Movement No. 20 is the second in a series of compound pulley arrangements (Movements 19 through 22) that systematically demonstrate how mechanical advantage can be multiplied by adding movable pulleys in sequence. In this arrangement, two movable pulleys are deployed — each one wrapped by its own dedicated cord. For each cord, one end is firmly anchored to a fixed point on the support structure above, while the other end connects to the axle of the next pulley below, ultimately supporting the load at the bottom of the chain. This is a fundamentally different design from the common block-and-tackle, where a single continuous rope threads through multiple sheaves. Here, each pulley operates as a fully independent force multiplier in its own right. The governing mathematical principle of the entire series is clear: mechanical advantage equals 2 raised to the power of the number of movable pulleys. With two movable pulleys in Movement No. 20, the mechanical advantage becomes 2² = 4. This means that an applied input force of just 1 unit is sufficient to lift a load of 4 units — four times the applied force. The trade-off is that the effort rope must be pulled through a distance four times greater than the distance through which the load rises. Movement No. 20 therefore represents a practical and powerful intermediate step between the single-pulley system of No. 19 (advantage of 2) and the more elaborate three-pulley arrangement of No. 21 (advantage of 8), illustrating the exponential scaling of mechanical advantage in this class of pulley system.
19, 20, 21 and 22. Are different arrangements of pulleys. The following rule applies to these pulleys:-In a system of pulleys where each pulley is embraced by a cord attached at one end to a fixed point and at the other to the center of the movable pulley, the effect of the whole will be = the number 2, multiplied by itself as many times as there are movable pulleys in the system.