This paper describes a physics laboratory activity aimed to illustrate Faraday's law in a hands-on fashion. By winding two coils of copper wire on a pen, students can construct a transformer in a short time. The induced emf in the secondary coil is observed as a function of the primary current for different waveforms. I describe how the device is constructed and show the experimental results obtained with the device.

See, for example, F. Sears, M. Zemansky, and H. Young, University Physics (Addison-Wesley, 1976), Chap. 33, pp. 564–569; R. Blum and D. E. Roller, Physics, Vol. 2: Electricity, Magnetism and Light (Holden-Day, 1982), chap. 36, pp. 1285–1288.
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