A copper wire carrying a current downwards pierces the card right through the letter O, and three rings of field strength spread round it across the card, with dashes running along them. Two arrows circle the wire along the rings, the middle ring breathes with a glow, and the slider of the scale on the right travels up to 0.2 oersted — what a current of one ampere gives at a distance of one centimetre.
Quantity data sheet
field strength · CGS sheet 1/1 · rev. 2026-09Oersted Oe · oersted
Unit outside the SI · quantity: magnetic field strength; a CGS unit, 1 Oe = 1000/4π A/m
The needle that twitched during a lecture
In the spring of 1820 Hans Christian Ørsted was giving an evening lecture in Copenhagen on galvanic electricity and, closing the circuit of a voltaic pile, noticed that a compass standing next to the wire had turned. The audience saw nothing special — the needle barely twitched — but that summer Ørsted repeated the experiment with a stronger battery and sent a short Latin note across Europe: a current flowing in a wire creates a magnetic action around it, directed neither towards the wire nor away from it, but round in a circle.
A hundred and ten years later Ørsted's name was given to the CGS unit of magnetic field strength — the very quantity that a current creates regardless of what surrounds the wire, be it air, iron or vacuum.
This sheet shows how the same oersteds of a coil turn into different numbers of gauss depending on the core, why four pi appears in the conversion to SI, and why oersteds still live on in magnet datasheets.
The symbol Oe is written with a capital O and a lower-case e, because it comes from Ørsted's surname, while the name of the unit is in lower case: oersted, kilooersted. The Danish letter ø is not carried into the symbol, so the unit looks the same in every language.
The oersted measures field strength H — what the current creates — while the gauss measures induction B — what results in the material. In vacuum the numbers coincide, but already in a steel core one oersted gives thousands of gauss, and one cannot be put in place of the other.
01 · Definition
ampere per metre · gilbert · gaussThe oersted is the unit of magnetic field strength in the electromagnetic CGS system: such a field strength is produced by a magnetomotive force of one gilbert per centimetre of path, and converted to SI it equals a thousand divided by four pi amperes per metre, that is about 79.6 A/m.
Field strength describes the field as currents make it and does not depend on the material: a coil with ten turns per centimetre carrying one ampere produces about twelve and a half oersteds inside, whether it holds air or steel. The induction this field strength produces, however, depends strongly on the material, because iron becomes magnetised and adds its own field, so that B = H + 4πM.
The four pi in converting the oersted to SI is a trace of how the CGS system was built: Coulomb's law for magnetic poles is written in it without a coefficient, and the spherical geometry of the field moved into the equations for currents, whereas SI removed it from Maxwell's equations into the definitions of the units.
Same oersteds, different gauss
On the left is a coil with ten turns per centimetre holding the chosen core: the current in the copper sets the field strength H, and the number and speed of the violet lines in the core show the resulting induction B. On the right are the magnetisation curves of all five materials on a logarithmic H scale — the bright one is the chosen one, the glowing dot is the current state, and the thin line at the bottom is vacuum, where B equals H numerically.
Why field strength is counted from current, not from the field
If you go round a wire along any closed path and add up the field strength along it, you get the current enclosed by that path times a constant factor, and nothing else: neither the core nor the air enters the sum. That is why an engineer calculates the field strength in a coil straight from turns and amperes, and only then finds the induction from the material's curve — and it is exactly this two-step logic that the oersted and the gauss preserve, hung on two different quantities.
02 · Conversion
units · fields and coils · coercivityFrom oersteds to amperes per metre
Enter a field strength in any unit, and the sheet will express it in oersteds and amperes per metre, show which currents and coils produce it, and compare it with the coercivity of materials — the field strength needed to demagnetise a substance. The field strength is the same in every tab.
Rule of thumb: an oersted is almost eighty amperes per metre, and a kiloampere per metre is a little over twelve and a half oersteds.
Magnets have two coercivities: the one for induction, Hcb, at which B vanishes, and the one for magnetisation, Hcj, at which the magnetisation itself vanishes, and in neodymium magnets the second can be twice the first. When comparing datasheets, check which one is given.
