8 



JUNIOR GRADE SCIENCE 



one of the impressions of the teeth. By dividing the result by twenty you 

 can find the distance between two successive teeth. Repeat the determin- 

 ation. Write in your note book : 



Distance between twenty impressions - - - mm. 



Therefore, distance between successive teeth - - mm. 



Now determine the length of a curved line by running the tracing wheel 

 along the line and counting the number of spaces between the impressions 

 it makes. This, multiplied by the distance between two teeth, gives the 

 length of the curve. 



iv. Opisometer. Obtain and examine an instrument called an Opiso- 

 meter. A small wheel with fine teeth fits upon a screw of fine pitch which 

 forms an axis, and as the wheel is turned it moves towards one end of the 

 axis. Turn the wheel until it reaches one end of the screw. Make a mark 

 upon it at the bottom. Place the mark at the starting point on the curve 

 you wish to measure, and carefully cause the wheel to travel along the 

 whole length of the curve. When it has completed the journey along the 

 curve, make a mark on the rim of the wheel touching the end of the curve 

 and lift the wheel off. Place this last mark over some division on your 

 millimetre scale, and run the wheel along the scale in the opposite direction 

 to which it formerly turned, so making it retrace its course along the screw 

 axis. When it arrives at the end of the axis notice the division on the 

 mm. scale. Record the length, and compare it with those obtained by the 

 string and dividers. 



v. Circumference of a cylinder. (a) Wrap a strip of paper closely round 

 a wooden cylinder (Fig. 5), and make a small hole with a pin at* a place 



Fia. 5. Measurement of the circumference of a cylinder. 



where the paper overlaps. Unroll the paper and measure the distance 

 between the two holes. This gives the distance round the cylinder, that 

 is, its circumference. 



Measure the circumference in another way by winding a piece of thread 

 round the cylinder several times side by side, and dividing the total length 

 of the thread by the number of coils. 



(6) Measure the diameter of the cylinder in several places by applying 

 the rule to one end and taking the greatest distance across. 



Repeat the preceding exercise with cylinders of different sizes, such, for 

 instance, as a tin canister, or a short piece of wide glass tubing, or a round 

 bottle. Make the measurements in centimetres. In each case divide the 

 length of the circumference by that of the diameter and notice the result 

 obtained. 



