In electromagnetism and electronics, inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current through a conductor creates a magnetic field around the conductor, whose strength depends on the magnitude of the current. A change in current causes a change in the magnetic field. From Faraday's law of induction, any change in magnetic field through a circuit induces an electromotive force (EMF) (voltage) in the conductors; this is known as electromagnetic induction. So the changing current induces a voltage in the conductor. This induced voltage is in a direction which tends to oppose the change in current (as stated by Lenz's law), so it is called a back EMF. Due to this back EMF, a conductor's inductance opposes any increase or decrease in electric current through it.
Inductance is defined as the ratio of the induced voltage to the rate of change of current causing it. It is a proportionality factor that depends on the geometry of circuit conductors and the magnetic permeability of nearby materials.^{[1]} An electronic component designed to add inductance to a circuit is called an inductor. It typically consists of a coil or helix of wire.
The term inductance was coined by Oliver Heaviside in 1886.^{[2]} It is customary to use the symbol for inductance, in honour of the physicist Heinrich Lenz.^{[3]}^{[4]} In the SI system, the unit of inductance is the henry (H), which is the amount of inductance that causes a voltage of one volt, when the current is changing at a rate of one ampere per second. It is named for Joseph Henry, who discovered inductance independently of Faraday.^{[5]}
The history of electromagnetic induction, a facet of electromagnetism, began with observations of the ancients: electric charge or static electricity (rubbing silk on amber), electric current (lightning), and magnetic attraction (lodestone). Understanding the unity of these forces of nature, and the scientific theory of electromagnetism began in the late 18th century.
Electromagnetic induction was first described by Michael Faraday in 1831.^{[6]}^{[7]} In Faraday's experiment, he wrapped two wires around opposite sides of an iron ring. He expected that, when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. Using a galvanometer, he observed a transient current flow in the second coil of wire each time that a battery was connected or disconnected from the first coil.^{[8]} This current was induced by the change in magnetic flux that occurred when the battery was connected and disconnected.^{[9]} Faraday found several other manifestations of electromagnetic induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he generated a steady (DC) current by rotating a copper disk near the bar magnet with a sliding electrical lead ("Faraday's disk").^{[10]}
A current flowing through a conductor generates a magnetic field around the conductor, which is described by Ampere's circuital law. The total magnetic flux through a circuit is equal to the product of the perpendicular component the magnetic field and the area of the surface spanning the current path. If the current varies, the magnetic flux through the circuit changes. By Faraday's law of induction, any change in flux through a circuit induces an electromotive force (EMF) or voltage in the circuit, proportional to the rate of change of flux
The negative sign in the equation indicates that the induced voltage is in a direction which opposes the change in current that created it; this is called Lenz's law. The potential is therefore called a back EMF. If the current is increasing, the voltage is positive at the end of the conductor through which the current enters and negative at the end through which it leaves, tending to reduce the current. If the current is decreasing, the voltage is positive at the end through which the current leaves the conductor, tending to maintain the current. Selfinductance, usually just called inductance, is the ratio between the induced voltage and the rate of change of the current
Thus, inductance is a property of a conductor or circuit, due to its magnetic field, which tends to oppose changes in current through the circuit. The unit of inductance in the SI system is the henry (H), named after American scientist Joseph Henry, which is the amount of inductance which generates a voltage of one volt when the current is changing at a rate of one ampere per second.
All conductors have some inductance, which may have either desirable or detrimental effects in practical electrical devices. The inductance of a circuit depends on the geometry of the current path, and on the magnetic permeability of nearby materials; ferromagnetic materials with a higher permeability like iron near a conductor tend to increase the magnetic field and inductance. Any alteration to a circuit which increases the flux (total magnetic field) through the circuit produced by a given current increases the inductance, because inductance is also equal to the ratio of magnetic flux to current^{[11]}^{[12]}^{[13]}^{[14]}
An inductor is an electrical component consisting of a conductor shaped to increase the magnetic flux, to add inductance to a circuit. Typically it consists of a wire wound into a coil or helix. A coiled wire has a higher inductance than a straight wire of the same length, because the magnetic field lines pass through the circuit multiple times, it has multiple flux linkages. The inductance is proportional to the square of the number of turns in the coil.
