STEAM ENGINES 



185 



Planimeter. A common form of planimeter is shown in 

 Fig. 1. It consists of two arms hinged together by a pivot 

 joint at j. One arm carries a recording wheel d, which rolls 

 on the surface to which the card is fastened, while the outline 

 of the diagram is being traced by the point /. The needle 

 point p is fixed in the paper or drawing board, and remains 

 stationary during the operation. 



The indicator card should be fastened to a smooth table or 

 a drawing board that has previously been covered with a 

 piece of heavy unglazed paper or cardboard. The point p 

 should be placed far enough from the card to enable the 

 wheel to roll on the unglazed paper without touching the card, 

 as it will slip if rolled over a smooth surface. Set the zero of 



FIG. 1 



the wheel d opposite the vernier e; then, with the tracing 

 point /, follow the line of the diagram carefully, going around 

 the diagram in the direction of the hands of a watch, and stop 

 exactly at the starting point. 



Reading the Vernier. The' area is read from the recording 

 wheel and vernier as follows: The circumference of the wheel 

 is divided into 10 equal spaces by long lines that are consecu- 

 tively numbered from to 9. Each of these spaces represents 

 an area of 1 sq. in. and is subdivided into 10 equal spaces, 

 each of which represents an area of .1 sq. in. Starting with 

 the zero line of the wheel opposite the zero line of the vernier 

 and moving the tracing point once around the diagram, the 

 zero of the vernier will be opposite some point on the wheel; 

 if it happens to be directly opposite one of the division lines 



