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A battery gives you a few volts. A spark that jumps 38 centimetres of air needs tens of thousands. In 1836 and 1837 Nicholas Callan, a priest teaching physics at the national seminary in Maynooth, Ireland, closed that gap with two coils of wire, an iron bar and a clockwork switch. This post covers who he was, what he built, the physics of why it works, a worked example, and what students can take from his methods.
Who was Nicholas Callan?
The Dictionary of Irish Biography gives his birth as 22 December 1799 at Darver, County Louth, and his death at Maynooth on 10 January 1864. He entered the national seminary at Maynooth in 1816, where Cornelius Denvir taught him electricity and magnetism, and was ordained in 1823. After a doctorate in divinity in Rome, he was appointed professor of natural and experimental philosophy at Maynooth on 15 September 1826. The DIB names three areas of his work: electromagnets and the induction coil, batteries, and electric motors.
What did he build in 1836?
Faraday had shown in 1831 that a changing magnetic field induces a current. Callan's contribution was to make the change large and fast.
His paper "A description of an Electro-magnetic Repeater", in Sturgeon's Annals of Electricity, volume 1 (page 229), describes a machine "by which the connexion between the Voltaic Battery and the helix of an Electro-magnet may be broken and renewed several thousand times in the space of one minute." A swing wheel turned by hand rocks a copper wire in and out of cups of mercury. In this version, the battery drives a thick wire wound on the iron bar; a thin wire, soldered to the end of the thick one, continues the winding; and the shock is taken between the start of the thick wire and the end of the thin one. Each break of the battery current, he wrote, excites "an electric current capable of giving a shock".
The DIB describes his first induction coil of 1836 as two coils on an iron core, with the high voltage appearing in the secondary when the primary current was interrupted, and notes that the faster he interrupted it, the bigger the spark. In 1837 he built a giant machine with a clock mechanism breaking the current 20 times a second, which gave sparks of 15 inches (38 cm) and an estimated 60,000 volts.
What did he find makes the spark stronger?
In a second paper in the same volume (page 295), on how to make an electromagnet for electrical purposes, Callan listed six things the induced current depends on. In his words, it increases with:
the magnetic power of the electromagnet;
the length of the helix;
the thinness of the wire;
closeness of the helix to the iron;
coiling at right angles to the bar, each turn close to the next;
insulation of each coil from the others, for which he used sealing-wax varnish.
These match what Class 12 physics predicts. More turns and closer coupling raise the mutual inductance, and good insulation stops the high voltage leaking between turns.
Worked example: why a fast break gives a big voltage
Mutual inductance and Faraday's law are in Unit 14 of the JEE (Main) 2026 syllabus. The EMF induced in the secondary is ε = M (ΔI/Δt), where M is the mutual inductance. Here are sample numbers for a laboratory coil.
Data. M = 2.0 H. The primary carries 5.0 A, and the break stops it in 1.0 ms.
Average EMF. ε = 2.0 × 5.0 / (1.0 × 10⁻³) = 1.0 × 10⁴ V, ten thousand volts.
Slower break. If the current takes 10 ms to fall, ε = 2.0 × 5.0 / 0.010 = 1,000 V. Ten times slower, ten times smaller.
The repeater's rate. Callan's wheel had 30 teeth, giving 60 breaks per turn. At one turn a second that is 60 breaks a second, or 3,600 a minute, as he wrote.
Notice what the example says. The voltage depends on how fast the current changes, not on how big the battery is. This is why the speed of the interrupter mattered so much to Callan.
How is this different from a transformer?
A transformer on AC mains steps up voltage in the ratio of the turns, N_s/N_p, because the current changes smoothly all the time. Our post on transformers and AC generators covers that. An induction coil runs on DC from a battery and relies on sudden breaks, so the output is a train of sharp pulses. The DIB calls the induction coil "the forerunner of the step-up transformer".
What mistakes do students make here?
"Callan discovered induction." Faraday did, in 1831. Callan engineered a device to exploit it.
Confusing self and mutual induction. The spark when a single coil is broken is self-induction, covered in our post on Joseph Henry. Callan's coil uses the change in one winding to drive another.
Using the turns ratio for a DC break. The ratio rule assumes steady AC. For a sudden break, use ε = M ΔI/Δt.
If the idea of a changing field inducing a current is hard to picture, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can a student take from how he worked?
Change one thing at a time. His six factors came from varying plates, wire length, thickness and distance one by one.
Measure with what you have. With no voltmeters, the DIB says, he judged his batteries by the weight their electromagnets could lift.
Respect safety. The DIB records that a student, William Walsh, later Archbishop of Dublin, was knocked unconscious by one of his coils. Do not repeat these experiments.
Publish your claim. At the 1857 British Association meeting in Dublin he set out his priority for the induction coil.
Next step: work out the EMF in the example if the same coil has M = 0.50 H and the break takes 0.20 ms.
