Peaucellier Cell

Peaucellier Cell is a seven-bar linkage invented in 1864 by a French officer, after whom it is named. The accompanying figure (Fig. 1) shows it in its simplest position. Four equal bars A P, A Q, Q B, ep are jointed to form a rhombus, the joints allowing it to be freely closed or opened. 0 A and o B are equal bars fixed at o, while c Q, the seventh bar, is half the length o Q, and is fixed at c. Q therefore is constrained to describe a circle whose radius is CQ, while A and E move in circles round O. If a' q' b' p' (Fig. 2) be any new position of the rhombus or "cell," it is easy to show that the product o q' x o p' is equal to o a'- - Q' A'2; but o a' and Q' A' i are fixed lengths; hence the product 0 0/ x o p' is constant for all positions of the linkage, and is therefore equal to o Q x 0 P. From this it follows that p' p is perpendicular to o p. Hence while Q describes a circle, p moves in a straight line, p and Q are called the "poles" of the cell.

The fixed point C may be anywhere on the line o p, and the point P will describe a circle, the radius of which depends upon the position of c; when, however, o Q is bisected at C, as in the case described, the radius becomes infinite, and so p describes a straight line.