992 Dr. G. A. Shakespear on some 



weights pull the cord to the lower ends of the slits, thus 

 keeping it taut and clear of the beam. To move the rider 

 the cord is grasped by both hands, one at each end, and is 

 raised. It thereupon automatically engages the hook of the 

 rider and lifts the latter from its beam. By sliding the cord 

 along, the rider can then be adjusted with ease and precision, 

 and the cord falls clear again when released. To facilitate 

 grasping with the fingers, the cord is made to pass over a small 

 projecting bar outside each slit, as shown in the figure at B. 



When the cord becomes worn, minute particles of silk 

 may occasionally stick to the rider. This is unimportant 

 unless the rider is very small, but the trouble can be obviated 

 entirely by replacing a short length of the cord with a piece 

 of thin wire which can always be brought into contact with 

 the rider in place of the silk. 



Pointer. — For rough adjustment, a projection at one end 

 of the beam movies near a fixed scale (as shown at 0). For 

 fine adjustment, a thin concave mirror is fixed to the back of 

 the beam, a diameter of the mirror being in line with the 

 axis of rotation of the beam. The plane of the mirror 

 may be vertical or inclined at an angle to the vertical. A 

 slightly inclined position, shown in the diagram at D, is 

 found convenient. A tube passes through the wall of the 

 case and reaches nearly to the mirror, the axis of the tube 

 nearly coinciding with that of the mirror. Sliding into this 

 tube is a shorter length which carries a plane piece of glass 

 at right angles to the axis of the tube. One half of this 

 glass is slightly ground and has engraved on it a scale. On 

 the other half is fixed a pointed piece of foil, the point 

 coming opposite to the middle of the scale (as shown at E). 



The small tube is slid into the larger one until the point 

 of the foil and the centre of the scale are at conjugate foci of 

 the mirror, and the image of the point is thus cast on to the 

 scale. To enable the image of the scale to be seen from the 

 front, an aperture is made in the front side of the tube and 

 a piece of plane mirror is put at an angle of 45° behind the 

 scale (as shown in plan at F). 



To illuminate the mirror the balance may be placed with 

 the tube directed towards a window ; but a plane mirror 

 capable of turning about an axis at right angles to that of the 

 tube and fitted to the latter by means of a collar which can 

 be turned about the outer end of the tube, enables light from 

 any source to be used (this is not shown in the figure). 



The arrestment of the balance should be of the type 

 (found in most good balances) which only raises the beam 

 a fraction of a millimetre. 



