APPARATUS 79 



A is the beam of the balance constructed in the form of two 

 triangles base to base, and made of fused quartz rod, 0-6 milli- 

 metre in diameter ; the whole weighing less than half a gram. 

 The frame thus formed oscillates about a central knife edge, 

 ground at the end of the vertical rod, and resting on a plane 

 quartz plate /. Attached to this beam at its centre (but not 

 shown here) is a tiny mirror, taking the place of the pointer in 

 an ordinary balance. 



At one arm of the balance is a quartz counterpoise, while at 

 the opposite end is suspended a small quartz bulb of known 

 internal volume, and sealed up at known temperature and 

 pressure. This is hung by means of a fine quartz fibre within a 

 tube which is connected air-tight to the bottom of the balance 

 case. A fine hook carries the quartz bulb, a, and below this a 

 quartz scale pan, 6, and a quartz counterpoise, c. The weight of 

 this suspended system is always adjusted to equilibrate the 

 counterpoise attached to the opposite end of the balance. 



The method of weighing is as follows : if the quantity of 

 substance to be weighed does not exceed the total weight of the 

 air contained in the bulb, the pressure inside the balance case 

 and the resting point having been taken with the scale pan 

 empty, the substance to be weighed is placed on the pan and the 

 pressure adjusted till the same resting peint is obtained. If w 

 is the total weight of air contained in the bulb, which was filled 

 at pressure P, and P' represents the difference in pressure 

 required to recover the original resting point, then the weight of 

 the substance is wP'/P. If the quantity of substance to be 

 weighed exceeds the weight of air contained in the bulb it is 

 necessary to prepare one or more counterpoises which must be 

 lighter than the original counterpoise c, and must differ from 

 each other by a known amount not exceeding w. 



The resting or zero point of the instrument is found by the 

 position taken by the image of a Nernst lamp reflected from the 

 mirror attached to the beam. The case is deprived of air by 

 means of a vacuum pump connected through the two-way stop- 

 cock x, and the pressure of the residual air is determined by 

 observing the height of the mercury column in the manometer, 

 which is read by means of a telescope and scale to a tenth of a 

 millimetre. 



The attachments for the release of the beam consist of two 

 V-shaped quartz rods which just centre the beam but do not 



