a. Starting at 25,000 rpm, decrease the flashing rate of the strob -
oscope until a single image is obtained. Record the RPM dial setting and
call it X.
b: Continue to decrease the RPM dial setting slowly. Watch the
changing images carefully, and stop when the next single image occurs.
Record the RPM dial setting as Y.
c. Calculate the harmonic number, n, by:
and round off the value of n to the nearest whole number.
d. Calculate the fundamental speed, Sf, by:
S f= nX
If X is 22,500 and Y is 16,800 , then:
= 2.95 = 3
and the fundamental speed is:
Sf = 3 x 22,500 = 67,500 rpm
2.8.4 LOW-SPEED OPERATION.
The measurement of speeds (below 600 rpm) on the low range of
the Type 1531 may be difficult because of flicker resulting from lack of
persistence-of -vision. It is recommended that these measurements be
made in a darkened environment, or that the operator wear dark glasses,
in order to reduce the confusing effect of high ambient room lighting on
the pattern observed.
Speeds below 110 rpm can be measured by means of multiple images.
For example, if the flashing rate of the stroboscope is twice the funda-
mental speed of the device, two images, 180 degrees apart, will appear.
At three times the fundamental speed, three images, 120 degrees apart
will appear. Refer to paragraph 3.1.2 for illustrations. This multiple-
image technique can also be used for higher speeds, within the range
of the Type 1531, where flicker makes it difficult to tell when the correct
flashing rate is obtained (for example, between 110 and 600 rpm).
2.8.5 SLOW -MOTION STUDIES,
High -speed motion can be reproduced by the stroboscope at an
apparently much lower speed if the rotating or reciprocating motion occurs
at a constant rate.
If the flashing rate of the instrument is set at a speed slightly lower
than the fundamental speed of the observed object, the object will appear
to move slowly in the same direction as the actual motion, at a speed