Lowell gage

[Phillip Ford]

>>>Phil -
>>>
>>>Thanks for taking the time to pursue the experiments.  I will try to 
>>>replicate them. A few comments:
>>>
>>>Part of my own soapbox, so to speak, relates to the way measurements are 
>>>taken with Lowell, though it would likely not alter your main 
>>>observations.  There are a number of ways the gauge readings can be 
>>>misread, but that's mostly a different discussion.
>>
>>Perhaps we can have that now.  What are the ways that the readings can be 
>>misread?
>
>
>I'll give it a try.  As already stated, pictures sometimes are much better 
>at conveying ideas.  Try making a drawing if my description is too 
>confusing (of course such a description could, if rendered accurately, 
>result in a confusing drawing).  Apart from any heavy-handedness in its 
>use, the traditional application has us either compare a zeroed reading on 
>speaking length to a reading on the length between bridge and string rest 
>(or visa versa), to make a conclusion regarding the implied angle.  A 
>positive angle would imply downbearing, a negative angle would imply 
>negative bearing, and no angle would imply zero downbearing.

This is how I normally use the gage.  Do you see a problem with that?

>   The "component approach" isolates the front and rear angles relative to 
> the top of the bridge.  We are instructed to read the surface of the 
> bridge with the gauge's feet spread just shy of the front and rear bridge 
> pins (to avoid binding between pins), and then compare  front and rear 
> readings, relative to bridge top.
>
>A basic mis-assumption in this latter method is that the bridge surface 
>is, in fact, flat.  By spreading the feet as described in the 
>instructions, the gauge assumes a straight line where, whether by the 
>actions of wood crushing or by the original set up, there is not such.  To 
>test this, move the feet of the gauge as close together as possible.  Zero 
>the bubble in the rear-most graduations (closer to rear pins than front) 
>and, making sure it is balanced on the string, slide it from contact with 
>front pin to rear.  You will see bubble movement, sometimes a few 
>thousandths, but more often (for me) in the area of 12 graduations, which, 
>I think, with the feet at about 1/4" would translate to about .009".

I would consider any measurements taken with the feet of the gage that 
close together to be questionable at best.  It's too easy to rock the 
gage.  It's not stable and the measurements are not repeatable, in my 
opinion.  I would consider 12 graduations to be a very large change.

>  Picture a diagram where the sounding length and the back scale are 
> exactly the same plane.  The gauge, zeroed on the first reads exactly the 
> same on the latter.  But instead of a flat bridge, the top is 
> symmetrically arched.  Either measuring method (front to rear or 
> pin-to-pin component) would, in theory, show zero downbearing.  This 
> reading would be distorted by the string angle being "artificially" 
> influenced by the bridge pins.  Without the clamping influence of angled 
> pins, there would be measurable downbearing, but the string contact with 
> the bridge would be considerably behind the notch edge and the pin.

What's artificial about it?  It is being influenced by the bridge pins.  In 
the situation you describe, if you take out the pins, you would in fact have 
large downbearing because the string would be describing a large angle over 
the bridge, with a large vertical component of force on the bridge, and the 
string would lose contact with the cap surface before it got to the notch 
edge, as you say.  Put the pins back in and the string exits the bridge 
horizontally, so there is no downbearing.  There is, however, a large 
internal load in the bridge itself, with the string pressing down on the 
cap (trying to crush itself into the cap) and pulling up on the pins.  For 
the bridge as a whole these loads are self equilibrating, so there is no 
net downbearing.  There are however, large stresses set up within the 
bridge itself.

Four points determine the downbearing, or lack thereof, on the bridge.  The 
front string termination (capo, agraffe, or whatever), the effective string 
end (aliquot, plate ridge, vertical hitch pin, etc.), and the two points 
where the string exits the bridge.  What the string does across the face of 
the cap is irrelevant to net load on the bridge.  So, as far as I'm 
concerned, Lowell's instructions with regard to 'Component Bearing' are 
correct.  The feet should be spread so that they are resting on the points 
where the string exits the bridge on the front side and on the back 
side.  In practice this is often impossible to do, since the string is 
often exiting the bridge at the edge of the notch and you can't put the 
foot there because the bridge pin is in the way.  Putting it on the string 
just behind the pin is about the best you can do.


>  I don't think we have a clear enough picture of the range of 
> configurations that can occur at the bridge, nor is it clear what the 
> acoustic differences are as the string / bridge notch interface moves 
> from the idealized "point", through the lessening of force as the angle 
> moves towards zero, through the phase where the force at the pin is 
> actually (slightly) upward, and, through each of these  phases, the 
> effect of the string's contact with the notch either remaining intact or 
> becoming compromised.

Are you referring to the dynamic situation here?  Does this have something 
to do with cap crushing, the strings finding themselves off the bridge, or 
other?

Phil Ford



>All of these concepts and conjecture need to be tested in a way that can 
>be considered definitive.  You certainly have offered a starting point.
>
>David Skolnik
>