Figure 1

© Trustees of the Science Museum Group/Science & Society Picture Library

Wheatstone’s Hatchment Dial. This specimen is organised in a diamond shape and marked with twenty letters, the letters C, J, Q, U, X and Z being omitted to save the expense of an additional needle and associated wire. In the five-needle model described in Sheet I of the joint patent of 1837, letters were indicated by the deflections of two needles in contrary directions, while numerals (not shown in this specimen) were indicated by the deflection of only one needle, and required the addition of a sixth wire. At rest, the needles were in a neutral (vertical) position.