The United States Torsion Balance Company, of New York, has recently
brought before the public a new form of balance which presents so many
ingenious and excellent features that we illustrate it below, on the
present page. The instrument in its simplest form is shown in Fig. 1. It
consists of a beam, A, which is firmly attached to a wire or band, B, at
right angles to it, and which wire is tightly stretched by any
convenient means. Then, since the wire and beam are both horizontal in
their normal position, and since the center of gravity of the beam is
immediately above or below the middle line of the wire, the torsional
resistance of the latter tends to keep the beam horizontal and to limit
its sensitiveness. When the beam is deflected out of its horizontal
position and the wire thereby twisted, the resistance to twisting
increases with the arc of rotation. To counteract this resistance and to
render the beam sensitive to a very slight excess of load at either end,
a poise, D, is attached to the beam by a standard, C, which poise
carries the center of gravity of the structure above the axis of
rotation. This high center of gravity tends to make the beam "top
heavy," or in unstable equilibrium. By properly proportioning the poise
and its distance above the wire to the resistance of the wire, the
top-heaviness may be made to exactly neutralize the torsional
resistance, and when this is done the beam is infinitely sensitive.
[Illustration: KENT'S TORSION BALANCE. Fig 1.]
The moment of the weight or its tendency to fall increases directly as
the sine of the arc of rotation, while the torsional resistance
increases as the arc, and for small angles the sine and the arc are
practically equal.
When arranged as in Fig. 1, the scale is balanced only when the center
of gravity of the structure is vertically above the middle line of the
wire, and the support of the scale must be leveled in the direction of
the beam, so as to cause the center of gravity to take this normal
position. After the scale is thus leveled, if from any cause whatever,
such as shifting the scale on a table, or shifting the table itself, the
scale support is thrown out of level, the center of gravity of the poise
and beam is shifted from the vertical line above the support, and its
moment immediately becomes greater than the torsional resistance, and
the beam tips out of balance, and cannot be used as a correct scale
until the support is again leveled.
[Illustration: KENT'S TORSION BALANCE. Fig 2.]
In spite of all the foregoing facts, it was reserved for the
"Encyclopedia Britannica," in its ninth edition, to use the following as
the result of its condensed wisdom:
"In the torsion balance proper, the wire is stretched out horizontally,
and supports a beam so fixed that the wire passes through the center of
gravity. Hence the elasticity of the wire plays the same part as the
weight of the beam does in the common balance. An instrument of this
sort was invented by Ritchie, for the measurement of very small weights,
and for this purpose it may offer certain advantages; but clearly if it
were ever to be used for measuring larger weights, the beam would have
to be supported by knife edges and bearing, and in regard to such
applications therefore (as in serious gravimetric work), it has no
_raison d'etre."_
[Illustration: KENT'S TORSION BALANCE. Fig 3.]
This would seem to settle the whole case, for if the encyclopedia says
it has no reason to be, then, like the edict of the Mikado, it is as
good as dead, and if that is the case, "Why not say so?" On the
contrary, the torsion balance seems very much alive. But as it is not
very generally known, perhaps the early history of this form of balance,
briefly sketched, may prove of interest.
One of the first forms of the torsion balance which met the disapproval
of the "Encyclopedia Britannica" was attended with the difficulty that
the pivoted wires were attached directly to the bifurcated ends of the
beam, and could not be tensioned without bending these ends unless the
beam was made so heavy as to interfere with its employment in delicate
weighing.
[Illustration: KENT'S TORSION BALANCE. Fig 4.]
The next step was the substitution of light forms stiffened by the wires
being tensioned over them. This was the invention of Professor Roeder,
recently deceased. The next step was the common counter scale, and then
that form of letter scale in which one of the bands acts as a fulcrum
and the other as a pivot.
After Professor Roeder's death, Dr. Alfred Springer, of Cincinnati,
continued perfecting this invention, and with marked success--scales not
intended for anything but the weighing of the ordinary articles of a
grocery store working so accurately that up to 50 lb. two grains would
turn the balance.
