Question

what does a spring balance measure? how would you demonstrate the principal on which it works?

Answer 1

Hii....
A spring balance is used to measure the weight of the body with in the elastic limits of its spring.
 
Spring balance works on the principle of Hook’s law.
Hooke's law to say that the strain (deformation) of an elastic object or material is proportional to the stress applied to it.

Answer 2

hlo may be this answer is helpful for you...

spring balance measure the weight of body take a spring and long it on nail and note the length, now long is small stone from the other and of the spring and the increase in its length of spring again a spring balance works on same principal