A relay is turning an electromagnet on and off. A coil is using the electromagnetic flux to pull a metal bar through the field. A relay is a magnet turning on and off that attracts the metal plate. I believe the purpose of the copper is to shape the electromagnetic field to better attract the plate. (Copper itself isn't a particularly magnetic metal, but can heavily influence electromagnetism)

It seems weird that they wouldn't still have a plunger going through the coil for max power (or minimal power needed since you could use a weaker coil), unless the slight extra complication just isn't worth it?

Perhaps those relays were the most easy to acquire. It's not like pinball was the only industry using similar style relays.

xsvtoys your relay summary is pretty good. Relay coils have steel cores because steel and other ferrous metals manipulate the magnetic field created by the coil of wire. In this case the steel core extends the concentrated, roughly parallel magnetic field lines beyond the edge of the coil windings because most of the magnetic field lines will tend to follow the steel rather than break away into the surrounding air.

The same thing can be done with a bar magnet. If you pick up something with a bar magnet, most of the magnetic field will extend through whatever it is you picked up, effectively making the magnet longer. You can often pick up several things one after the other before enough of the magnetic field leaks away and the end of the extended magnet isn't strong enough to pick up anything else.

Similar things happen in a score motor (usually a shaded pole induction motor). The coil of wire is wrapped around a stack of C shaped steel plates. Those plates carry most of the magnetic field from the center of the coil around to the ends of the C to concentrate the magnetic field where the rotor is.

Another point to understand is that ferrous metals are attracted by the density of the magnetic field, not by the direction of the magnetic field. Magnets have north and south poles that attract only opposite poles. Unless magnetized, steel doesn't have poles. Since EM solenoid and relay coils use AC current, the direction of the current through the coil, and of the magnetic field change 120 times per second. If the plunger on a solenoid or the armature on a relay were a magnet instead of just steel, the changing magnetic field would try to repel the plunger half the time (like a speaker does). A steel plunger or armature doesn't care about the direction of the magnetic field. It's attracted to any magnetic field or to both north and south poles of a magnet.

The reason a relay extends its magnetic field with a steel slug is probably to make room for the copper loop on the end. While copper isn't very magnetic, it is very conductive. What happens when the relay creates a changing magnetic field (due to the AC current passing through it) is that a similar changing current is induced in the copper loop (in the same way that one loop can induce current in another loop in a transformer). That induced current itself creates a small magnetic field. The magic with this arrangement however is that the magnetic field induced in the copper loop is 90 degrees (?) out of phase with the magnetic field created by the relay:
Relay coils (resized).jpg
In this plot the green line shows the changing current or magnetic field of the relay coil. The red line shows the induced current or magnetic field in the copper loop. Notice how it is much smaller, and slightly delayed.

This becomes important when the magnetic field in the relay coil gets very small as the AC current through the coil changes direction. As the relay's current and magnetic field (green line) approach zero (black line) the armature spring can overpower the magnetic field and pull the armature away before the current and magnetic field increase again. This is what can cause relay buzzing.

But with the copper loop in place, there is a small delayed magnetic field (red line) created just as the relay's magnetic field drops to zero that is intended to hold the armature against the relay and keep the spring from pulling it away. This induced magnetic field is what prevents relay buzzing. Since the armature doesn't care which direction the magnetic field is (north vs south) it sees a combined magnetic field that looks like the blue line which never reaches zero as long as there is AC current running through the relay coil.

The same kind of copper coil is in coil stops of AC solenoids too BTW, also to minimize buzzing.

/Mark

Edit: Thinking about it some more I'm not sure about the 90 degrees out of phase business. However I do think that the two magnetic fields are not aligned, so that they're never both zero at the same time. The general idea that the smaller magnetic field covers the gaps when the larger magnetic field goes to zero is true I believe.

An extremely learned expose Mark-thks.

Looks like Mark G is now The Electromagnetic Man!

Solenoid- the coil core moves in and out. Relay - the core is stationary (it's there, all the way through) but another steel part of the magnetic circuit (the armature) moves.

But if you sketch out the magnetic circuit (steel parts) of either the solenoid or relay you see they are arranged to conduct the flux generated by the coil (Amps x number of wire turns) efficiently, but leave a moveable gap that performs a useful function. A spring on the moving parts enables the designer to make the device useful when energized or de-energized.

Think of steel as a magnetic conductor, air as a magnetic insulator. The air gaps they can't eliminate (b/c moving parts) are real wasters of flux.

Mark's explanation of the shading pole (copper slug) is dead on.

These open frame relays are ancient- literally. They were developed for the telephone technology of the late 19th and early 20th centuries. Steppers too! They are the thing that counted the pulses from a rotary dial telephone.

Don C.

I like that analogy. Clean, simple, easy to understand. I'll have to remember that.

Relays go back even further. They were first widely used in the telegraph.

When I take my exhibit to shows I often tell folks that EM pinball technology wasn't developed just for games. It ran the telephone network through the 1970s and even 1980s in some places. EM technology is still used in many applications like anti lock brakes which are essentially just pulsed solenoids.

Another thing to add to the in depth descriptions above is the amount of movement the armature actually needs which is very little. And the job it does, just needs less moving parts and less things to wear out.

Yep, almost all the EMs I have owned or worked on even as far back as the 40s rarely need any new mechanical parts beyond the usual flipper, pop bumper parts, bulbs and rubbers.

Occasionally I get a burned up coil, or a worn out part in a score reel, but I've got a few backups and most coils are readily available. Once I go thru them they work solid until they need rubbers again, with maybe a minor adj here or there.

The stuff that can be a challenge are playfields, backglasses, and plastics. Fortunately some are made, unfortunately most are not.

With these kind of relays, I have encountered several now where the armature has become magnetized, not releasing right away, and that can cause problems that can make you pull your hair out trying to diagnose. But I know what to look for now and they are few and far between and easy to fix.

I worked on Strowger and crossbar telephone exchanges in the UK during the early 80's. Mind blowing to think that a telephone call could actually be made when you consider all the switches and relays involved.