An induction coil or "spark coil" (archaically known as an inductorium or Ruhmkorff coil after Heinrich Rühmkorff) is a type of transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply. To create the flux changes necessary to induce voltage in the secondary coil, the direct current in the primary coil is repeatedly interrupted by a vibrating mechanical contact called an interrupter. Invented in 1836 by the Irish-Catholic priest Nicholas Callan, also independently by American inventor Charles Grafton Page, the induction coil was the first type of transformer. It was widely used in x-ray machines, spark-gap radio transmitters, 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.
Construction and function
An induction coil consists of two coils of insulated wire wound around a common iron core (M). One coil, called the primary winding (P), is made from relatively few (tens or hundreds) turns of coarse wire. The other coil, the secondary winding, (S) typically consists of up to a million turns of fine wire (up to 40 gauge). An electric current is passed through the primary, creating a magnetic field. Because of the common core, most of the primary's magnetic field couples with the secondary winding. The primary behaves as an inductor, storing energy in the associated magnetic field. When the primary current is suddenly interrupted, the magnetic field rapidly collapses. This causes a high voltage pulse to be developed across the secondary terminals through electromagnetic induction. Because of the large number of turns in the secondary coil, the secondary voltage pulse is typically many thousands of volts. This voltage is often sufficient to cause an electric spark, to jump across an air gap (G) separating the secondary's output terminals. For this reason, induction coils were called spark coils. An induction coil is traditionally characterised by the length of spark it can produce; a '4 inch' (10 cm) induction coil could produce a 4 inch spark. Until the development of the cathode ray oscilloscope, this was the most reliable measurement of peak voltage of such asymmetric waveforms. The relationship between spark length and voltage is linear within a wide range:
4 inches (10 cm) = 110kV; 8 inches (20 cm) = 150kV; 12 inches (30 cm) = 190kV; 16 inches (41 cm) = 230kV Curves supplied by a 1984 reference agree closely with those values.
Interrupter
To operate the coil continually, the direct current must be repeatedly connected and disconnected to create the magnetic field changes needed for induction. To do that, induction coils use a magnetically activated vibrating arm called an interrupter or break (A) to rapidly connect and break the current flowing into the primary coil. The interrupter is mounted on the end of the coil next to the iron core. When the power is turned on, the increasing current in the primary coil produces an increasing magnetic field, the magnetic field attracts the interrupter's iron armature (A). After a time, the magnetic attraction overcomes the armature's spring force, and the armature begins to move. When the armature has moved far enough, the pair of contacts (K) in the primary circuit open and disconnect the primary current. Disconnecting the current causes the magnetic field to collapse and create the spark. Also, the collapsed field no longer attracts the armature, so the spring force accelerates the armature toward its initial position. A short time later the contacts reconnect, and the current starts building the magnetic field again. The whole process starts over and repeats many times per second. The secondary voltage v2 (red) is roughly proportional to the rate of change of primary current i1 (blue). Opposite potentials are induced in the secondary when the interrupter breaks the circuit and closes the circuit. However, the current change in the primary is much more abrupt when the interrupter breaks. When the contacts close, the current builds up slowly in the primary because the supply voltage has a limited ability to force current through the coil's inductance. In contrast, when the interrupter contacts open, the current falls to zero suddenly. So the pulse of voltage induced in the secondary at break is much larger than the pulse induced at close, it is the break that generates the coil's high-voltage output.
Capacitor An arc forms at the interrupter contacts on break which has undesirable effects: the arc consumes energy stored in the magnetic field, reduces the output voltage, and damages the contacts. 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. The capacitor and primary winding together form a tuned circuit, so on break, a damped sinusoidal wave of current flows in the primary and likewise induces a damped wave in the secondary. As a result, the high-voltage output consists of a series of damped waves.
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