An electric field resistor includes N coils. N coils encircle a common center in sequence, and each of the coils has a first terminal and a second terminal, wherein the first terminal of the first coil receives a first reference voltage, the second terminal of the Nth coil receives a second reference voltage. The second terminal of the ith coil is coupled to the first terminal of the (i+1)th coil, wherein N is a positive integer greater than 1 and 1≦i