Hal Murray wrote:
> It's energy loss in both cases.
>
> Is there a term other than Q that is used to describe the rate of energy loss
> for things that aren't oscillators?
I've seen "energy dissipation" or "energy decrement".
Before Q was used to describe line width of atomic and optical clocks,
before Q was used to describe performance of simple harmonic oscillators and
quartz crystals,
before or while Q was used to describe the ratio of reactance to resistance in
a coil,
horologists that worked with precision pendulum clocks needed a way to
characterize the amount of energy lost per period.
This was very important because the energy lost each swing (due to all forms of
friction) had to be replaced in order to keep the pendulum oscillator running.
And the timekeeping performance seemed to be related to how small or how
consistent this energy was. I'm sure "decrement" was used before then, but I'm
not well-read enough in science history to have any idea. Still...
What I do know is this abstract from a 1938 paper [1], written by an early time
nut:
THE DISSIPATION OF ENERGY BY A PENDULUM SWINGING IN AIR, By E. C. ATKINSON
------
ABSTRACT. The decrement of a pendulum falls slowly with the amplitude:
hence the
need for determinations based on small changes of angle. The resulting
errors of observation
lead to erratic values but not to systematic error. The result of
measurements with a
seconds pendulum enclosed in a case is shown by a smoothed curve, the
departure from
observed times being expressed by smoothing fractions, and a smoothing
figure is a
measure of this departure for the whole or part of the experiment. From the
decrement
the rate of loss of energy is calculated. This 7 kg. pendulum with
amplitude 53' dissipates
a Board of Trade Unit (which serves a 70 w. lamp for 14 hours) in rather
over 100,000
years. Experiments with different pendulums are described by which the
component
losses due to suspension, rod, and bob are found. Suspension springs made
from thin
strip clamped in chaps dissipate large and variable amounts of energy
compared with
springs made from thick strip ground thin in the middle. The variable
losses are associated
with variable rates of the pendulum. The cylindrical case adds considerably
to the air
resistance. The measured loss due to a gravity impulse lever is little in
excess of the
computed loss from collision with the pendulum: for a seconds pendulum
1/2000 part of
the free pendulum loss.
------
This article is also a favorite of mine because it talks about a "root mean
square of a series of such fractions may be called the smoothing figure for the
series, and its smallness is an indication of the confidence which may be
placed in the result." -- a 1938 precursor of the two-sample variance, or Allan
deviation.
/tvb
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