88 EXPERIMENTAL PHYSIOLOGY 



the spring balances in the two end holes. Pull down on 

 each, keeping the bar horizontal. Compare the pulls regis- 

 tered by the balances. What is their relation? Attach one 

 balance halfway between the end and middle holes, keeping 

 the second balance in the other end hole. Pull until the bar 



f \ \ \ 

 -^ "^ p ^ "^ i» ^ 



\ 



A. B. C. 



Fig. 43. — Forms of Levers: A, 1st class; B, 2d class; C, 3d class; W, weight; F, 

 fulcrum or pivot; P, pull. 



is level as before. What is the relation of the registered 

 pulls now? Verify the following law by changing the posi- 

 tion of the two balances. 



Weight X perpendicular distance from the pivot equals 

 Pull X perpendicular distance from the pivot. (Perpendic- 

 ular distance is measured from the pivot at right angles to 

 the direction in which the force is acting.) 



This arrangament of lever is called a lever of the first class. 



B, Support the bar by one end hole, and at the extreme 

 end attach a weight so that the bar will balance level; then 

 insert the balances in the other two holes (see Fig. 43, jB). 

 Pull down with the one nearest the pivot (Weight), and up 

 with the one at the end (Pull). Record the pull and weight 

 when the bar is level, measure the distances from the pivot, 

 and see if the law of A still holds. This arrangement is 

 called a lever of the second class. 



C If the pull nearest the pivot be called the Pull and the 



