9.  Below the balance pans are two pan-arrests operated by a button from the front of the case.  These arrests exert a very slight upward pressure upon the pans and minimize the displacement of the beam when objects or weights are being placed upon the pans.

10.  A movable rod, operated from one end of the balance case, extends over the balance beam and carries a small wire weight, called a rider.  By means of this rod the rider can be placed upon any desired division of the scale on the balance beam.  Each numbered division on the beam corresponds to one milligram, and the use of the rider obviates the placing of very small fractional weights on the balance pan.

If a new rider is purchased, or an old one replaced, care must be taken that its weight corresponds to the graduations on the beam of the balance on which it is to be used.  The weight of the rider in milligrams must be equal to the number of large divisions (5, 6, 10, or 12) between the central knife-edge and the knife-edge at the end of the beam.  It should be noted that on some balances the last division bears no number.  Each new rider should be tested against a 5 or 10-milligram weight.

In some of the most recent forms of the balance a chain device replaces the smaller weights and the use of the rider as just described.

Before using a balance, it is always best to test its adjustment.  This is absolutely necessary if the balance is used by several workers; it is always a wise precaution under any conditions.  For this purpose, brush off the balance pans with a soft camel’s hair brush.  Then note (1) whether the balance is level; (2) that the mechanism for raising and lowering the beams works smoothly; (3) that the pan-arrests touch the pans when the beam is lowered; and (4) that the needle swings equal distances on either side of the zero-point when set in motion without any load on the pans.  If the latter condition is not fulfilled, the balance should be adjusted by means of the adjusting screw at the end of the beam unless the variation is not more than one division on the scale; it is often better to make a proper allowance for this small zero error than to disturb the balance by an attempt at correction.  Unless a student thoroughly understands the construction of a balance he should never attempt to make adjustments, but should apply to the instructor in charge.

The object to be weighed should be placed on the left-hand balance pan and the weights upon the right-hand pan.  Every substance which could attack the metal of the balance pan should be weighed upon a watch-glass, and all objects must be dry and cold.  A warm body gives rise to air currents which vitiate the accuracy of the weighing.