The inductance of a coil can be increased by placing a magnetic core of ferromagnetic material in the hole in the center. The magnetic field of the coil magnetizes the material of the core, aligning its magnetic domains, and the magnetic field of the core adds to that of the coil, increasing the flux through the coil. This is called a ferromagnetic core inductor. A magnetic core can increase the inductance of a coil by thousands of times.
If multiple electric circuits are located close to each other, the magnetic field of one can pass through the other; in this case the circuits are said to be inductively coupled. Due to Faraday's law of induction, a change in current in one circuit can cause a change in magnetic flux in another circuit and thus induce a voltage in another circuit. The concept of inductance can be generalized in this case by defining the mutual inductance of circuit and circuit as the ratio of voltage induced in circuit to the rate of change of current in circuit . This is the principle behind a transformer. The property describing the effect of one conductor on itself is more precisely called selfinductance, and the properties describing the effects of one conductor with changing current on nearby conductors is called mutual inductance.^{[15]}
If the current through a conductor with inductance is increasing, a voltage will be induced across the conductor with a polarity which opposes the current, as described above (this is in addition to any voltage drop caused by the conductor's resistance). The charges flowing through the circuit lose potential energy moving from the higher voltage to the lower voltage end. The energy from the external circuit required to overcome this "potential hill" is being stored in the increased magnetic field around the conductor. Therefore, any inductance with a current through it stores energy in its magnetic field. At any given time the power flowing into the magnetic field, which is equal to the rate of change of the stored energy , is the product of the current and voltage across the conductor^{[16]}^{[17]}^{[18]}
From (1) above
When there is no current, there is no magnetic field and the stored energy is zero. Neglecting resistive losses, the energy (measured in joules, in SI) stored by an inductance with a current through it is equal to the amount of work required to establish the current through the inductance from zero, and therefore the magnetic field. This is given by:
If the inductance is constant over the current range, the stored energy is^{[16]}^{[17]}^{[18]}
So therefore inductance is also proportional to how much energy is stored in the magnetic field for a given current. This energy is stored as long as the current remains constant. If the current decreases, the magnetic field will decrease, inducing a voltage in the conductor in the opposite direction, negative at the end through which current enters and positive at the end through which it leaves. This will return stored magnetic energy to the external circuit.
If ferromagnetic materials are located near the conductor, such as in an inductor with a magnetic core, the constant inductance equation above is only valid for linear regions of the magnetic flux, at currents below the level at which the ferromagnetic material saturates, where the inductance is approximately constant. If the magnetic field in the inductor approaches the level at which the core saturates, the inductance will begin to change with current, and the integral equation must be used.
When a sinusoidal alternating current (AC) is passing through a linear inductance, the induced backEMF will also be sinusoidal. If the current through the inductance is , from (1) above the voltage across it will be
where is the amplitude (peak value) of the sinusoidal current in amperes, is the angular frequency of the alternating current, with being its frequency in hertz, and is the inductance.
Thus the amplitude (peak value) of the voltage across the inductance will be
Inductive reactance is the opposition of an inductor to an alternating current.^{[19]} It is defined analogously to electrical resistance in a resistor, as the ratio of the amplitude (peak value) of the alternating voltage to current in the component
Reactance has units of ohms. It can be seen that inductive reactance of an inductor increases proportionally with frequency , so an inductor conducts less current for a given applied AC voltage as the frequency increases. Because the induced voltage is greatest when the current is increasing, the voltage and current waveforms are out of phase; the voltage peaks occur earlier in each cycle than the current peaks. The phase difference between the current and the induced voltage is radians or 90 degrees, showing that in an ideal inductor the current lags the voltage by 90°.
In the most general case, inductance can be calculated from Maxwell's equations. Many important cases can be solved using simplifications. Where high frequency currents are considered, with skin effect, the surface current densities and magnetic field may be obtained by solving the Laplace equation. Where the conductors are thin wires, selfinductance still depends on the wire radius and the distribution of the current in the wire. This current distribution is approximately constant (on the surface or in the volume of the wire) for a wire radius much smaller than other length scales.
