Whether the hemispherical bowl is raised or hollowed, it will be observed that the centre portion of the disc is stretched, whilst that part which is nearer to the edges will contract; there must, therefore, be a part of the disc or bowl which is neutral; that is, neither stretches nor contracts. This will be a circle lying on the surface of the bowl, and shown by the line N C in Fig. 224. To determine the position of this circle, a distance equal to the diameter of the hemisphere divided by 3.1416 is set down from the top, and a line parallel to the top of bowl then drawn; this gives the diameter of the neutral circle. Taking a hemispherical bowl of 5 in. diameter, the distance down of the neutral circle will be d / ╥ = 5 / 3.1416 = 1.6 in.
In the working up, this circle should remain of constant diameter, and therefore gives us the boundary line where the wrinkles from the edge of disc should die away. In copper bottoms, and such like work, the wrinkling circle should be marked on the circular plate, and the wrinkles put in on the outside of this circle.