From my knowledge, there needs to be an alternating current for the transformer to be able to create magnetomotive force throughout the core.

Various nerve testers (say for standard school projects) include an audio transformer ran on a 9V DC circuit though, with nothing looking like it would create an AC.

Sample circuit

Now I see a pushbutton? over an SCR, however in simple thinking that should not do too much, just act like transistor switch?

What on earth makes the circuit work to have a high enough voltage?

Something has popped in to my mind actually, tranformers actually do work with DC, only for a miniscule period of time when power runs through it, is that why it shocks? And why an audio transformer? Cheaper?

  • \$\begingroup\$ transformers work with changes in current. power transformers do what they do with 50 or 60 Hz b/c mains power is sinusoidal, the rate-of-change for which is a phase-shifted sinusoidal. you're right in guessing that when you switch the DC on, you get an output for a moment; the step function input induces a spike output. \$\endgroup\$ – JustJeff Nov 1 '11 at 0:08
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    \$\begingroup\$ @JustJeff, I am a little unclear, what do you mean by step function? Just "stepping up" to a voltage while the circuit is completed? \$\endgroup\$ – Kim N. Nov 1 '11 at 0:52
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    \$\begingroup\$ sorry about that - a step function means stepping instantly from one constant level to another. The mathematical ideal is actually instant, but in real life, the very quick change you get from a switch is a fair approximation, as is making contact between two conductors such as the wand and the maze. \$\endgroup\$ – JustJeff Nov 1 '11 at 11:00

As you pretty much guessed, then transformer works when DC is applied initially. This causes a change in current through the windings.
As the formula V = L(di/dt) shows, a change in current will produce a voltage across the inductor, proportional to how quickly the current changes.
So when you touch the wand to the maze, there is a step change in voltage across the primary winding which falls off as the current rises, and this appears stepped up (assuming a step up transformer) across the secondary winding. The same effect will occur but with polarity reversed when the wand is pulled away from the maze and the current flow is stopped abruptly.

  • \$\begingroup\$ Thank you, there are just so many things to learn, it is very encouraging. \$\endgroup\$ – Kim N. Nov 1 '11 at 1:51

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