Bipolar Junction Transistors wired as a current source will pass a certain amount of current, based on the voltage across the emitter resistor.
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Kit looks fun, especially for high school age.
1KΩ emitter resistor makes the math easy. For each volt across it, it will pass 1mA of current. Connect a wire or load to Vcc (positive supply power of the voltage you are/will be using) and measure the voltage across the 1KΩ resistor. 3V across it, for example, will mean that it passes 3mA of current, as will pass through the transistor and load. Remember that the NPN BJT base to emitter drops about 0.6V of the trimpot output voltage from reaching the emitter resistor in this circuits.
This process is demonstrated in the video below.
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