. Modifications in an ordinary Balance. 991 



read, and (d) the magnitude of the smallest measurable 

 deflexion of the beam. Weighings are often made, even 

 by skilled observers, in which details (a) and (b) are not suf- 

 ficiently regarded, and a degree of accuracy which is quite 

 illusory is apparently obtained. When a centigramme rider 

 is used, it is generally possible to estimate the small changes 

 of weight, due to changing the position of the rider, to a 

 degree of accuracy far beyond that which is warranted in 

 view of the uncertainty as to the conditions (a) and (b). 

 Theoretically, the sensitiveness may be increased so that the 

 smallest appreciable change of position of the rider produces 

 an observable deflexion of the beam. In practice, however, 

 it is found advisable to have a degree of sensitiveness for 

 which a comparatively large change of position of the 

 centigramme rider produces no appreciable change in 

 deflexion. In other words, the rider is usually too small 

 and the means of measuring deflexion not sufficiently deli- 

 cate. In the balance to be described the rider is therefore 

 comparatively large (either *5 gm. or *05 gm.), and the 

 deflexion is observed optically by using a concave mirror 

 attached to the beam. 



The effect is to provide a pointer of considerable length 

 but with small mass and moment of inertia, so that the period 

 of oscillation is appreciably reduced. There is, moreover, 

 no parallax. 



Rider Beam. — This runs from end to end of the beam, the 

 upper edge being in the plane of the knife-edges. It is 

 divided into ten equal divisions each of which is subdivided 

 into ten smaller divisions. 



Rider. — This is in the form of an elongated ring which 

 surrounds the rider beam (thus preventing possibility of 

 falling off) and provided with a hook at its upper part. 

 When the balance is unloaded and in equilibrium the rider 

 is at zero mark on the left side of its beam. It may be 

 either *5 gm. or *05 gm., giving respectively 1 cgm. or 

 1 mgm. for one small division ; the smallest weights in the 

 box are then 1 gm. or \l gm. Tenths of a division can, of 

 course, be estimated. 



Method of moving the Rider. — The rider is moved by means 

 of a thin silk cord (plaited fishing-line is good) passing- 

 through a slit at each end of the case. The shape of the 

 slit is shown at A in the figure, the upper part being vertical 

 and in the same plane as the rider beam but rising above 

 the level of the top of the rider. The lower part of the 

 slit slopes downwards and forwards. The silk cord has a 

 small stretching-weight at each end, and when at rest these 



