Experimental measurements of the roughness of brittle cracks

We record the height of the crack surface as a function of position along one-dimensional cuts orthogonal to the crack for six different brittle materials. We find that the width w of this one-dimensional trace as a function of its length L behaves as w{~}L^ζ, where {ζ}=0.87(7). This result is in agreement with recent conjectures of a universal roughness exponent {ζ} for these materials.