Hooke's law doesn't work well with my spring?

I'm trying to make an application for Hooke's law using a spring, but the law doesn't give any correct result with my spring, because when I hang a $100\,\mathrm{g}$ object on the spring it's elongates about $0.3\,\mathrm{cm}$ and when I hang a $200\,\mathrm{g}$ object the spring elongates about $1\,\mathrm{cm}$ while it should elongates only $0.6\,\mathrm{cm}$ .

I'm sure that the problem is with the spring design.

The spring I'm using is just like in picture below:

• do you have any other data points or just the two?
– Jim
Jul 25, 2013 at 15:50