Understanding Inductors and Solenoids A Basic Introduction

                Inductors are ubiquitous in the world of Electronics.  Arriving in a multitude of shapes and sizes, existing in your car radio, used in dishwashers, power plants, the vehicle we are driving, all the way to space ships and satellites majestically orbiting the earth.  In such devices their functional use varies widely, from creating tuned circuits in the radio to operate at discrete frequencies, mechanical relays used to turn other circuits on or off.  Mechanically used to drive valves in the dishwasher, diverting water from one place to another or draining/filling the dishwashers interior.  They are even used for safety circuits such as the GFCI outlets in our homes and offices.

                An inductor though the name may give way to complicated devices is nothing more than a coil of wire.  Every wire is surrounded by a magnetic field, the strength of the magnetic field is directly proportional to the magnitude of the current traveling through it.  The earth has such a magnetic field creating the magnetic north and south poles, the creation of this magnetic field is not known.  Magnets interact with such fields as to align themselves with them.  How they align themselves is based on the shape and direction of the field.

 For a single wire the magnetic field generates a circular pattern around the wire that extends outward from the wire decreasing in overall strength the further the field is from the wire as shown here:

Note the direction of the magnetic field.  Magnetic field lines travel from the north pole of a magnet to the south pole.  For a wire the direction is determined using the “Right Hand Rule”.  Using your right hand, if you place your thumb in the direction of current travel in the wire your fingers will curve around the wire in the direction of the magnetic field.  This is for ‘conventional current’ that is current traveling from the positive potential to a negative potential.

A coil of wire strengthens the magnetic field and changes the geometry of that field as shown here:

In (a) is a standard bar magnet notice that the direction of the magnetic field is from North to South.  In (b) we effectively create a similar situation with the current flowing from right to left the effective magnetic north pole will be at the left side of the coil as shown.  Notice the lines are closer together in the center of the coil of wire, the closer the magnetic field lines are the stronger the field is.

When you coil a wire, each turn joins with the circular magnetic field of the other creating a stronger field, the strength of which is the sum now of the two, thus two turns the strength is doubled, three, three times the strength, etc.

How close the magnetic lines are to one another will depend directly on the type of material the magnetic field is flowing through.  For a vacuum the permeability is:

Permeability, is how strong the magnetic field is in comparison to the current

That is Henrys per Meter.  I am not going to get into the details of the units.  Wikipedia has some good articles on this, the following one talks more about the units and the fields:


Relative permeability is the permeability of a material with respect to a vacuum and is a more useful piece of information (IMHO), as it gives you an idea how much a material can improve the strength of a magnetic field using a coil of wire.  For example, air has a relative permeability of 1.00000037.  Notice this is practically the same as a vacuum (1.0).  Thus, the atmosphere does not help much in this regard.  But the relative permeability of steel varies considerably on the type of steel, but is typically 100.0 thus it can increase the strength of a magnetic field by a factor of 100 compared to a vacuum.

Because of an effect called ‘Para magnetism’, materials with a relative permeability greater than 1 will have a force applied to them when in the presence of a magnetic field.  The force tends to drive the material to the area that will maximize the magnetic field strength of the field.

For a coil of wire and a steel rod, that implies the steel rod will be driven towards the center of the coil of wire, as shown here:

Any device that is constructed to pull a steel rod into the center of a coil of wire is called a solenoid.  Solenoids are ubiquitous in electrical equipment that has to turn electrical energy into mechanical energy.  Solenoids are used for a wide dynamic range of applications; turning valves on and off in a dishwasher, thus diverting water into or out of the fill tank, locking or unlocking doors for a safe, diverting air via an air valve for compressed air tools.  The electronic locks on a car door a typically driven by solenoids.  Solenoids are used in GFCI circuits, like the ones you buy for your house outlets, when a current flowing to ground and not down the neutral wire is detected, a solenoid is used to pull back a latch that latches the GFCI circuit into the off position.  It is assumed that any current coming into the hot wire that is not flowing back out the neutral wire but to the ground wire is a path being created most likely by someone getting shocked from the 120VAC power.  Thus, the device turns off the power as fast as possible to protect people from hazardous shocks.  I recently had one of these fail at my home and had to replace it, taking the old one apart I removed the solenoid.  Here is a picture of the solenoid used in the GFCI circuit, with a quarter placed beside it for size perspective:

Notice the steel rod pointing out on the right, this device has a spring built into the device that keeps the steel rod out of the center of the coil when the coil does not have current flowing through it, the following picture makes the spring mechanism more apparent:

Notice the large number of turns of fine wire.  In order to create a magnetic field strong enough to do any useful mechanical work, a large number of turns is required.  The fine wire is required to keep the wires close together to make a compact unit and to help strengthen the field.  The insulating material, being a very thin layer of typically polyurethane or polyamide.  This wire is available on the web and referred to as ‘magnetic wire’.  The typical gage is small 22 gauge and less.  The following link shows some from amazon’s web site:



Because of the small gauge wire and the high number of turns, most inductors have a fairly high resistance, because of the total length of the wire.  For example, 32-gauge copper wire has a resistance of 0.538 ohms/meter.  Using a DMM I measured the resistance of 53.3 ohms for the GFCI inductor.   Assuming this device uses 32-gauge wire, dividing 53.3 ohms by 0.538 ohms per meter, indicates a total length of wire for my inductor being 99 Meters or roughly 100 feet of wire to make this solenoid.  Using a pair of calipers, the outer diameter of my coil is about 0.5 inches, the inner diameter where the spool is appears to be around 0.25 inches giving an average diameter of this coil of about 0.38 inches.  With the circumference of a circle being roughly 3 times it’s diameter, this comes out to a circumference of 1.14 inches or 0.095 feet.  Being that we have 100 feet of wire then the number of turns would be 100 Feet/0.095 feet per turn or 1,052 turns of wire.  Creating such an inductor by hand wrapping the wire around the core would take a considerable amount of time.  If you decide to create your own inductors, solenoid’s etc.  I would recommend making a small devise that will spin the coil using a motor, then you can feed the wire to the spindle and reduce the time to make such a device.

Testing this device, I attached an adjustable power supply and slowly increased the voltage.  I found the device operated properly at around 12Vdc.  The current being V/R is about 225mA.  With power being VI this device is consuming about 2.7 Watts to hold the solenoid in the on position.  I noticed this was enough to make the device slightly warm to the touch when left on for periods of time exceeding a few seconds.

The video shown here is me connecting and disconnecting the 12V power source to the device.  Notice the metal coil overcomes the spring and pulls to the center when the voltage is applied:

To prove the magnetic field has a directional aspect to it the following video show it deflecting a nearby compass from true north, here we slowly increase the current from 0 to around 225mA, as the current increases the deflection of the needle is more pronounced:

Here we have changed the polarity of the power supply, thus reversing the direction of the current and reversing the direction of the magnetic field, note now how the compass is deflected in the opposing direction, again the current is increased slowly so the amount of deflection increases slowly as well:

                This article was mostly about solenoids, that is inductors that pull a metal shaft towards their center when a current is passed through them.  A compass is basically a small bar magnet.  The magnet wants to align itself with the earths magnetic field.  Under the influence of our solenoid, it has a conflict and now desires to align itself with the direction of the magnetic field of the solenoid.

                My goal was to help inspire you to learn more about such devices by experimenting on your own with these nifty, useful little fellows.

                If you have questions, please feel free to contact me at:




Author: Dennis Bingaman

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