When the load is at F', the reaction at B' is m/l and the moment at C' is m(l-x)ll, which will be reckoned positive, when it resists a tendency of the right-hand part of the girder to turn counter-clockwise.
This, however, is not generally retained in one plane, and the spire projects, little by little, on the side which was originally left, but finally becomes right (in dextral forms, with a clockwise direction, if viewed from the side of the spire; but counter-clockwise in sinistral forms).
Thus in A the twist may be right-handed or left-handed; in B the polarity of a given end may become north or south; in C the circular magnetization may be clockwise or counter-clockwise; in D the length may be increased or diminished; in E the magnetization may become stronger or weaker.
Most of the solar vortices indicate clockwise rotation in the southern hemisphere and counter-clockwise rotation in the northern, as in the case of terrestrial cyclones.
If we mark off a segment AB along the line of action so as to represent the force completely, the A moment is represented as to magni FIG 15 tude by twice the area of the triangle OAB, and the usual convention as, .o sign is that the area is to be reckoned positive or negative iccording as the letters 0, A, B, occur in counter-clockwise or clockwise order.
