I beg to differ.  A standard open coil spring (where there is space 
between the coil turns) has a finite lenght with zero tension.  The 
tension increases with  stretch, but that tenssion starts wth that 
offset length.  On the other hand a spring such as some types of screen 
door closer springs which are wound such that that even from the start 
position there is  tension.  If a plot of the tension vs length is done, 
it is possible for the tension to go through zero at an imaginary zero 
length, hence the term "zero length spring.." 
On the other hand, a clock spring such as you describe may be the 
reverse -- a constant force spring -- wherein at any length, it is 
exhibiting constant force.  The only versions of these I'm aware of is a 
type of roll up spring where the spring is slightly crowned to give it a 
constant straightening force.  I believe is is also called a "negator" 
spring.  But I also believe that watch springs are in general just stand 
wind-up helical coils which really follow a small positive spring curve, 
i.e., increasing force for increasing wind-up and since it starts out 
too big and open, it fails the definition of a zero-lenght spring.
Regards,
Charles Patton

ChrisAtUpw@....... wrote:
> In a message dated 2006/11/30, tchannel@.............. writes:
>
>> I think its k that I am unclear of. 
>
>
> Hi Ted,
>
>        'k' is the force constant of the spring in say, pounds per inch 
> of stretch. This is constant until you get to very large extensions 
> which change the spiral.
>
>        An example of a zero length spring is an ordinary clock spring 
> when you pull it along it's shaft axis.
>
>        Regards,
>
>        Chris Chapman

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