Induction coil
[1] Invented in 1836 by the Irish-Catholic priest Nicholas Callan, also independently by American inventor Charles Grafton Page,[1] the induction coil was the first type of transformer.
It was widely used in x-ray machines,[1][6] spark-gap radio transmitters,[1][6] arc lighting and quack medical electrotherapy devices from the 1880s to the 1920s.
Today its only common use is as the ignition coils in internal combustion engines and in physics education to demonstrate induction.
This voltage is often sufficient to cause an electric spark, to jump across an air gap (G) separating the secondary's output terminals.
The relationship between spark length and voltage is linear within a wide range: Curves supplied by a 1984 reference agree closely with those values.
[10] To operate the coil continually, the DC supply current must be repeatedly connected and disconnected to create the magnetic field changes needed for induction.
The secondary voltage v2 (red, left), is roughly proportional to the rate of change of primary current i1 (blue).
[11] To prevent this, a quenching capacitor (C) of 0.5 to 15 μF is connected across the primary coil to slow the rise in the voltage after a break.
In one widely used technique, the secondary coil is wound in many thin flat pancake-shaped sections (called "pies"), connected in series.
[12][1] The primary coil is first wound on the iron core and insulated from the secondary with a thick paper or rubber coating.
[1] The eddy currents, which flow in loops in the core perpendicular to the magnetic axis, are blocked by the layers of insulation.
During its development between 1836 and the 1860s, mostly by trial and error, researchers discovered many of the principles that governed all transformers, such as the proportionality between turns and output voltage and the use of a "divided" iron core to reduce eddy current losses.
[15] The induction coil was invented by the American physician Charles Grafton Page in 1836[16][17] and independently by Irish scientist and Catholic priest Nicholas Callan in the same year at the St. Patrick's College, Maynooth[1][18][19][20][21] and improved by William Sturgeon.
Prof. James William MacGauley (1838) of Dublin, Ireland,[16][26] Johann Philipp Wagner (1839), and Christian Ernst Neeff (1847).
[1][29][30] Heinrich Ruhmkorff generated higher voltages by greatly increasing the length of the secondary,[1] in some coils using 5 or 6 miles (10 km) of wire and produced sparks up to 16 inches.
In the early 1850s, American inventor Edward Samuel Ritchie introduced the divided secondary construction to improve insulation.
