A curve of constant curvature and zero torsion must be a circle

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From Elementary Differential Geometry by Pressley

I don't understand the last paragraph. Why does it show $\gamma$ lies on the sphere $\mathcal S$ with center $\mathbf a$ and radius $1/\kappa$?

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$$ \|\gamma - \mathbf{a}\| = {1\over\kappa}$$ says that the distance from $\gamma$ to $\mathbf{a}$ is the constant value $1\over \kappa$.

Now, what is the definition of a sphere $S$ of radius $1\over\kappa$ about $\mathbf a$?