Electrical Wizards: Hands-on with a hand-crank DC generator, some LEDs, and a voltmeter

We’ve wrapped up our reading of Electrical Wizard: How Nikola Tesla Lit Up the World, an informative biography of Serbian-American inventor and electrical pioneer Nikola Tesla, whose reputation has enjoyed a revival of fortunes in recent years. Author Elizabeth Rusch guides readers through roughly the first half of the great man’s life, highlighting his creation of a useful motor powered by AC current, his alliance with AC proponent George Westinghouse, and the struggle that decided whether DC, promoted by Edison’s companies, or AC would dominate large scale electrical generation and distribution during the earliest phase of electrification in the United States. Spoiler: AC triumphed and has remained supreme, though DC never went away and DC-powered devices are increasingly ubiquitous within homes and offices (e.g. mains-powered LED bulbs contain circuitry that converts the AC they receive into DC for the actual LED or LEDs inside the bulb).

At the end of this volume, published by Candlewick Press, Rusch has thoughtfully included a Scientific Notes featuring explanations of some of the key technical concepts mentioned within the preceding biographical narrative, including a young-person accessible explanation of the difference between AC (alternating current) and DC (direct current) and of how an AC motor works.

We carved out enough time last week for what proved to be quite an enjoyable activity related to the text. After distributing hand-crank DC generators, LEDs, and voltmeters and guiding each student in stripping both ends of several lengths of solid-core jumper wire, we walked everyone through the hookup process. The meters, lights, and generators all featured finger-tightenable screw post terminals, making the physical process of wiring them together reasonably straightforward and speedy and obviating the need for needle-nose pliers or other hand tools.

The voltmeters allowed us to show that negative voltages were possible and that what constituted a positive or negative voltage depended on which posts on the generator were connected to which posts on the meter and also on which direction the generator’s crank was turned. It made the small leap to the explanation that AC current jumped rapidly back and forth between flowing in one way and then the other a bit more comprehensible, or so we hope! In the video that caps this post, you can see, one participant, Javier Y., producing enough current to intermittently power a rainblow-blinky LED.

An excellent question posed by one student elicited no small amount of interest from their classmates: why did the incandescent bulb (included, as an add-on, with each generator) emit light regardless of the cranking direction while the LEDs, once they were connected to the generator, only lit up when the crank was turned in one direction? In broad strokes, it’s because the filament in an incandescent bulb acts as a resistor and emits light (along with a lot of heat) when current flows through it regardless of the direction whereas the third letter in the acronym LED stands for diode and diodes only permit current to flow in one direction. Both descriptions are gross oversimplifications, but correct enough for the time being.