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## Linearity correction

We will first take a look at the linearity correction. The deflection coil is not ideal, but has a certain (wire) resistance. The voltage across this resistance rises as the current in it increases. So the voltage across the actual coil is less than it should be. The current in the coil will not increase as fast as it should. To prevent this, we need a negative resistance that compensates for the DC resistance of the deflection coil. Unfortunately, we cannot buy a negative resistor in a shop. But we can create one using an inductor with a ferrite core (Lsat). As the curent in this inductor increases, its core will become more and more magnetic, until its limit is reached. Because a (small) change in current does not affect the magnetic field anymore, the impedance has become zero. Of course it doesn't matter in which direction the current is flowing. By pre-megnetising the core, we'll end up with an inductor which inductance increases as the current becomes more negative and increases when the current becomes more positive. The voltage across Lsat equals VLsat=Lsat∙dI/dt. Because the inductance of Lsat is far less than the inductance of the deflection coil, dI/dt will remain (almost) the same. However, Lsat decreases when the current rises. This means that VLsat decreases when the current increases. So Lsat behaves like a negative resistance. It's obvious that when Lsat becomes defective, it must be replaced with an original part from the manufacturer.

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