The apparatus consist of a small split cylinder of spring brass or other non-corrodible metal of 0.5 mm thickness forming a mould 30 mm internal diameter and 30 mm high.
On either side of the split,
Two indicators are brazed suitably with pointed ends made of 2 mm diameter brass wire in such a way that the distance of these ends to the centre of the cylinder is 165 mm.
The split cylinder will be kept between two glass plates.