Page 323 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

302 Maritime Archaeology: A Technical Handbook, Second Edition
provided they are read correctly, simplify the drawing process; for example, 34.81m at a scale of 1:5 requires a division and then the use of a ruler, whereas a scale rule requires only the use of the rule.
Measuring the outside and inside diameter of objects ranging from ceramic pots to gun barrels can present some problems. Vernier calipers are useful for outside diameters and just inside the necks of tubes. Otherwise large double-arc calipers can be bought or made for use on outside and inside diameters, provided in the latter case you can extract the caliper without altering the reading. It is worth having mild steel calipers chromed to prevent rusting. This is particularly important if they are taken anywhere near the sea, otherwise without the chrome they rust very badly.
There are various ways of measuring in places where a direct measure- ment cannot be made. For example, it is not normally possible to measure the wall thickness inside a jar with conventional calipers because when the calipers are extracted, the separation changes. Home-made scissor- or X- calipers are one method that can be used in this situation (Figure 11.4). The point of separation on the bottom caliper is duplicated on the top, thus while the inaccessible bottom one is in the inside position the reading can be taken from the top. Alternatively, it is possible to utilize ordinary calipers with a small block placed against the outside surface of the vessel. The calipers measure the thickness of the wall of the object together with the block, the increased caliper jaw separation allows the calipers
Figure 11.4 X-calipers used to measure the wall thickness inside jars and pots. The calipers are set so that the gap is identical at each end of the caliper.