WEIGHTS AND MEASURES 11 



WEIGHING 



_ In weighing a body we simplj balance the force it exerts by 

 Its gravity against another known force. There are several types 

 of balances or scales,— spring, equal arm, unequal arm, and torsion, 

 ihe hrst is not very exact but is handy for coarse weighing. In 

 weighing small amounts delicate instruments should be used. A 

 few simple rules should always be followed. 



1. Balance pans before starting weighing. 



2. Drugs should not be placed directly on scale pans. These 

 should first be covered with pieces of paper. The opposite pan 

 should be balanced by a piece of paper of equal weight. Unless 

 the pans are equally balanced in this manner, serious mistakes may 

 occurin weighing small amounts. The object of the paper on the 

 pans is two fold. It keeps the pan clean and insures freedom from 

 contamination with some previously weighed drug. 



^ 3. Balance accurately with material to be weighed. When a 

 pointer is provided on the balance, it should swing an equal distance 

 each side of the center or zero. 



4. Always throw balance off center when through weighing. This 

 stops the movements of the balance and consequently stops its wear. 



5. In weighing liquids, tare (weigh) or balance the container 

 first. 



MEASURING 



This is done in graduated vessels (graduates), graduated or 

 measuring flasks and pipettes. The wider the vessel at the place of 

 reading the greater is the liability to error. On this account greater 

 accuracy can be obtained if the vessel is as narrow as possible where 

 the reading is taken. The minim graduate is not accurate for 

 measuring small amounts, as a considerable amount of the measured 

 liquid is retained in the vessel by capillarity. A minim pipette is 

 to be preferred -for this purpose and also for the fact that there is 

 less error in pouring more or less than the required amount into the 

 graduate. 



The cylindrical graduate has the advantage that equal accuracy 

 can be obtained throughout while the conical shaped graduate has 

 the advantage of greater accuracy for small amounts. A few rules 

 for measuring should always be followed. 



1. Hold the graduate so that the top of the liquid is a horizontal 

 plane perpendicular to the long axis of the graduate and have the 

 top of the liquid on a level with the eye. 



2. On account of capillarity, the surface is always cupped, 

 forming a meniscus. The reading should always be taken at the 

 lowest level of the meniscus. 



