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loads are in equilibrium there, must be no swinging of the pointer
at release of the pans, the* beam rests being clown. Kquilibrium
is then destroyed by adjusting one of the screws on the bourn end,
so that at release the pointer will swing X to 7 scale divisions
in one direction. The point on the scale which tin* pointer
reaches on its first excursion is taken as the "xoro point," f-ho
pans having first boon steadied to stop lateral swinging.
In weighing, the weights are adjusted as by arty other method,
the rider finally being placed so that when, the pans are released
the pointer will reach the same* " xero point," on its first, excursion,
that was first determined.
Although this method would seem, at, first, to be essentially
incorrect in principle, it is capable of giving accurate results in
the hands of a careful analyst,, with the following limitations:
1. It cannot be used with, balances having a single control,
releasing beam, and pans at one* operation.
2. The pan rests are cleaned, if necessary, with alcohol to
prevent sticking to the parts, an otherwi.se a swinging impulse
would be given by release* of the hit tor.
3. Most balance's show a variation of sensibility with variation
of load. The "xero point" must then be determined at the
approximate load that is to ho weighed, if a single weighing is to
be made, or at both loads in case* of differential weighing, unless
the singlo load or the difference between the two loads is quite
small. One of these two conditions is met in most unalytieul
work, Sample weights or weights of precipitates are l«»ss than
one gram, in the majority of canon. If a sample is to bo weighed
on counterpoised glasses it in sufficient to determine the point
reached on the first swing, with the empty glasses. If if, in
to be weighed front a weighing bottle, or if the precipitate in
to be weighed in a crucible, the point reached when the filled
weighing bottle or the empty crucible, roHpoeftvoIy, in firing
weighed, i« taken as the xero point for that particular pair of
weighings.
4. It in obviouft that a single observation given no cheek upon
chance causes of variation, sueh us vibration or air current*
within the balance cane.
The method is useful, especially for rapid work, if proj>or cure
and consideration arc exercised, hi any evimt tht* hftiumw mnnt