# Where'd the current go?

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#### puzzled

##### Newbie level 6
This is a very basic question that confounds me.
I am trying to measure the current in a coil that has a generated EMF pulse
travelling through. It's the standard stuff according to the classic equation;
number of coils, magnetic field strength, time interval, cross-section of magnetic
field, etc.
I'm generating, typically, 20 millisecond 2+ vac pulses of using a powerful neodymium cylinder magnet , which match nicely with the equation. Elementary stuff.
I'm assuming there's nothing magical going on that nullifies the basic IR=V
law, so why do I measure transient current far below expectations when the
total resistance of the winding is very low.
I've tried everything , including a very sensitive A/C galvanometer, but alway
read a tiny fraction of the current that should be generated by several volts
travelling through a couple of ohms resistance.
This is def Electronics 101, I realize, but what am I missing?

#### FvM

##### Super Moderator
Staff member
Did you consider that the current is also limited by the coil inductance?

#### puzzled

##### Newbie level 6
I generally understand that concept, but no textbook example of EMF generated in a coil that I've found suggests that would reduce the current by such a huge factor. Also, when I look at datasheets for solenoids, etc., they seem to suggest current effects in line with the basic EMF-coil equation.

#### chuckey

Have you worked out the time constant for your circuit. Try slowing the pulses down as I think you are not waiting long enough for the current to reach its V/R level.
Frank

puzzled

### puzzled

Points: 2

#### puzzled

##### Newbie level 6
Have you worked out the time constant for your circuit. Try slowing the pulses down as I think you are not waiting long enough for the current to reach its V/R level.
Frank

Verrry interesting, will pursue that- thank you!

#### dick_freebird

If you are generating the voltage / current using a magnet,
the speed at which its field lines moves through the coil and
the proximity, both matter hugely. Yet you mention nothing

You might remove some of these variables from the quandary
by making a 1:1 replica coil and linking it with a good known-
attributes core, getting a 1:1 transformer whose voltage
transfer qualities ought to be more predictable. You can stimulate
your circuit through the new (primary) winding from some handy
pulse generator and observe w/ oscilloscope. Then you can
assess "everything but the magnet" and once that is well in
hand, go back to your flying magnet case with stuff better
understood.

#### puzzled

##### Newbie level 6
"If you are generating the voltage / current using a magnet,
the speed at which its field lines moves through the coil and
the proximity, both matter hugely. Yet you mention nothing

I'm more sad about the equations not living up to my expectations,
rather than vice versa.

Field lines and proximity I can understand, and results obtained
to date have been very consistent in terms of voltage (only) with the
standard equations. I have actually been using thin < 1 mil
copper tracings (rectangular in cross-section and 5 mils wide),
insulated, wrapped belt-like around a cylindrical conduit in which
a linear array of spaced neodymium magnets ( with magnets spaced sufficiently far apart not to interfere with each magnetic field) travels at up to 20 m/s (no, it's not a coil gun).
The concept behind using flat wiring with the greater-area planes parallel to the travel of the magnets was to present more conductive material
at closer promixity to the magnetic field than would be achieved by using circular cross-sectional wiring (nominal gains based on 1/r^2 field strength attenuation). This does produce a higher voltage, according to the o-scope.
Seemed like a smart idea, but it looks like I've inadvertently introduced capacitive and inductive effects through all that parallel surface area on the flat windings.....

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