As a practical matter, longer wires have more inductance, and thicker wires have less, analogous to their electrical resistance (although the relationships aren't linear, and are different in kind from the relationships that length and diameter bear to resistance).
Separating the wire from the other parts of the circuit introduces some unavoidable error in any formulas' results. These inductances are often referred to as "partial inductances", in part to encourage consideration of the other contributions to wholecircuit inductance which are omitted.
As an essential component of coils and circuits, understanding what the inductance of a wire is, is essential. Yet, there is no simple, unambiguous definition of the inductance of a wire. The reasons are both practical and epistemological.
There are practical difficulties in measuring wire inductance. A short straight segment of singleconductor wire has some inductance, which in our ordinary experience is intangible because it is so small that it is effectively undetectable: It is too small to be readily be measured at low frequencies, and at high frequencies becomes entangled with the inductance of the wires of any meter connected to measure it. A long straight wire like an electric transmission line hundreds of kilometers long has substantial inductance, and there is no problem at all measuring it.
Difficulties with epistemology also trouble the calculations: There is no unambiguous definition of the inductance of a straight wire. If we consider the wire in isolation we ignore the question of how the current gets into the wire. That current will affect the flux which is developed in the vicinity of the wire. But this flux should also be a part of the definition. A consequence of Maxwell's equations is that we cannot define the inductance of only a portion of a circuit, we can only define the inductance of a whole circuit, which includes how the current gets to the wire and how it returns to the source. The magnetic flux incident to the whole circuit determines the inductance of the circuit and of any part of it. The magnetic flux is an indivisible entity, yet we wish to consider only a part of it, the part incident to the wire, between whatever we define to be the "ends" of the wire.
For derivation of the formulas below, see Rosa (1908).^{[20]} The total low frequency inductance (interior plus exterior) of a straight wire is:
where
The constant 0.75 is just one parameter value among several; different frequency ranges, different shapes, or extremely long wire lengths require a slightly different constant (see below). This result is based on the assumption that the radius is much less than the length , which is the common case for wires and rods. Disks or thick cylinders have slightly different formulas.
For sufficiently high frequencies skin effects cause the interior currents to vanish, leaving only the currents on the surface of the conductor; the inductance for alternating current, is then given by a very similar formula:
where the variables and are the same as above; note the changed constant term now 1, formerly 0.75 .
In an example from everyday experience, just one of the conductors of a lamp cord 10 m long, made of 18 gauge wire, would only have an inductance of about 19 µH if stretched out straight.
There are two cases to consider:
Currents in the wires need not be equal, though they often are, as in the case of a complete circuit, where one wire is the source and the other the return.
This is the generalized case of the paradigmatic twoloop cylindrical coil carrying a uniform low frequency current; the loops are independent closed circuits that can have different lengths, any orientation in space, and carry different currents. Nonetheless, the error terms, which are not included in the integral will only be small if the geometries of the loops are mostly smooth and convex: they do not have too many kinks, sharp corners, coils, crossovers, parallel segments, concave cavities or other topological "close" deformations. A necessary predicate for the reduction of the 3dimensional manifold integration formula to a double curve integral is that the current paths be filamentary circuits, i.e. thin wires where the radius of the wire is negligible compared to its length.
The mutual inductance by a filamentary circuit on a filamentary circuit is given by the double integral Neumann formula^{[21]}
where
where
where
Stokes' theorem has been used for the 3rd equality step.
For the last equality step, we used the Retarded potential expression for and we ignore the effect of the retarded time (assuming the geometry of the circuits is small enough compared to the wavelength of the current they carry). It is actually an approximation step, and is valid only for local circuits made of thin wires.
Formally, the selfinductance of a wire loop would be given by the above equation with . However, here becomes infinite, leading to a logarithmically divergent integral.^{[a]} This necessitates taking the finite wire radius and the distribution of the current in the wire into account. There remains the contribution from the integral over all points and a correction term,^{[23]}
where
A solenoid is a long, thin coil; i.e., a coil whose length is much greater than its diameter. Under these conditions, and without any magnetic material used, the magnetic flux density within the coil is practically constant and is given by
where is the magnetic constant, the number of turns, the current and the length of the coil. Ignoring end effects, the total magnetic flux through the coil is obtained by multiplying the flux density by the crosssection area :
When this is combined with the definition of inductance , it follows that the inductance of a solenoid is given by:
Therefore, for aircore coils, inductance is a function of coil geometry and number of turns, and is independent of current.
