16 Domestic Science 



coil of five strands. Remove the wire, straighten it 

 carefully by gentle stretching, and measure the dis- 

 tance between the two marks by bringing the wire 

 in contact with the edge of a metre scale. The length 

 read off is evidently five times that of the circumference 

 of the bottle. Find the length of the circumference 

 by division, and calculate the length of the diameter, 

 making use of the result obtained in the previous 

 experiment. Confirm the values obtained by per- 

 forming two more determinations of the length of the 

 circumference at the same part of the bottle, making 

 coils of three and seven turns respectively. 



The value found for the diameter may be readily 

 checked by measuring it directly with a pair of calipers 

 (see Fig. 5B). Open the instrument so that the points 

 of the jaws are separated by a distance slightly greater 

 than the length of the diameter as found by calcula- 

 tion. Place the points on the circumference of the 

 bottle in the region where the coil was applied and move 

 the calipers to and fro on the surface, adjusting the 

 distance between the points meanwhile so that they 

 just slip over the outside of the bottle with the line 

 between the points parallel to its base. Remove the 

 calipers, place the points of the jaws alongside the 

 metre scale, and read off the distance between them. 

 Make two further determinations, placing the jaws in 

 each case at different parts of the circumference. 

 Find the mean of the three values obtained and com- 

 pare it with the mean of those already found. 



In this experiment no allowance has been made for 

 the thickness of the wire. Should this be considerable 

 as compared with the diameter of the bottle, the length 

 of the diameter as determined must be corrected by 

 subtracting the thickness of the wire. 



