# Power spike in secondary solenoid

Some students were performing an experiment involving 2 solenoids in a line, a power source attached to the primary, and a globe attached to the secondary solenoid.

A crude depiction of the setup is as below.

/////// /////// <---- Rod inserted here
Pri . . . Sec

An iron rod was inserted through both solenoids and the globe became exceptionally bright when the iron rod was fully through the secondary and half way into the primary. This effect was persistent, not temporary, or fluctuating.

I cannot figure out why, any help would be appreciated.

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At first glance, it appears you are making an iron core transformer from the solenoids.

Iron is often used to enhance transformer efficiency. You shouldnt have to google too long to find out details on "iron core transformers". The "iron" is usually in the form of laminated sheets, to reduce eddie current losses.

It is a simple iron rod, and yes it is a transformer experiment a senior physics class is doing.

The point of the question is that (beginning at the point the rod is totally through the secondary coil, and entering the primary) the light is dull gets to a brightness peak (rod midway through the primary) and then dulls down again (rod fully through the primary). And to boot if the rod is left half way through the intensity is sustained.

Why does the intensity of the light peak when the rod is present in half of the primary solenoid ?

It is a simple iron rod, and yes it is a transformer experiment a senior physics class is doing.

The point of the question is that (beginning at the point the rod is totally through the secondary coil, and entering the primary) the light is dull gets to a brightness peak (rod midway through the primary) and then dulls down again (rod fully through the primary). And to boot if the rod is left half way through the intensity is sustained.

Why does the intensity of the light peak when the rod is present in half of the primary solenoid ?

Others may have a better(more specific) answer for you, I had a University inductor lab, where we did similar experiments. If I remember correctly, your basicly altering the "efficiency" of the transformer.

Why is 1/2 way in optimal? Perhaps more than that and "eddie currents" produce a point of diminishing returns?

Thanks, I was leaning towards that description myself, but wanted a sure answer if I could get one.

I do believe the rod was hollow, not solid.

Anyone have a more technical and more definate answer ?

Maybe the changing rate of magnetic flux peaks when the rod is present in half of the primary solenoid

If that is the case i woulld like to know why