Let the inner conductor have radius and permeability , let the dielectric between the inner and outer conductor have permeability , and let the outer conductor have inner radius , outer radius , and permeability . However, for a typical coaxial line application, we are interested in passing (nonDC) signals at frequencies for which the resistive skin effect cannot be neglected. In most cases, the inner and outer conductor terms are negligible, in which case one may approximate
Most practical aircore inductors are multilayer cylindrical coils with square crosssections to minimize average distance between turns (circular cross sections would be better but harder to form).
Many inductors include a magnetic core at the center of or partly surrounding the winding. Over a large enough range these exhibit a nonlinear permeability with effects such as magnetic saturation. Saturation makes the resulting inductance a function of the applied current.
The secant or largesignal inductance is used in flux calculations. It is defined as:
The differential or smallsignal inductance, on the other hand, is used in calculating voltage. It is defined as:
The circuit voltage for a nonlinear inductor is obtained via the differential inductance as shown by Faraday's Law and the chain rule of calculus.
Similar definitions may be derived for nonlinear mutual inductance.
The inductance equations above are a consequence of Maxwell's equations. For the important case of electrical circuits consisting of thin wires, the derivation is straightforward.
In a system of wire loops, each with one or several wire turns, the flux linkage of loop , , is given by
Here denotes the number of turns in loop ; is the magnetic flux through loop ; and are some constants described below. This equation follows from Ampere's law: magnetic fields and fluxes are linear functions of the currents. By Faraday's law of induction, we have
where denotes the voltage induced in circuit . This agrees with the definition of inductance above if the coefficients are identified with the coefficients of inductance. Because the total currents contribute to it also follows that is proportional to the product of turns .
Multiplying the equation for v_{m} above with i_{m}dt and summing over m gives the energy transferred to the system in the time interval dt,
This must agree with the change of the magnetic field energy, W, caused by the currents.^{[24]} The integrability condition
requires L_{m,n} = L_{n,m}. The inductance matrix, L_{m,n}, thus is symmetric. The integral of the energy transfer is the magnetic field energy as a function of the currents,
This equation also is a direct consequence of the linearity of Maxwell's equations. It is helpful to associate changing electric currents with a buildup or decrease of magnetic field energy. The corresponding energy transfer requires or generates a voltage. A mechanical analogy in the K = 1 case with magnetic field energy (1/2)Li^{2} is a body with mass M, velocity u and kinetic energy (1/2)Mu^{2}. The rate of change of velocity (current) multiplied with mass (inductance) requires or generates a force (an electrical voltage).
Mutual inductance occurs when the change in current in one inductor induces a voltage in another nearby inductor. It is important as the mechanism by which transformers work, but it can also cause unwanted coupling between conductors in a circuit.
The mutual inductance, , is also a measure of the coupling between two inductors. The mutual inductance by circuit on circuit is given by the double integral Neumann formula, see calculation techniques
The mutual inductance also has the relationship:
where
Once the mutual inductance, , is determined, it can be used to predict the behavior of a circuit:
where
The minus sign arises because of the sense the current has been defined in the diagram. With both currents defined going into the dots the sign of will be positive (the equation would read with a plus sign instead).^{[25]}
The coupling coefficient is the ratio of the opencircuit actual voltage ratio to the ratio that would obtain if all the flux coupled from one circuit to the other. The coupling coefficient is related to mutual inductance and self inductances in the following way. From the two simultaneous equations expressed in the twoport matrix the opencircuit voltage ratio is found to be:
while the ratio if all the flux is coupled is the ratio of the turns, hence the ratio of the square root of the inductances
thus,
where
The coupling coefficient is a convenient way to specify the relationship between a certain orientation of inductors with arbitrary inductance. Most authors define the range as , but some^{[26]} define it as Allowing negative values of captures phase inversions of the coil connections and the direction of the windings.^{[27]}
Mutually coupled inductors can be described by any of the twoport network parameter matrix representations. The most direct are the z parameters, which are given by
where is the complex frequency variable, and are the inductances of the primary and secondary coil, respectively, and is the mutual inductance between the coils.
