MEASUREMENT OF THE DIAMETER, 



(3) The movable arm must move with sufficient ease along 

 the rule. 



Callipers of iron would be too heavy and too cold in winter, 

 hence they are made of wood. As wood alters with the 

 degree of humidity, the movable arm is liable to jam at one 



o e 



CD 







Fig. 2. 



time, or to move too easily at others. To avoid this drawback, 

 various constructions have been adopted, resulting in a number 

 of callipers, of which the following two deserve to be specially 

 mentioned : 



Gustav Heyers Calliper. The distinguishing feature of this 

 instrument is that the rule is given, in section, the shape of a 

 trapezium, and that it is pressed up or down in the movable 

 arm by means of a wedge, so as to counteract the swelling or 

 shrinking of the wood. In Fig. 3, a represents the cross 

 section of the rule, b the wedge, and c the section of the mov- 

 able arm. The wedge is fastened to a screw, which can be 

 moved by a key at d. On moving the wedge from left to right, 

 it presses the rule upwards and thus tightens it ; on moving the 



