If the radius of the circle to which the inner chamber level tube is ground is large, a small vertical movement of one end of the tube will cause a large displacement of the bubble; if the radius is small, the displacement will be small. Thus, the radius of the tube is a measure of its sensitivity. The sensitivity generally is expressed in seconds of the central angle, whose arc is one division of the tube. For most instruments, the length of a divisions is 2mm. The sensitivity expressed in seconds of arc is not a definite measure unless the spacing of graduations is known.

Sensitiveness of bubble tubes, expressed in seconds per 2mm. division, vary from 1″ to 2″ for precise levels up to 10″ to 30″ for engineer’s levels.

Should determination of bubble tube sensitivity be necessary, proceed as follows:

\[ \alpha_s = \rho \frac{d}{an} \] in which $d$ is the average increment on the rod, $a$ is the distance from the level to rod in compatible units, $n$ is the number of divisions the bubble tube moved, and $\rho = \frac{206 264.8″}{rad.}$.

- align the bubble tube axis with a pair of diagonally opposite level screws
- hold a rod in a vertical position at a measured distance from the level
- observe the rod reading;
- tilt the telescope by manipulating the level screws, moving the bubble tube through $n$ divisions; and
- observe the rod reading.

\[ \alpha_s = \rho \frac{d}{an} \] in which $d$ is the average increment on the rod, $a$ is the distance from the level to rod in compatible units, $n$ is the number of divisions the bubble tube moved, and $\rho = \frac{206 264.8″}{rad.}$.

**Radius of Curvature:**\[ R = \rho \frac{n}{\alpha_s} \]