[CW] Regarding the slowing down of bugs......

[Richard Knoppow via CW]

----- Original Message ----- 
From: "Steve WD8DAS via CW" <cw at mailman.qth.net>
To: <cw at mailman.qth.net>
Sent: Sunday, December 21, 2014 9:07 AM
Subject: Re: [CW] Regarding the slowing down of bugs......



I've never understood the idea of slowing down a bug by 
adding weight.  The swinging dit arm is a pendulum, and the 
period of a pendulum is controlled by the length of the arm, 
not the weight.  You move the weight farther back to make 
the pendulum arm longer and slow it down.  When you've 
reached the end you have to add an extension to the arm 
(which I did as a kid using the shank of a blank house key) 
to go even slower.

On a pendulum wityh a small amplitude of swing like on a 
bug, adding weight without lengthening the arm will make it 
close the dit contact for a longer time, changing the length 
of the dits,but the speed would not be changed.


    The vibrator of a bug is not a pendulum even though its 
called one. In a clock pendulum the restoring force is 
gravity so in an ideal pendulum the period is controlled by 
the length and the force of gravity. Since the force of 
gravity varies with the mass the two cancel and there is no 
effect from mass.  The device in a bug is really what is 
physics is called a clamped bar. The restoring force is the 
elasticity of the main spring and the resonance or period 
depends on the elasticity (Young's modulus) and the density 
or mass of the bar. The effective mass varies with the 
length of the bar and with the added mass of the weight. The 
effective length is a combination of the position of the 
weight and the mass of the bar but usually the mass of the 
weight dominates.
    I could not find a web site with a good description of 
the clamped bar with added mass but this does appear in many 
physics books and in books on acoustics and mechanical 
vibration. There is a little at:
 http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

     Nearly all bugs rely on the position and mass of the 
weight to control the period but variation of the effective 
restoring force of the spring will also work. The only 
practical application of this I know of is the LaHiff 
variable spring used on some Vibroplex keys. This is simply 
an adjustable clamp allowing the length of the spring to be 
varied which varies its stiffness. The elements of stiffness 
and mass in mechanics are approximately analagous to 
capacitance and inductance in an electrical circuit and it 
is common to see an equivalent electrical circuit made up 
for a mechanical or acoustic device.
    Probably the term "vibrator" would be better than 
"pendulum" for the device in a bug.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com