16 



ELEMENTS OF LABORATORY WORK 



when the divided circle is at zero. Small vertical distances 

 and curvatures are measured by finding the new position of 



Fig. 4. 



contact for all four legs by the sense of touch. The screw- 

 gauge (fig. 4) also illustrates this method of measuring linear 

 distances by the movement of a screw. Measurements should 

 be taken with each instrument. 



15. Other Methods of Measuring Density. When a body 

 is weighed while suspended in water or other liquid, it may be 

 counterpoised by a smaller quantity of matter than when 

 weighed in the air. Experiments show that this apparent loss 

 is exactly equal to the quantity of water or other liquid dis- 

 placed by the body i.e. to the mass of the volume of the liquid 

 equal to its own volume. In order to determine the density 

 of a body, suspend it by a fibre of silk or by a thin wire to 

 the hook at the end of a balance-beam. Weigh it in this 

 position, and then support a vessel containing water over the 

 pan of the balance, so that the body may now be weighed 

 when immersed in water. Care must be taken that no air 

 adheres to the body when in the water, and that the density 

 of the water at the temperature of observation is used in 

 the calculation, not that of water at the standard tempera- 

 ture (0 C.). 



T. ., True mass Mass 



Density =- : - n = ==-. r 



Apparent loss of mass V olume. 



in water. 



