Most of the motion and computation in electromechanical (EM) pinball machines is done with solenoids and relays. These devices use electric current to generate a magnetic field which attracts a steel plunger or armature and produces motion. The motion is typically used by solenoids to move mechanical devices and by relays to open and close switches.
But how do these devices convert electric current to motion? The answer lies in the relationship between electricity and magnetism.
Imagine a straight piece of copper wire with no current flowing through it. The wire just sits there passively and has no side effects. But if an electric current passes through the wire, an invisible magnetic field develops around the wire as shown in the video below. The red rings represent the electric current through the wire and the blue features represent the magnetic field generated by the electric current.
Notice that the magnetic field has small red arrows indicating its direction. The direction of the magnetic field is determined by the direction of the electric current. If current were to flow in the opposite direction, the magnetic field would rotate in the opposite direction as well. An easy way to remember which way the magnetic field rotates is called the right hand rule. If you use your right hand and point your thumb in the direction that the electric current is flowing, then your curved fingers will point in the direction of the generated magnetic field.
The strength of the magnetic field is affected by the distance from the wire and by the amount of current flowing through the wire. The closer you get to the wire, the stronger the magnetic field is. The magnetic field also gets stronger as more current passes through the wire. But generally speaking the magnetic field from a single wire is very weak and has little effect on nearby pieces of steel. Also, since the magnetic field develops uniformly along the entire length of wire, any magnetic attraction to the wire would be felt equally anywhere along the length of the wire.
Once we have a magnetic field, how can it be made strong enough to move a steel plunger or armature? You could place the plunger closer to the wire, but there might not be enough space between the wire and the plunger to make the motion useful. You could put more current through the wire, but that could heat up and possibly melt the wire. A better solution is to form a loop with the wire as shown in the video below.
The blue field lines in the video above help to visualize the density of the magnetic field. They don't represent a specific part of the magnetic field, they're just equally spaced along the wire to give a better idea of where the magnetic field is. The magnetic field lines are similar to the lines on a topographic map: the closer together they are, the steeper the terrain. With magnetic field lines the closer they are, the stronger the magnetic field is in that area.
Looking at the magnetic field lines in the video you can see that there is a higher density or concentration of magnetic field lines inside the loop than outside it which represents the stronger magnetic field inside the loop. There aren't any more magnetic field lines inside the loop than outside it, but the fact that they're packed in closer together shows that there is a stronger magnetic field inside the loop to attract the steel plunger or armature.
Solenoids and relays used in pinball machines are built with hundreds or thousands of loops of wire. In a solenoid the loops are wrapped around a central (non magnetic) tube that receives the steel plunger. In a relay, the loops are wrapped around a fixed steel core that attracts the armature plate. The loops are wrapped snugly to minimize the distance between the wire and the steel plunger or core. The magnetic fields from each of the loops of wire combine to form a much stronger magnetic field than a straight wire or single loop of wire could generate. The video below models a simple solenoid and the magnetic field lines that it might create.
Note how the magnetic fields from each wire loop in the coil combine to form a stronger magnetic field. Inside the coil of wire the field lines are closely packed together and nearly parallel. This magnetic field closely resembles the magnetic field you might find in an ordinary bar magnet but it can be turned on and off with electric current.
You may wonder whether the electric current in a pinball machine is direct current (DC) or alternating current (AC) and how that affects solenoids and relays. As it turns out both DC and AC have been used in pinball machines and the relays and solenoids work either way. The videos above demonstrate the relationship between electric current and magnetic field using direct current (DC).
Alternating current implies that the electric current through the wire changes direction periodically (usually 100 or 120 times per second). As it changes direction it gradually decreases from the maximum in one direction to zero, then gradually increases from zero to the maximum in the other direction. While it is true that if the electric current changes direction, so does the magnetic field (refer to the right hand rule from above), it is the concentration or strength of the magnetic field (represented by the density of magnetic field lines) that determines how attracted a piece of steel will be, not the direction of the magnetic field.
One advantage for solenoids and relays of direct current (DC) over alternating current (AC) is that direct current offers a constant magnetic field once electric current is applied. Since alternating current switches back and forth there are moments where the current flowing through the wire is very low or even zero, and the magnetic field is similarly very weak or missing many times per second. Normally though the steel plunger or armature is too heavy to react to such frequent changes to the magnetic field and tends to react more to the average magnetic field density over time.
The video below demonstrates the Solenoid from the Fun with Pinball exhibit. When the button is pressed electric current runs through the wire loops of the solenoid which creates a magnetic field that is strongest inside the hollow center of the solenoid. The magnetic field is strong enough to lift the steel plunger off its base and into the solenoid. This solenoid is barely strong enough to lift the plunger. It was purposely chosen for the exhibit to slow down the motion of the solenoid.
If you look carefully you may notice that the plunger isn't lifted immediately into its final resting position inside the solenoid. Instead it rises, then falls and oscillates a bit before stopping. The plunger is being drawn into the solenoid so quickly that it overshoots and starts to emerge from the top of the solenoid. At this point the solenoid pulls the plunger down and it overshoots again. The process repeats until the plunger stabilizes inside the solenoid at a point where the magnetic force lifting the plunger is equal to the gravitational force pulling down on the plunger. The oscillating behavior of the plunger is similar to the oscillation of the ball in the electromagnet video.
The two videos below show a typical solenoid and relay where the action is much faster. The magnetic field created by the electric current draws in the steel plunger and armature instantly. When the current stops and the magnetic field collapses both the plunger and the armature are returned to their resting positions by springs which were stretched when the solenoid and relay were activated.
The solenoid below is more typical than the example above because it has a plug at one end to prevent the plunger from passing through the solenoid. The plunger is drawn into the solenoid until it hits the plug where it stops instantly with no oscillation.
Find more background information about other pinball machine devices is on the Things to Learn page.