normalized magnetic permeability to 1; the ESU system normalized electric permittivity to 1; the Gaussian system mixed both. Once Maxwell unified electromagnetism, this fragmentation became unnecessary — but the systems persisted in literature for a century.

They introduced "practical" units — the ampere, volt, and ohm — as decimal multiples of CGS-EMU quantities. The ampere was defined as 0.1 abampere (CGS e.m. unit). This practical system eventually became SI, while the "absolute" CGS units faded. The factor of 10 was chosen for human-scale convenience.

The gauss (magnetic flux density, = 10⁻⁴ tesla) remains surprisingly common — refrigerator magnets are rated in gauss, and MRI field strengths are often quoted in both tesla and gauss. The oersted (magnetic field strength) appears in materials science. These CGS-EMU holdouts persist because their numerical values are more convenient for everyday magnets.

The SI was officially adopted in 1960, but the transition took decades. Most physics journals required SI by the 1970s, though astrophysics and plasma physics held onto Gaussian CGS into the 2000s. Some subfields never fully switched — you can still find new papers using gauss and oersted alongside tesla and A/m.

A CR2032 coin cell has about 225 mAh capacity. At 10 μA continuous draw, it lasts roughly 225,000 / 10 = 22,500 hours — about 2.5 years. At 1 μA, theoretical life exceeds 25 years, though self-discharge limits practical life to about 10 years.

Not from shock — the perception threshold is about 500 μA (0.5 mA) for DC and 1,000 μA for AC at 60 Hz. However, microampere currents applied directly to the heart (e.g., through a catheter) can cause ventricular fibrillation at as little as 50–100 μA, which is why medical device safety standards are so strict.

A glass pH electrode has an internal resistance of 10–1,000 megaohms. Drawing more than a few microamperes would cause voltage drops across this resistance, shifting the reading. Modern pH meters use high-input-impedance amplifiers that draw under 1 μA to avoid disturbing the electrochemical potential being measured.

Quiescent current (Iq) is what an IC draws when powered on but doing nothing — no signal processing, no load driving. For battery-powered designs, low Iq is critical. A voltage regulator with 1 μA Iq wastes far less standby power than one with 100 μA, directly extending battery life in always-on devices.

Pacemakers use constant-current output stages that regulate pulse amplitude to within ±5 μA. The pulse is typically 100–500 μA for 0.4–1.5 ms, just enough to depolarise heart tissue and trigger a contraction. Modern devices automatically adjust the current to the minimum needed, conserving the battery for its 8–12 year design life.