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