410 



CHAPTER IX. 



STANDARDISATION AND CALIBRATION OF APPARATUS. 



I. Weights. — A good set of weights is a sine qua non in a laboratory ; 

 they should consist of the following : — 



100, 50, 20, 10, 10, 5, 2, 1, 1, 1, grammes, and 

 Oo, 0-2, 0-1, 0-1, 005, 002, 001, 001 gramme, 

 and some riders each O'Ol gramme. 



Select one of the weights, preferably a 10 gramme, as a standard ; mark 

 one of the 10 grammes and one of the 1 gramme with a mark (') by means 

 of a fine steel point; mark another 1 gramme with a mark ("); turn up 

 one corner of a O'l granyne and of a O'Ol gramme. By this means the 

 weights can all be distinguished from each other. 



See that the balance is in adjustment by swinging it without an3' 

 weights in the pans ; if the pointer does not travel to an equal distance 

 on both sides, alter the adjustment till this end is attained. After the 

 adjustment, leave the balance for at least ons hour and see if it is still 

 in adjustment ; if not, repeat the process, handling the beam, &c., as little 

 as possible. 



When the balance is in proper adjustment, place the 10-gramme weight 

 on the right-hand pan, and the lO'-gramme weight on the left-hand pan; 

 they should very nearly balance, and the pointer should swing nearly 

 eqiiallj' on both sides ; if they do not balance, place the rider so that the 

 balance is restored. The value of the lO'-grarame weight can now be 

 obtained in terms of the 10-gramme weight, by adding the readings of the 

 rider, if on the right arm, and subtracting, if on the left arm. Now 

 reverse the weights, placing the 10-gramme weight on the left-hand pan, 

 and the lO'-gramnie weight on the right-hand pan, and repeat the weighing ; 

 the value of the lO'-gramme weight can be obtained in terms of the 

 lO-gramme weight by adding the readings of the rider, if on the left arm, 

 and subtracting, if on the right arm. Owing to minute differences in the - 

 lengths of the arms it is not unusual to find a difference between the two 

 values. 



The true value may be found by adding the two values together and 

 dividing by 2. (It is more correct, theoretically, to multiply the two- 

 values and take the square root, but the values thus obtained are practi- 

 cally identical with the arithmetical mean. ) 



The total value of the 5 -I- 2 -|- 1 -)- 1' -f 1" weights are similarly obtained. 



weig 



+ ] 



the mean of the three values ("which should not differ appreciably). 



The value of the 50-gramme weight is obtained bv weighing it against 

 the 20 4- 10 -t- 10' -I- 5 -h 2 -H -I- 1' -H" weights. -J = o 5 



The value of the 100-gramme weight is obtained by weighing it against 

 the 50 -I- 20 -I- 10 -I- 10' -I- 5 -1- 2 -f l-H r -I- 1" weights. 



The 5-grarame weight is now taken, temporarily, as a standard, and the 

 2 + 1 + V + 1" weights are >\-eiglied against that, and the value of the series 

 obtained in terms of the 5-gramme weight. 



