More about oscillator
When you finish wiring this project, switch power ON and press S1. You'll hear a sound from the speaker.
Now rotate the control back and forth. What do you now hear from the speaker?
Oscillators make use of feedback. That means part of the output of the circuit is sent back into the input.
Look carefully at the schematic for this project. Can you see where some of the output goes back into the input?
Let's look how the current flows. When you press the key, current flows from the (+) terminals of the batteries through the transistor's emitter, then to its base. Also current flows through the collector of Q1 and enters the base of Q2 through the 22K and 50K resistances, then it returns to the batteries.
Current also flows in the transformer the speaker is connected to. Think that electromagnetism might come into play here?
You're right - an electric current is set up from the top of the transformer coil through the 3.3µF capacitor, the transistor's base and emitter, the key, batteries and back to the transformer by the connection at the middle of the its coil (that's called a tap).
As you might expect, charging and discharging of the 3.3µF capacitor is the key to this project's operation. When the 3.3µF capacitor discharges, it does so into the base of the transistor Q2. It just happens that this current is opposite from the emitter to base current (it's + instead of -).
You guessed it - it shuts off the emitter to base current. But when the capacitor discharges low enough, emitter-to-base current again flows, the 3.3|xF capacitor recharges, and the whole cycle starts again. That's how feedback works. And feedback is necessary to make an oscillator go!.