To operate the coil continually, the DC supply 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 (red, left), is roughly proportional to the rate of change of primary current (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