Another example of a lever of the first class is the use of the fire poker with the bar of the grate serving as a fulcrum. When a lever consists of two parts fastened by a rivet, it is called a double lever. Scissors, pincers, and forceps are all examples of such a lever; the rivet serves as a fulcrum.
The scale beam used in weighing is also a simple lever. The arms on each side are of equal length and are suspended over the center of support. The axis at the point of suspension is sharpened to a very fine, sharp edge, so that when weights are placed in the scales, the beam may turn with as little friction as possible. When the arms are not of equal length, the scales cannot weigh accurately, although the beam may seem fairly balanced and the weights true. If one arm is 8 in. long and the other only 7 1/2 in. the scale will balance with a 1-lb. weight on the short arm and 15-oz. on the long arm. Thus the customer of a merchant who uses such a scale loses an ounce in every pound. The deceit can, of course, be discovered by changing the weight and material to the opposite scales. In some cases where the beams of scales are not accurate, the articles to be weighed are put in one pan and balanced by weights; the article is then put in the other pan and balanced again. The correct weight is found by taking the square root of the product of the two weights.