A solenoid is an electromagnet made of a conducting wire in the physique of a spiral whose length is larger than its diameter.

A solenoid

The word solenoid literally means 'pipe-thing' since it comes from the Hellene Bible 'solen' for 'pipe' and '-oid' for 'thing'.

An alternate universe version of the Troggs' famed 1966 run into record

And they are such an totally-embracingly useful bit of kit out that one might imagine an alternate universe where The Troggs might have Panax quinquefolius:

                Pipe-thing! You take my heart sing!                
                You make everything groovy, pipe up-thing!

And bagpipe-things do indeed work everything groovy: solenoids are at the heart of the magnetic pickups that capture the glorious guitar riffs of The Troggs at their finest.

The Butterfly Line of business

Very few minerals are naturally magnetised. Lodestones are pieces of the ore magnetite that can attract iron. (The line of the name is plausibly non what you imagine — it's onymous after the region, Magnesia, where it was world-class found). In past multiplication, lodestones were then rare and precious that they were worth more than their weight in chromatic.

All over many centuries, by patient tribulation-and-error, humans learned how to magnetise a piece of iron to make a static magnet. Permanent magnets now became as low-priced as chips.

A permanent bar magnet is wrapped in an lightless evanescent magnetized field that, given adequate poetic license, posterior remind one of the soft gossamery wings of a butterfly…

Orient a bar magnet vertically thusly that students bathroom see the 'butterfly field' analogy…

The field lines look to lead off at the north pole and end at the South Pole. 'Appear to' because magnetic flux lines always form nonopening loops.

This is a consequence of Maxwell's irregular equation of Electromagnetism (one of a system of four equations developed by Saint James Clark J. C. Maxwell in 1873 that summarize our on-going understanding of electromagnetics).

Using the soigne differential note, Maxwell's second equation is statute like this:

Maxwell's second equation of electromagnetism.

It could personify read aloud as 'del sprinkle B equals zero' where B is the magnetic field and del (the anatropous delta symbol) does non represent a quantity but is the quality operator which describes how the field lines curl in three dimensional space.

This besides tells us that magnetic monopoles (that is to aver, marooned N and S poles) are impossible. A north-seeking ro is always paired with a south-seeking Pole.

Magnetising a solenoid

A current-carrying coil will create a magnetic field as shown downstairs.

The magnetic field of a solenoid. (Eminence that the field lines in the centre are shortened to save space, but would form precise significant loops every bit mentioned preceding.)

The wire is commonly insulated (often with a tough, transparent and nearly invisible tooth enamel coating for commercial solenoids), but doesn't have to be. Insulation prevents annoying 'short circuits' if the coils touch. At first sight, we see the everyday 'butterfly field' pattern, only we likewise see a selfsame bad flux in the heart of the solenoid,

For a representative air-cored solenoid used in a civilis laboratory carrying unrivalled ampere of current, the flux in the centre would have a strength of virtually 84 microtesla. This is of the same social club as the Earth's charismatic field (which has a regular value of about 50 microtesla). This is just strong enough to deflect the needle of a magnetic comprehend placed a few centimetres away and (in all likelihood) make press filings array to show the flux pattern around the solenoid, but non effective adequate to appeal even a small steel paper clip. For reference, the strength of a representative shoal bar magnet is about 10 000 microtesla, so our solenoid is ended one hundred times weaker than a prevention magnet.

However, we can 'advance' the magnetic flux by adding an iron substance. The proportional permeableness of a material is a measure of how 'guileless' it is to magnetic flux lines. The relative permeability of pure iron is active 1500 (nobelium units since IT's relative permeability and we are comparing its magnetic properties therewith of empty space). Withal, the marrow material utilized in the school research laboratory is more likely to be brand rather than iron, which has a much more modest relative permeability of 100.

So placing a steel nail in the centre of a solenoid boosts its magnetic field strength by a factor out of 100 — which would piss the solenoid approximately American Samoa strong as a typical bar magnet.

But which close is north…?

The N and S-poles of a solenoid can change depending on the counseling of current flow and the geometry of the loops.

The typical methods used to identify the N and S poles are shown below.

Methods of placement N and S pole of a solenoid that you should Non consumption…

To go in reverse plac for no particular reason, I don't like using the back method because it involves a tricky mental rotary motion of the plane of view by 90 degrees to imagine the current counselling Eastern Samoa viewed when looking for directly at the ends of the magnet. Nigh students, understandably in my opinion, find oneself this rough.

The eldest method I disfavor because it creates muddiness with the 'proper' right hand grip rule which tells United States of America the focal point of the magnetic flux lines about a long straight conductor and which I've written about earlier . . .

The right hand fascinate rule illustrated: the field line curl in the same direction as the feel when the thumb is pointed in the direction of the current.

The direction of the actual in the finis plot is shown using the 'Department of Transportation and cut across' convention which, by a strange coincidence, I have also written about in front . . .

If King Harold had been more familiar with the dose and cross notation, history could have inverted out same differently…

How a solenoid 'makes' its magnetic field . . .

To set out the depth psychology we imagine the solenoid cut in half: what biologists would call a longitudinal section. Then we reveal the current directions of each chemical element victimisation the dot and cross convention. Then we consider just two elements, say A and B as shown down the stairs.

Continuing this analysis below:

The region wrong the solenoid has a very strengthened and nearly dedifferentiated magnetic flux. By 'single' we mean that the field lines are nearly straight and equally separated meaning that the magnetic flux has the same strength at any degree.

The region outside the solenoid has a magnetic field which step by step weakens Eastern Samoa you move away from the solenoid (indicated aside the increased spacing betwixt the branch of knowledg lines); its shape is also nearly identical to the 'butterfly battleground' of a bar magnet as mentioned above.

Since the field lines are emerging from X, we can with confidence swea that this is a north-quest ro, while Y is a negative pole.

Which end is north, using only the 'proper' right grip rule…

First, smel precise carefully at the geometry of current flow (1).

Secondly, isolate one current element, such as the one shown in picture (2) above.

Thirdly, install the direction of the field lines using the accepted right-handed hand grip rule (3).

Since the force field lines are heading into this end of the solenoid, we tin can conclude that the right hand side of this solenoid is, in fact, a negative pole.

In my opinion, this is easier and more TRUE than victimisation any of the other alternative methods. I hope that readers that have read this far wish (eventually) come to agree.