How does the voltage regulator adjust the pressure?

How does the voltage regulator adjust the pressure?

调压器的构造与自耦变压器相同，只是将铁芯作成环形线圈就绕在环形铁芯上。次级线圈抽头用一个可以滑动的电刷触头，使触头沿线圈表面环形滑动，达到平滑的调节电压作用。

The structure of the voltage regulator is the same as that of the autotransformer, except that the iron core is made into a ring coil and wound on the ring core. A sliding brush contact is used for the tap of the secondary coil to make the contact ring-shaped along the coil surface to achieve smooth voltage regulation.