Experience in monitoring movement across tension cracks in a slate quarry in Cornwall suggested a need for two types of instrument. One would be sufficiently sensitive for the measurement of daily variations used to predict failure on the basis of acceleration. The other would provide enough detail for the calculation of the direction and magnitude of movements across the crack.

The two instruments are an Invar bar extensometer developed by the Cement and Concrete Association and a large vernier caliper fitted with a clinometer and modified for measurements between steel pegs topped with phosphor bronze balls.

Performance figures are given along with some of the design considerations. A statistical evaluation of precision and an analysis of the errors inherent in ball-in-cone measurements form appendices to the paper.

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