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Every scrap-yard crane, relay and electric bell uses the same trick: a coil of wire around soft iron makes a strong magnet that you can switch on and off. One of the earliest published descriptions of that trick came from William Sturgeon, an experimenter living at Woolwich, near London. This post tells the story of William Sturgeon's 1825 apparatus, explains the physics with NCERT's own formulas, works through a solenoid example, and ends with what his way of working can teach you.
Who was William Sturgeon?
Sturgeon lived from 1783 to 1850, as his entry in the Oxford Dictionary of National Biography records. By the late 1830s he was "Lecturer on Experimental Philosophy" at the East India Company's Military Seminary at Addiscombe, as the title page of The Annals of Electricity, Magnetism, and Chemistry states. He edited that journal; its first volume covers October 1836 to October 1837.
What did Sturgeon build in 1825?
The primary record is in the Transactions of the Society for the Encouragement of Arts, Manufactures, and Commerce, volume 43, under the heading Improved Electro-Magnetic Apparatus. It says the Society presented "the Large Silver Medal and Thirty Guineas" to "Mr. W. STURGEON" of Artillery Place, Woolwich, and placed a set of his instruments in its repository.
Sturgeon's own letter in that volume explains his aim. Electro-magnetic experiments of the time needed large, expensive batteries and were so small that only "a very near observer" could see them in a lecture hall. He wanted apparatus that worked "on a large scale", and found that "the galvanic force may be reduced to almost any degree, provided the magnetic be sufficiently powerful". His battery had a surface of only 130 square inches.
His description of figures 13 and 14 is the part every physics student will recognise. A bent iron wire wound round with copper wire was set in two cups of mercury. When current flowed, "the iron wire becomes a strong horse-shoe magnet, and will support a heavy bar of iron". Lift one connecting wire out of its cup and "the weight immediately drops". Restore the connection and the power returns. Reverse the connections, or wind the wire the other way, and the north and south poles swap.
Why does an iron core make the magnet so much stronger?
The NCERT Class 12 chapter Moving Charges and Magnetism derives the field inside a long solenoid from Ampere's law: B = μ0nI, where n is the number of turns per metre and I the current. It then adds that "a large field is possible by inserting a soft iron core inside the solenoid".
The next chapter, Magnetism and Matter, explains why. In a ferromagnet, atomic dipoles line up in domains, and an applied field turns and grows those domains. With a core, the field becomes B = μr μ0 n I, and NCERT notes that for ferromagnets the relative permeability is greater than 1000. It also explains the second half of Sturgeon's demonstration: in soft iron "the magnetisation disappears on removal of the external field". That is why his weight dropped when the circuit broke, and why hard magnetic materials such as alnico, which keep their magnetism, suit permanent magnets instead.
Worked example: how much does a core help?
Sample numbers: a coil 0.20 m long has 300 turns and carries 1.5 A.
Turns per metre. n = 300 ÷ 0.20 = 1500 turns per metre.
Air-core field. B = μ0nI = (4π × 10^-7) × 1500 × 1.5 = 2.83 × 10^-3 T, or about 2.8 mT.
Add a core. NCERT's Example 5.5 uses a core with μr = 400. Then B = 400 × 2.83 × 10^-3 = 1.13 T.
Compare. The same current gives about 400 times the field. That is Sturgeon's point: a modest battery plus iron beats a huge battery without it.
What mistakes do students make with solenoids?
Using total turns for n. n is turns per unit length. A 300-turn coil 0.20 m long has n = 1500 per metre, not 300.
Forgetting μr. With a core, B = μr μ0 n I. Without it, μr = 1.
Mixing up soft and hard iron. Soft iron loses its magnetism when the current stops; that is the point of an electromagnet. Hard materials such as alnico keep it.
Getting the poles backwards. Use the right-hand rule: curl your fingers along the current and your thumb points to the north pole. Reversing the current or the winding swaps the poles, exactly as Sturgeon reported.
The JEE (Main) 2026 syllabus lists Ampere's law for a solenoid, the bar magnet as an equivalent solenoid and ferromagnetic substances, so this is exam material. Check the current year's syllabus before you plan. For the Class 10 version of these ideas, see our guide to magnetic effects of electric current.
What can a student take from how Sturgeon worked?
Design for the audience. He built his apparatus so a whole lecture hall could see it.
Question the standard approach. Others added battery power; he asked whether magnetic power could do the work instead, and tested it.
Make it easy to repeat. His battery could be charged by "merely pouring in the liquor from a common lipped jug".
Share it. His instruments went into a public repository, and he later edited a journal that carried other people's work.
A practice routine: solve the example again for 500 turns over 0.25 m at 2 A, with and without a μr = 400 core; then sketch a coil, mark the current, and find its north pole with the right-hand rule. If you get stuck on how to set up solenoid problems, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than just printing an answer.
Next step: wind a few turns of insulated wire round an iron nail, connect a cell briefly, and see how many paper clips it lifts.
