Hi,
First it is a very classical question in DC ELECTRICS. Voltage regulator of a DC generator.
The fundamental of the system is in the state of the surface of a washer of carbon: made of compressed powder, quite soft, irregular, contact depending upon the crushing of the grains. With no current, a spring maintains the stack of carbon washers compressed, as a consequence, the contact between the washers is quite good and the total resistance of the stack is low.
When the current is established, an electromagnet (a coil) is working against the spring and tends to extends the stack, so doing, the contact of the carbon washers become worse, and the resistance of the carbon pile increases.
Best regards
Malaysian eaglet
Malaysian Flying Academy