Jump Rope Generator Is it possible to generate electricity, however inefficient, by swinging a long extension cord connected to a circuit along Earth's magnetic field like a jump rope?
 A: In principle yes but not really. Just to get a sense of what kind of voltages we are dealing with here let's use the simplified model of a long straight conductor of length $\ell$ moving perpendicular to a magnetic field $B$ at a velocity $v$. The induced voltage between the ends of the wire $\varepsilon$, by convention called the electromotive force, is*
$$\varepsilon = v B \ell$$
Let's say the wire is $10$m long, the velocity $10$m/s, and the earth magnetic field around $50 \, \mu \text{T} = 5 \times 10^{-5}\,\text{T}$.
$$\varepsilon = 10 \times (5 \times 10^{-5}) \times 10\, \text{V} = 5 \, \text{mV}$$
While this is certainly measurable it's not really a useful voltage as a 10 m wire already has a fair deal of resistance and if the wire is bend and not moving at right angles with the magnetic field the voltages will be even smaller. The problem is really that the magnetic field of the earthis very weak which is why motors use magnets to get a stronger field at the wires.
*http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/genwir2.html
