  We define a set of $2^{n-1}-1$ entanglement monotones for $n$ qubits and give
a single measure of entanglement in terms of these. This measure is zero except
on globally entangled (fully inseparable) states. This measure is compared to
the Meyer-Wallach measure for two, three, and four qubits. We determine the
four-qubit state, symmetric under exchange of qubit labels, which maximizes
this measure. It is also shown how the elementary monotones may be computed as
a function of observable quantities. We compute the magnitude of our measure
for the ground state of the four-qubit superconducting experimental system
investigated in [M. Grajcar et al., Phys. Rev. Lett._96_, 047006 (2006)], and
thus confirm the presence of global entanglement in the ground state.
