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\ensuremath{\sim}${\mathit{L}}^{\mathrm{\ensuremath{\zeta}}}$, where \ensuremath{\zeta}=0.87(7). This result is in agreement with recent conjectures of a universal roughness exponent \ensuremath{\zeta} for these materials.