Chap. XL.] DYNAMOMETERS. 489 



velocity of 32 '1889 feet ; and we have seen that 

 force, can be measured by the velocity produced in 

 unit mass in unit time. So that the gravitation 

 measurement can become absolute measure. At 

 London, the pound weight produces 32-1889 units of 

 force. It is to be noted that the action of gravity 

 differs in amount in different places (page 504), so 

 that for the same body the force differs at different 

 parts of the earth's surface. 



Dynamometers are instruments for measuring 

 forces in pounds or kilogrs. Fig. 199 shows one form. 

 It consists of two steel arcs 

 AB and CD, connected together 

 at the extremities. The in- 

 strument is suspended by the 

 ring R, and a weight is at- 

 tached to the opposite hook. 

 The curves of the arcs are 

 increased by the weight, the 

 action being resisted by the Fig-. 199. Dynamometer, 

 elasticity of the steel. The 



/ 



amount by which the arcs are separated in the 

 middle is measured by the graduated bars, one 

 being attached to the middle of each arc. The bars 

 slide on one another, and are graduated by hanging 

 on various known weights, which mark the extent 

 of separation effected. An unknown force can 

 then be estimated in terms of the previous graduation. 

 Another form of the same instrument is made for 

 estimating force exerted not by traction, but by pres- 

 sure. 



For instance, such a form is made for estimating 

 the pressure that can be exerted by the hand in 

 squeezing. The instrument is grasped in the hand and 

 the arcs pressed together. Between the arcp is a dial 

 plate and an indicator, which travels a greater or less 

 distance over the dial plate according to the pressure 



