A 
‘Oct. 30, 1873] 
‘care is required in the adjustment of the knife edges. 
_ the action of the balance very much depends. 
_ tral knife edge rests upon an agate or polished steel plane, 
_ whilst the two pans are suspended from agate or steel 
_ moveable in a vertical direction upon it. 
_ means of a lever handle, and the knife edges are brought 
successive weighings with a balance of precision arises 
t 
They are first made quite sharp, and are then slightly 
rounded with a fine hone or a piece of buff leather. On 
the regular form of this rounded edge, the excellence of 
The cen- 
planes bearing upon the knife edges at the ends of the 
m. In order to preserve the nice adjustment of the 
knife edges, they are never allowed to rest upon their 
bearings, except when weighings are made. At all other 
times, the beam and pans are separately supported upon 
a brass frame attached to the column of the balance, but 
When required 
to be put in action the support is gradually lowered by 
upon their bearings. 
The principal cause of discordances in the results of 
from the risk of the knife-edges not being brought again 
to exactly the same position on the plane bearings, after 
the balance has been stopped and again set in action. 
NATURE 
The most perfect balance is that which varies least in the 
points of contact between the knife-edges and their bear- 
ings during successive weighings. For the attainment of 
this very important requirement, the supporting frame 
is furnished at eack of its extremities with two pins termi- 
nating in cones and miade to fit exactly into corresponding, 
conical holes in the plane bearings, at each of the extremi- 
ties of the beam. The pins and holes are in a linenormal 
to the axis of the beam. The points of these four cones are: 
allin the same horizontal plane. As the movement of the 
supporting frame in a well-constructed balance of precisiom 
is always in the same vertical line, being guided by a ver- 
tical rod fitted to a cylindrically drilled hole in the column 
of the balance, the knife-edges and their bearings are 
always brought into contact in the same relative positions. 
Balances of precision are always enclosed in plate glass: 
cases, with a view both to their preservation, and to keep 
the balances as far as possible from being affected in their 
ge by draughts of air, alternations of temperature, 
cc, 
As to the theory of the relative positions of the centre 
of motion and the centre of gravity of a balance, it is to 
Beer 
x 
Fic. 16.~Index Scale, &c., of No.73 Balance of Standard Department. 
353 
be remarked, (a) If the fulcrum be placed in the centre | the fulcrum be below the line joining the points of sus- 
_ of gravity of the beam, and the three edges be all in the | pension, and these be loaded, the beam will upset, unless 
same right line, the beam of the balance will have no| prevented by the weight of the beam tending to produce 
tendency to one position more than another, but will rest | a horizontal position, as shown in (c). In such case, small 
a 
in any position in which it may be placed, whether the 
pans be suspended to it, or not, and whether the pans 
be empty or equally loaded. (4) If the centre of gravity 
of the beam, when level, be immediately above the ful- 
crum, it will upset with the smallest action ; that is to 
say, the end which is lowest will descend; and it will 
descend with the greater velocity, according as the centre [ 
of gravity is higher, and the points of suspension less | 
loaded. (c) But if the centre of gravity of the beam be 
immediately below the fulcrum, the beam will not rest |, 
in any position but when level; and if disturbed from 
that level position, it will vibrate, and at last come to 
rest in a horizontal position. Its vibrations will be 
quicker, and its tendency to the horizontal position 
stronger, the lower the centre of gravity, and the less the 
weight upon the points of suspension, 
Again, as to the relative position of the central knife 
_ edge, which constitutes the fulcrum of the beam with the 
line joining the two outer knife edges, which form the | 
weights will form an equipoise. In case of (a), a certain 
exact weight will rest in any position of the beam; and 
all greater weights will cause the beam to upset, as in (4). 
(2), If the fulcrum be above the line joining the points of 
suspension, the beam will come to its horizontal position, 
unless prevented by its own weight, as in (6). (3) If the 
centre of gravity be nearly in the fulcrum, all the vibra- 
tions of the loaded beam will be made in lines nearly 
| equal, unless the weights be very small, when they will 
be slower. The higher the fulcrum the quicker will be 
| the vibrations of balances, and the stronger the horizontal 
tendency. 
It is thus-evident that the nearer the centre of gravity 
of the beam is to the centre of motion, the more delicate 
will be the balance, and the slower the vibrations. The 
tendency toa horizontal position is therefore increased 
by lowering the centre of gravity, in which case it will 
also require a greater additional weight to cause it to tura 
or incline to any given angle, and it is therefore less 
points of suspension, it is further to be remarked, (1) If | sensible with a greater load. The fixing of the centre of 
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