CHAPTEK XXX. 



STANDARDISATION AND CALIBRATION OF APPARATUS. 



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

 they should consist of the following : 



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



0-5, 0-2, 0-1, 0-1, 0-05, 0-02, 0-01, 0-01 gramme, 



and some riders each 0-01 gramme. 



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

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

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

 one corner of a 0-1 gramme and of a 0-01 gramme. By this means the 

 weights can all be distinguished from each other. 



See that the balance is in adjustment by swinging it without any 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 one hour and see if it is still in adjustment ; 

 if not, repeat the process, handling the beam, etc., 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 

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

 balance is restored. The value of the lO'-gramme 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'-gramme weight on the right-hand pan, and repeat the weighing : 

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

 10-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 + 2+l + l'4-l" weights are similarly obtained. 



The value of the 20-gramme weight is obtained in a similar manner by 

 weighing it against the 10 + 10', 10 + 5 + 2 + 1 + 1'+!", or the 

 10' -}- 5 + 2 + 1 + 1' + 1" or, preferably, by weighing against all three 

 series and taking the mean of the three values (which should not differ 

 appreicably). 



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

 the 20 + 10 + 10' + 5 + 2 + 1 + 1' + 1" weights. 



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

 the 50 + 20 + 10 + 10' + 5 + 2 + 1 + 1' + 1" ~ 



462 



