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N.A.C.A. Technical Memorandum No. 291 2
damp the vibrations, can be employed.
2. The recording device must register the vibrations trans-
mitted to it without appreciable diminution and shifting of the
phases.*
The vibration period and damping of the recording device deter-
mine its accuracy. The smaller the vibration period and hence the
larger the vibration number, the more accurate will be the recording
of rapid vibrations. If the recording instrument has an apprecia-
ble mass and a long vibration period and is not damped, false curves
will be recorded, especially for vibrations of abrupt wave-form.
Such records have recently been published and have resulttdd in a
number of further computations, which have given wrong results, be-
cause the curves employed, due to the fault of the recording appa-
ratus, did not represent what they were supposed to-
The vibration numbers may be very large. If, for example, an
electric motor with 3000 R.P.M. drives a machine over a pinion
with 20 teeth which do not work smoothly, so that every tooth causes
a jolt, there is produced a vibration number of 1000 r-p.s. in the
motor shaft and, under unfavorable circumstances, with vibrations
of considerable amplitude. In order to record such vibrations with
sufficient accuracy, we need a recording instrument whose own vibra-
tion number is considerably larger and which does not damp the vi-
brations recorded. These conditions cannot be fulfilled by mechan-
ical recording devices. On the contrary, we possess, in the oscil-
logaph, an instrument which satisfies all requirements and even
*very freely oscillating recording device gives some error in am-
plitude and phase-
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Elsässer, R. Measuring Vibration and Torque With the Oscillograph, report, December 1924; (https://digital.library.unt.edu/ark:/67531/metadc277385/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.