Amplifing the output of a current transformer sensor
Your toroidal sensor is basically a coil that converts the magnetic field lines generated by the current being sensed, into a voltage signal. Note that the current has to be AC in order for you to be able to measure it using an inductive sensor such as this.

Faraday's Law:
$$|V|=N\frac{d\phi}{dt}$$
$$|V|: \text{magnitude of induced voltage}$$
$$N: \text{number of turns in receiving coil}$$
$$\phi: \text{magnetic flux}$$

According to the Faraday's equation, you can increase the induced voltage several ways:


Increase the number of turns in toroidal coil by using smaller diameter wire or increasing the number of layers of windings.
Increase the change in flux by increasing the cross-sectional area of your toroidal coil by using a longer or thicker core material. As long as it makes a single wind of the coil use up more wire, that is increasing the cross-sectional area.
Increase the change in flux by increasing the strength of the B field induced by the current in the conductor. You can do this by using a core material with a higher magnetic permeability (mu) value.