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...
In your example the twin that went on a space trip could be as little as 1 year old on return, depending on how close to c it was travelling.
I have often pondered the possibility of a twin returning at 25 years of age to find his or her twin dead of old age.
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...