Mutually coupled inductors can equivalently be represented by a Tcircuit of inductors as shown. If the coupling is strong and the inductors are of unequal values then the series inductor on the stepdown side may take on a negative value.
This can be analyzed as a two port network. With the output terminated with some arbitrary impedance, , the voltage gain, , is given by,
where is the coupling constant and is the complex frequency variable, as above. For tightly coupled inductors where this reduces to
which is independent of the load impedance. If the inductors are wound on the same core and with the same geometry, then this expression is equal to the turns ratio of the two inductors because inductance is proportional to the square of turns ratio.
The input impedance of the network is given by,
For this reduces to
Thus, the current gain, is not independent of load unless the further condition
is met, in which case,
and
Alternatively, two coupled inductors can be modelled using a ? equivalent circuit with optional ideal transformers at each port. While the circuit is more complicated than a Tcircuit, it can be generalized ^{[28]} to circuits consisting of more than two coupled inductors. Equivalent circuit elements , have physical meaning, modelling respectively magnetic reluctances of coupling paths and magnetic reluctances of leakage paths. For example, electric currents flowing through these elements correspond to coupling and leakage magnetic fluxes. Ideal transformers normalize all selfinductances to 1 Henry to simplify mathematical formulas.
Equivalent circuit element values can be calculated from coupling coefficients with
where coupling coefficient matrix and its cofactors are defined as
For two coupled inductors, these formulas simplify to
and for three coupled inductors (for brevity shown only for and )
When a capacitor is connected across one winding of a transformer, making the winding a tuned circuit (resonant circuit) it is called a singletuned transformer. When a capacitor is connected across each winding, it is called a double tuned transformer. These resonant transformers can store oscillating electrical energy similar to a resonant circuit and thus function as a bandpass filter, allowing frequencies near their resonant frequency to pass from the primary to secondary winding, but blocking other frequencies. The amount of mutual inductance between the two windings, together with the Q factor of the circuit, determine the shape of the frequency response curve. The advantage of the double tuned transformer is that it can have a narrower bandwidth than a simple tuned circuit. The coupling of doubletuned circuits is described as loose, critical, or overcoupled depending on the value of the coupling coefficient . When two tuned circuits are loosely coupled through mutual inductance, the bandwidth will be narrow. As the amount of mutual inductance increases, the bandwidth continues to grow. When the mutual inductance is increased beyond the critical coupling, the peak in the frequency response curve splits into two peaks, and as the coupling is increased the two peaks move further apart. This is known as overcoupling.
When , the inductor is referred to as being closely coupled. If in addition, the selfinductances go to infinity, the inductor becomes an ideal transformer. In this case the voltages, currents, and number of turns can be related in the following way:
where
Conversely the current:
where
The power through one inductor is the same as the power through the other. These equations neglect any forcing by current sources or voltage sources.
The table below lists formulas for the selfinductance of various simple shapes made of thin cylindrical conductors (wires). In general these are only accurate if the wire radius is much smaller than the dimensions of the shape, and if no ferromagnetic materials are nearby (no magnetic core).
Type  Inductance  Comment 

Single layer solenoid 
The wellknown Wheeler's approximation formula for currentsheet model aircore coil:^{[29]}^{[30]} (English) (cgs) 

Coaxial cable (HF) 
: Outer cond.'s inside radius : Inner conductor's radius : Length : see table footnote.  
Circular loop^{[31]}  : Loop radius : Wire radius : see table footnotes.  
Rectangle made of round wire^{[32]} 

: Border length : Wire radius : see table footnotes. 
Pair of parallel wires 
: Wire radius : Separation distance, : Length of pair : see table footnotes.  
Pair of parallel wires (HF) 
: Wire radius : Separation distance, : Length of pair : see table footnote. 