I'd like to offer a little more background into the arcing part of this discussion for anyone who may want to understand why and when it happens.
Solenoids and coils work because passing an electric current through a conductor creates a magnetic field around the wire. When you wrap a wire into a coil the magnetic field from each loop gets combined and concentrated inside the loop which effectively forms a magnet that attracts the armature or plunger towards the center of the coil. (More info and video here: funwithpinball.com/learn/solenoids-relays-and-electromagnetism).
The magnetic field created by passing electric current through the coil is a form of stored energy analogous to the voltage stored in a capacitor. And just like a capacitor where the voltage can't change instantly (it needs to drain over a short period of time) the current through a coil of wire can't change instantly either. Think of the stored energy as a flywheel that can't be started or stopped instantly; you need to overcome its inertia.
When you open a switch that has been supplying current to a coil you open the circuit leaving the current through the coil with nowhere to go. Since the current through the coil can't change instantly, it continues to flow for a short time after the switch opens, effectively piling up charge or voltage on one side of the switch relative to the other side. This is called the back electromotive force (EMF). If enough current flows from the collapsing magnetic field in the coil to one side of the switch the voltage climbs to a point where it can break down and jump across the air gap of the switch causing the arcing and pitting in the contacts.
Air generally requires about 3000 volts per millimeter to break down and arc so the voltages required to arc across switch contacts are likely in the 1000s of volts. About the same as shuffling across a carpet and touching a metal object, BTW.
DC solenoids generally have a diode (which only conducts current in one direction) across their solder lugs to prevent arcing across the switch contacts, or inside the drivers on solid state games. The diode doesn't prevent the back EMF, it just provides a closed loop for the current to flow through when it does happen. The current flows from the coil through the diode and back through the coil until it eventually decays.
AC solenoids don't have diodes because the current changes direction constantly. A diode would provide a short circuit across the coil half the time. But AC coils don't arc every time a switch opens either. Since the current changes direction, the magnitude of the current decreases from the maximum in one direction to zero then increases to the maximum in the other direction before decreasing and going through zero again. If the switch happens to open when the current through the coil is close to zero, there won't be enough back EMF or accumulated voltage to arc across the switch.
/Mark