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04 · Measuring instruments
needle · Helmholtz · loop · clampHow oersteds are set and read
A wire over a compass
The wire is stretched from south to north right above the needle, and when a current is passed through it the needle swings across, while moving the wire underneath or reversing the current turns it the other way. Within a few weeks this simple experiment gave rise to Schweigger's multiplier — the first galvanometer — and later to the tangent galvanometer, in which the angle of the needle was read as the strength of the current.
Field strength calculated from dimensions
Two identical coils set apart by a distance equal to their radius create between them a field that is almost uniform over a wide region, and its strength is calculated from the number of turns, the current and the radius with no fitting at all. That is why Helmholtz coils serve as a field standard: sensors are calibrated in them, the Earth's field is cancelled, and exactly as many oersteds are set as a test method requires.
The loop where both names live
The sample is given two windings: an alternating current is passed through the first and the field strength in oersteds is calculated from it, while the voltage induced in the second is integrated into induction in gauss. The dot travels round the hysteresis loop, and everything a designer needs is read from it: the remanence, where the loop crosses the B axis, and the coercivity, where it crosses the H axis.
A current found by going round
A clamp meter encircles the wire with a ring of core and, by Ampère's circuital law, adds up the field strength all the way round, so the reading does not depend on where the conductor lies inside the ring. This is a direct legacy of Ørsted's experiment: the current is measured without breaking the circuit, from the circle of field strength it creates around itself alone.
05 · Writing rules
symbol · H and B · magnet datasheetCoercivity in oersteds, not in gauss
The oersted goes only with field strength, the gauss only with induction, and when converting oersteds to SI one does not forget the factor of a thousand over four pi.
The first line on the right measures field strength in a unit of induction, the second loses the factor of eighty, and the third takes a formula from the absolute system, where current is in abamperes, and is out by a factor of ten. The fourth invents a non-existent unit instead of the megagauss-oersted, the fifth spoils the case of the symbol, and the sixth carries an equality that holds only in vacuum into a core, where the induction is thousands of times larger.
06 · Neighbouring units
ampere per metre · gauss · gilbertThe oersted has three neighbours: the ampere per metre, which measures the same field strength in SI; the gauss, with which it coincides in vacuum; and the gilbert, the magnetomotive force from which the oersted is obtained by dividing by a centimetre of path.
Field strength in SI, which never got a name of its own.
Induction in CGS; in vacuum the oersted and the gauss are expressed by the same number.
Magnetomotive force in CGS; a gilbert per centimetre is the oersted.
07 · Historical section
archive · from Copenhagen to SIHow field strength got a name and lost it
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How many oersteds it takes to demagnetise
The height of each bar is the coercivity Hcb of a typical magnet of its time on a logarithmic scale from 10 to 20.000 Oe: hardened steel, alnico, ferrite, samarium–cobalt and neodymium. The frame visits the generations in turn; in eighty years the resistance of magnets grew almost two-hundredfold.
Manufacturers still print all these numbers in kilooersteds, and in this trade the oersted lives on more confidently than anywhere else.
A quantity left without a name
Almost every electromagnetic quantity in SI has a name of its own: volt, ohm, weber, tesla, henry. Magnetic field strength did not make it into this row and remained the ampere per metre, although in the CGS system it had a separate name — and this is not carelessness but a direct consequence of how SI is built: the ampere is a base unit in it, and field strength, which in essence is current per length of path, is expressed through it without any intermediary.
The oersted, however, lived in a system that had no base electrical units at all, and there field strength simply had no shorter notation. When CGS was abandoned in 1960, the name became superfluous, but the trade of permanent magnets, which had counted in oersteds for decades, kept it together with the gauss, and now almost every neodymium magnet datasheet is printed in both systems at once.
The two notations are reconciled by a simple factor: a kiloampere per metre is 12.566 oersteds, and manufacturers have long put it in their tables, so the buyer need not remember the four pi.