USE OF THE BALANCE 5 



two arms of the beam. To obtain the true weight of a substance 

 when extreme accuracy is required, counterbalance it with sand 

 or weights and then replace the object with weights; or weigh in 

 one pan and then exchange the substance and weights, weigh 

 again, and take the mean of the weights so obtained. 



The beam and pans must always be supported before adding or 

 removing weights, and the weights must be handled only with 

 forceps. 



To avoid errors in noting weights always count them twice: 

 (1) by noting those missing from the box; (2) by noting the 

 weights as they are taken from the pan and replaced in the box. 



All objects must be at room temperature when weighed. 

 Warm objects cannot be weighed accurately as currents of air are 

 caused which introduce an error. 



Crucibles should be cooled in a desiccator when the precipitates 

 weighed in them take up water from the air. If the crucible 

 gains weight measurably during the weighing it should be reheated 

 and weighed again very quickly. The weights to balance the 

 crucible approximately are in this case placed on the pan before 

 the crucible is removed from the desiccator. 



A platinum crucible should remain in the desiccator ten to 

 fifteen minutes and a porcelain crucible twenty to twenty-five 

 minutes before weighing. 



When, as in the case of barium sulphate, the precipitate is not 

 hygroscopic a desiccator should not be used. Accurate results are 

 more readily obtained when the crucible, both before and after the 

 precipitate is in it, can be cooled in the open. The time required 

 for cooling is about half as long as in a desiccator. 



The supports of beam and pans must be lowered gently to avoid 

 injury to the knife edges. 



The balance case must never be left open or with the beam 

 unsupported, and the rider must be removed from the beam. 

 When not in use the balance should be protected from fumes and 

 dust by the regular use of a rubber covering placed over the case. 



Be careful to avoid spilling the substance to be weighed on the 

 pans or on the floor of the balance case. If this happens, remove 

 at once by dusting carefully with a camel's-hair brush. 



Liquids must be weighed in closed weighing bottles, and solids 

 in weighing bottles, watch-glasses, or aluminum pans. 



From time to time determine the true zero point of the scale 



