CHAPTER IV 

 LEVERAGE 



27. The Principle of the Lever. Many tools are based 

 upon the principle of the lever. A lever is a rigid bar, straight 

 or bent, free to turn about a point called a fulcrum. Levers 

 are generally divided into three kinds or classes, the class be- 

 ing determined by the position of the fulcrum in relation ^o 

 the applied force or effort and the resisting force, i.e., the 

 weight. The mechanical principle of the lever was discovered 

 by a Greek named Archimedes, who lived in the third cen- 

 tury. He stated that if he had a lever long enough and a 

 place to stand, he could move the earth. 



28. Mechanical Advantage. Since a lever is a tool, its 

 object is to assist in distributing strength or speed to the best 

 advantage. Suppose a lever is used in moving a heavy stone. 

 By what means can the amount of assistance rendered by 

 it be determined? This assistance, called the mechanical 

 advantage, is obtained by dividing the force arm or effort arm 

 (the perpendicular distance from the fulcrum to the direc- 

 tion of the force), by the weight arm (the perpendicular dis- 

 tance between the fulcrum and the weight), or by dividing 

 the resistance or weight by the effort or applied force. In other- 

 words, there are two ways by which a lever car) be made 

 to be of more service : first, by lengthening or increasing the 

 force arm; second, by shortening or decreasing the weight arm. 



If a mechanic, for example, desires to have more advantage, 

 or, as he usually says, more " leverage," he may increase the 

 length of the force arm by taking a tool with a longer handle. 



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