Measurement of Length 13 



the length of this line in inches, making at least three 

 different determinations. Find the mean value of the 

 length in inches, and then calculate 



(a) the number of millimetres in one inch, 

 (6) the number of inches in one decimetre. 



Compare the results obtained for (a) and (6) with 

 those obtained in Experiment 1 for the number of 

 centimetres in an inch and the number of inches in a 

 centimetre. 



13. Hitherto we have considered only the measure- 

 ment of straight lines. Curved lines must be measured 

 by special methods, such as those described in the 

 following experiments. 



EXPERIMENT 3. Draw a circle BEF with a radius 

 of 5 centimetres by means of a pair of compasses 

 provided with a sharply pointed pencil. Draw any 

 radius AB (Fig. 4). By means of a millimetre scale 

 set the points of a pair of dividers so that they are 

 2 mm. apart. To test the accuracy of the setting, 

 place one point of the dividers at A, and bring the 

 other into contact with the line AB at (7. Keeping 

 this latter point in position, remove the first from 

 A and rotate the dividers about C until the point 

 originally at A is in contact with AB at D. Continue 

 this " stepping " along AB until B is reached. If the 

 dividers have been correctly set, exactly 25 steps will 

 be required to reach B. Should this not be the case, 

 reset the dividers and once more step out a radius of 

 the circle, repeating this test till satisfied with the 

 accuracy of the setting. Next step out the circum- 

 ference BEF with the dividers as set, beginning at B. 

 Note the number of steps required to go completely 



