How to verify that the Resistance of a Superconductor is Actually Zero?

One can readily draw a graph (see Figs. 1 and 4) resting on a curve against the abscissa axis, but it is much more difficult to make sure that the resistance is not merely very low, but actually zero. When measuring resistance, a physicist uses a device sensitive enough to register the expected value. If the measured quantity suddenly decreases 10 or even 100 times, the pointer stops deflecting. It is precisely due to this fact that the temperature dependence of the resistance, first obtained by Onnes, looked so uneven. He needed nearly a year to make certain that the resistance of a superconducting substance was lower than one that could be registered by the most sensitive device of that time. This, however, does not prove that the resistance is strictly zero, but such a rigorous experimental proof is inaccessible. A physical quantity may be assumed equal to zero if its possible deviation from the theoretically calculated zero is so small that it cannot be recorded by any measurement.

Onnes conducted the following experiment. Into a vessel with liquid helium, which served as a cooler, he immersed a superconducting ring with a current circulating in it (see Fig. 7). If the ring had a resistance other than zero, the current in the ring would decrease and the magnetic field that created such a current would change. The magnetic field could be registered outside the vessel filled with liquid...