Dave Dameron wrote: more _how_ a As the list's resident electrical (as opposed to electronic) engineer, I feel I must make a stab at answering this. Especially since my first job at PowerGen was research on transformers. (Not at this fundamental level though - I was looking at fault detection systems) As I see it, when you have a winding linked with a magnetic field, you induce a voltage in the winding proportional to the rate of change of the field. If you like to visualise lines of magnetic flux, the voltage is related to the number of flux lines that actually cut the wires of the winding, not the flux linked with the winding at any time. This means, among other things, that (for sinusoidal ac) the flux is zero when the voltage is at its peak and vice versa. The transformer, though, is not a differentiator since the magnetic field is proportional to the integral of voltage in the primary. That is, flux = integral (Volts in).dt; (Volts out) = d(flux)/dt (assume = means "is proportional to") Note also that the flux depends only on voltage, NOT CURRENT. You associate magnetic fields with currents (well, I do, anyway), and as you increase the primary current you expect the field to increase. But it doesn't. An equal and opposite effect from the secondary current exactly balances this. This has a number of implications that are quite important for my current (pun not intended) job, modelling power systems. 1. The equivalent circuit of a transformer has a branch in parallel with the primary, the magnetising branch. This is an inductance representing the magnetic field (and it's in parallel, so it depends only on voltage as above) and a resistance representing "iron losses" 2. No transformer is perfect. There is regulation - inductance and resistance apparently in series with it. The resistance represents "copper loss" - the physical resistance of the windings. The inductance represents flux in each winding that _does not_ link with the other, and therefore is not backed off by current in the other winding. But in an electrical system model, the simplest representation of a tranformer is a series inductance connecting two sides of the transformer. Ironically, this represents physically the magnetic field that does not connect the two sides of the transformer! 3. For metering, protection and the rest, you see a lot of current transformers. One of these consists of a high current, low voltage primary - usually a bar running through the middle of the toroidal core - and one or more multi-turn (low current) secondaries. The equation is simple. The mmf (magnetomotive force = amps * turns) of all the windings add up to zero (equal and opposite effect of secondary current again). Because the measuring kit acts as an effective short circuit on the secondary - or drops a few volts at most - the magnetising current is almost zero, and you can thus make very accurate measurememnts. Is this the answer you wanted? Final note. What do you mean by "the primary winding's magnetic field" as distinct from that of the secondary? To a good approximation, the magnetic field is the same at both the primary and the secondary. It may be close to zero, for reasons I described. But the difference between the mag. field at the two windings contributes only to the equivalent series impedance. It is not something to look at when discussing the detailed operation of the tranformer. series leads the boards Aargh! I've not heard of that (throwing the whole string away) before but I can well believe it. Philip.