As will be noted, this balance dispenses entirely with knife edges, and
this statement carries with it the gist of its entire merit. There is no
friction, and the elegance of the work and the nice adjustments of the
parts struck the writer at once.
[Illustration: KENT'S TORSION BALANCE. Fig 5.]
The prescription scale and the proportional scale (see Fig. 4) are
particularly interesting. The former is sensitive to 1/64 of a grain,
and the latter, invented by Mr. Kent, is a most ingenious method for
weighing, by which, in a small compass (10½ in. by 4¼ in. by 3¾ in.), we
have a balance capable of weighing 3 lb. avoirdupois by thirty-seconds
of an ounce.
For ordinary balances on the torsion system, in which extreme
sensitiveness is not needed, the trouble caused by change of level of
the scale is insignificant; but it becomes a matter of importance in
more sensitive scales, such as fine analytical balances in places where
it is impossible to keep the table or support of the scale level, for
instance on shipboard.
To counteract this effect of the change of level, Dr. Alfred Springer
devised the system which is shown in its most elementary form in Fig. 2.
An additional beam, E, with wire, F, and poise, H, on support, C, were
added to the balance, and connected to it by a jointed connecting piece,
J. The moment of the structure, E C H, about its center of rotation was
made equal to the moment of A C D about the center. The wires, B and F,
are attached at their ends to supports which are both rigidly connected
to the same base or foundation. If this base, the normal position of
which is horizontal, is tipped slightly, the weights, C and H, will both
tend to fall in the same direction. But suppose the right hand end of
the base is raised, causing both of the weights to tip to the left of
the vertical, D, tending to fall over, the left tends to raise the right
hand end of the beam, and the connecting piece, J H, also tending to
fall to the left, tends to lower the left hand end of E and the piece,
J. The moments of the structure, E C H, and A B D being equal, and one
tending to raise J and the other to lower it, the effect will be zero,
and J will remain in its normal position.
It is not at all necessary, however, to have the weights and dimensions
of the structure, E C H, equal to those of A B D. All that is necessary
is that the components of the weight of each part of the structure which
act vertically on J shall be equal and opposite. For, if the left end of
the beam, E, is made shorter than the right end of the beam, A, a given
angle of rotation of the beam, A, will cause a greater-angle of rotation
of E, consequently will tip the weight, H, further from the vertical
than the weight, D, is tipped, and in that case the weight, D, must be
made smaller than H, to produce an equal and opposite effect upon J. In
practice it is convenient to make the beam, E, only one-fifth to
one-twentieth as long as A, and to correspondingly reduce the weight, H,
relatively to D. In this case, on account of the angle of rotation of
the beam, E, being greater than the angle of rotation of A, the beam, E,
becomes a multiplier of the indications of the primary beam, A.
Mr. Kent has devised a modification of Dr. Springer's system, which is
shown in Fig. 3. It is applied in those varieties of the torsion balance
in which there are two parallel beams, connected by either four or six
wires. The wire, F, carrying the secondary beam, E, and poise, H,
instead of being carried on an independent support, rigidly attached to
the base, as above described, is attached directly to a moving part of
the balance itself, and preferably to the two beams. In Fig. 3, T T T
are trusses over which are tightly stretched the wires, B B B. A A' are
two beams rigidly clamped to the wires; _t_ is another truss with
stretched wire, F F¹. The upper wire, F', is attached by means of a
flexible spring and standard, S, to the upper beam, and the lower wire
is attached either directly or through a standard to the lower beam. The
secondary poise, H, is rigidly attached to the truss, _t_. The secondary
beam, E, is also rigidly attached to the truss, and acts as a
multiplying beam. The secondary structure thus completely fills two
functions: First, that of multiplying the angle of rotation and thereby
increasing the apparent sensitiveness of the scale, and, second, that of
overcoming the effect of change of level. The secondary beam may be
dispensed with if a multiplier is not needed, and the secondary truss,
_t_, with its standard and counterpoise, H, used alone to counteract the
effect of change of level. Fig. 5 shows a modification of this extremely
ingenious arrangement.--_Engineering_.
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