building an electric bell-ringer

A piece of steel is probably close enough. Some of these things used to use a collection of iron wires as the yoke. A steel bolt might also work OK. But easiest nowadays might be to find some old transformers and take the laminations out of those.

I remember having fun with that stuff.

The material is not critical at all. Any bolt or part of one will work fine, same thing with the armature a piece of steel 1/16-1/8" thick would be good. A somewhat harder part may be the piece of spring steel or spring to use and the contacts. If you can find some steel strapping used to bind lumber together that works great for a leaf spring.

You don't have to buy any of that stuff just use what you find in the trash.

The armature needs a spring, of course, but so do the points. If you just make fixed (unmovable) points on the armature, you wind up with a high pitched buzzer. There has to be another spring on one of the contact points to allow inertia and over-travel to slow the action of the armature and clapper.

One source of iron (steel) are railroad spikes. They are about 5/8" square and make a great electromagnet. Almost no residual magnetism when the power is removed. There are lots of 120 V shaded pole induction motors with the same 5/8" square bobbin and coil. A short walk on a track is all it takes to find a few. Hacksaw will cut them and they can be drilled and tapped.

American Science and surplus is a good source for unusual junk and they usually have some induction motors priced cheap - great paper catalog too.

Love those Lindsay reprints . . . . I have a collection. I find I have a better appreciation of electronics and history from reading and building the stuff. I made numerous Tesla coils, a very large induction coil, and several static generators using instructions from the turn of the century, or earlier.

Really interesting reading Maxwell and his theories of electricity and magnetism. (Dover Books for Maxwell and Gilbert reprints) He's coming up with all manner of mechanical contraptions in an effort to explain what he thinks is happening in physics. Struggling to explain induction, inductive reactance, resonance, transformers, etc., but without the words and conceptual framework we now take for granted.

You're not likely to find cotton covered wire, gutta percha covered wire, hard rubber sheet, rod, or tubing, but modern wire and plastics work just fine.

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Mild steel is iron. In fact low carbon, low manganese steels, AISI 1000 series grades are purer iron than the stuff they used to call iron and cast iron has way more carbon than virtually any steel, go figure! Aside from the pure element iron, the irons of commerce were really a statement of processes used to make them and their lack of purity than anything else. The word steel implied highly refined iron, controlled process, heatreatment, and specific alloys. For your purposes mild steel that hardware store bolts are made of should work fine. Bob