Graduation and Calibration of Apparatus 147 



as time passes, and to such an extent that thermometers, for 

 example, are commonly kept from three to five years before 

 graduating and marketing. In these instruments the fixed 

 points are determined, the distance between divided into a 

 hundred parts and the scale duly marked. The assumption, 

 therefore, is that the tube is of equal diameter throughout, and 

 care has been taken to ensure this, any tube in which this is 

 not the case being rejected in the manufacture of thermo- 

 meters. Similarly with burettes, the zero is commonly marked, 

 water is filled in to the mark, 50 grams run off, a new mark 

 made, and the distance divided into fifty equal spaces. In 

 modern instruments successive 5-c.c. are taken, and one may 

 frequently trace a faint line at successive 5-c.c. marks, where 

 the graduation has been checked. It will be seen, however, 

 that to graduate a tube according to its true calibration would 

 entail an enormous amount of work, and would lead to very 

 expensive apparatus (which, after all, no chemist would trust ; 

 he would surely recalibrate his instruments), consequently the 

 method adopted is to divide the extremes into an equal number 

 of parts, and find the true value of each say, once a year or so. 



It is not uncommon to graduate a tube in millimetres, 

 as this can easily be copied from a steel scale in the manner 

 described below, and to calibrate afterwards. 



We may for our purpose describe the method of graduation 

 and calibration of (a) a pipette ; (b) a burette ; (c) a thermo- 

 meter. From these, it will be easy to arrange the details of 

 any other calibration exercise that may fall to the lot of an 

 experimenter. 



(a) A pipette. A pipette is an instrument for delivering 

 the amount of liquid engraved upon it, and may easily be 

 made by a .student of glass working. Since it has to deliver, 

 and not to contain its rated amount, calibration can only be 

 accurately carried out by treating the instrument in the way 

 in which it has to be used. Thus it is of no value to weigh 

 the pipette full and empty, but it must be used to deliver the 

 exact amount of its rating, and this must be measured; the 

 delivery taking place exactly in the same manner in which 

 the instrument is to be subsequently used, 



