'Articles of faith' again Pat!
The easiest way to see how it all operates is if you have the dynamo on a bench, a battery handy, and a meter and / or bulb and some play time. A dynamo works like a motor in reverse, and can be made to behave like a motor. An inefficient motor, because the design isn't for it to BE a motor, but the principles are much the same. (Is there a fridges and aircon parallel . . . dunno!). Anyway -
Dyning:
If it is a working dynamo, then you can bridge your F and D leads, stuff a meter and/or bulb (12v, say 21W brake light bulb, is easiest because the filament will light up more easily due to its being thinner to heat up than on a 6v, which needs more current/amps) between the bridged wires and the body (earth), and spin the dynamo fast in the direction you reckon it should go. Battery-powered drills are no good at all, you'll need way over 1000 rpm often to get the thing to excite and kick in, especially 'first-time'.
If it deigns to start up, you'll see volts, rapidly rising, on the meter. Starts with far less than a volt, using the 'residual magnetism' so-called, which is the magnetic trace left in the iron pole the field coil wraps round on the inside since it last operated; then it builds up with speed once it's got going (what's called the 'avalanche' stage). The volts you'll see will be either positive or negative on the meter. With revs, an E3L will shoot up to maybe 20v or so depending on how fast you go, but it's not a great plan to leave them running like that for any length of time.
The reason the magic happens is because the residual magnetism is sufficient to get the armature, turning in this very weak magnetic field, to produce a really small output - this output is fed directly back into the field coil from D, and when that happens, the output shoots up because there is now a proper strong electro-magnet instead of a few miserable traces of residual magnetism. Under the influence of this new magnetic mojo the armature creates a feeding frenzy by supplying the field coil with as much as there is by way of output because F and D are connected.
On the Other Hand . . . If it doesn't do anything - and it might well not! - try turning it the other way . . . it might go. If it did, you could see, again, whether the volts are + or -.
If you want to see how rotation is affected by the wiring of the field coil or by the brushes, you can play, swapping them around. Whereupon you will find that it works one direction with the field wires connected one way, and the other way if you swap 'em. Or swap the brushes - comes to the same thing. Swap both, and you're back to where you started.
Motoring:
If you want to see from another perspective how the thing is affected by swapping wires and brushes, then you can play with the battery, and use it to drive the dynamo as a motor. Bridging F and D again, and putting battery live (your choice) to the bridge and the battery earth to the body, it'll run in one direction or the other.
If you want to change just the polarity, swap the battery connections to the bridge and to earth, and it will be repolarised to run the other earth - but it will definitely go in the same direction. You have just, effectively, 'flashed the field'. But if you want to change the direction, swap the field wires (or the brushes) and seemingly miraculously it will run the other way. Lots of (very basic) reversible electric motors do their reversing exactly like this - by switching the field coil wires over.
The thing can run either way, with either polarity. Rotation requires intervention to reverse it, but polarity can be swapped just by telling the field which polarity you want it to have. Which is why you can swap from pos to neg earth in a heartbeat by flashing the field, as long as you have a regulator that works with the chosen earth (diodes and all that modern malarkey). But you have to do some fettling if you want things to go the other way.
The point I made before about some regulators needing different connections is quite a serious one though - I get a lot of bikes in for checks that 'don't charge' - but they do when the field is connected for a regulator designed to work what I call the 'Miller Way' - ie between F and D, with a brush to D and a brush to earth. Lucas were pretty much alone in connecting the field from the F terminal to E in fact, and it confuses anyone used to dealing with Bosch and other equipment, which follows well-established standards. And the number of people outside the UK and the Empire (ha!) who run Positive Earth is pretty small too. It's rare in France where I am to be asked to set a system up Positive Earth for anyone, although my own bikes are because it's what I grew up with.
