50 



ELEMENTARY GENERAL SCIENCE 



CHAP. 



obtained by multiplying this force by the vertical distance 

 between the point and the line of action of the force. 



But it must always be remembered that the force and the 

 vertical distance between its line of action and the point about 

 which moments are taken must be expressed in suitable units. 



The Balance. The balance is really another form of the 

 supported lath in Expt. 50. All the parts are very care- 

 fully made, and every means is taken to have very delicate 

 supports and accurate adjustments. Fig. 22 shows a simple 

 form of balance which is very suitable for easy experiments, 

 such as are described in this book, which can all be accurately 

 done by its means. Instead of the wooden lath in the experi- 



FIG. 22. The Students Balance. 



ment we have a brass beam supported at its middle line by a 

 knife-edge of hard steel, which, when the balance is in use, 

 rests on a true surface of similar steel. The hooks to which the 

 pans are attached are similarly provided with a V-shaped de- 

 pression of hard steel, which also, when the balance is in use 

 rests upon knife-edges on the upper parts of the beam. To the 

 middle of the beam is attached a pointer, whose end moves over 

 an ivory scale fixed at the bottom of the upright which carries 

 the beam. When not in use the beam and hooks are lifted off 

 the knife-edges by moving the handle C. 



Fig. 23 shows a very delicate form of balance used by chemists. 

 The principle of its action is precisely the same as in simpler 

 kinds, but, to ensure greater accuracy, the knife-edges and the 



