COAST. I ARTILLERY JOURNAL, - Air Defense Artillery

444 THE COAST ARTILLERY JOURNAL
This error causes failure of the optical axis to intercept accurately
the line of flight and to pass over or under it. This error is less apt to
manifest itself by a less sharply defined outline of the projectile than by.
a misrepresentation of the "rise," because the trajectory then does not
remain parallel to the axis of the film drum but makes a slight angle
with it.
With the focal. length as applied, the error in setting may reach to
+ 0.08 mm. It has therefore to be reckoned with when the rise is
small (up to about 10 mm.). If the rise comes to about 40 mm. it can
generally be disregarded because it is then less than 0.1/{.
One should, therefore, when using the instrument, take care to see
that the velocity of the film is so chosen that the time for one revolution
of the film requires only about three times as much time as is needed hy
the projectile to pass through the sighting field of the camera. This is
attained by increasing or reducing the motion of the motor by regulation
of the tension. In all cases a rough offhand calculation of the
time is sufficient, since accurate measure of time is determined by the
tuning fork or the falling staff. The shut-off of the object glass must
then also be adjusted to the time of revolution of the film drum in
order to avoid excessive light on the film.
f. Measuring Errors of Time. The measurement errors above referred
to, especially those of erroneous setting of the objective and
those of erroneous setting of the lenses, give rise, in the main, to inaccuracy
in time measurement, since they misrepresent the measure of
the "rise" of the path of the projectile.
By revolving the focussing glass, with the cross wires that should be
set on the center of the gun muzzle one may readily make errors of the
fractional part of a millimeter if the muzzle is large in diameter and the
apparatus is standing near the gun. It is then expedient to indicate the
center of the muzzle with a target with a black and white square field,
in order that this error shall not exceed 0.05 mm. One can focus on
this with an assurance of sufficient accuracy since the dimension of the
cross wires are only 0.02 mm.
With d3.ytime exposures of the projectile without light fuses the
measuring errors of the "rise," and with them of the time, are to be
feared when, on account of deficient light, we obtain only a dim impression
of the path of the projectile and we are obliged to mark this
barely yisible trace at some points with prick points of a fine needle,
without using a microscope. In such cases the measuring accuracy
will reach a yalue of ~ 0.2 mm. only, and the measure of time must
become correspondingly inaccurate_ (With a "rise" of about 40 mm.
at about -.:.... 0.5~(.1.