In fact, this is a very small value, which can be easily achieved in general coating matching without surface roughness. The purpose of painting is to harmonize the iron mesh with the nature; it is usually painted green; the wire mesh in the cable TV line is the shielding mesh.
The function of the shielding network is to shield the stray radio signals in the external environment. TV signal is easy to be interfered by some of its frequencies, which leads to the increase of picture noise and even poor synchronization. Some high-quality wires, in addition to the screen, are also wrapped with a layer of gold-plated film, which is also used for shielding.
The overhead cable is connected to the power supply network of the electrified railway, which is divided into two types: flexible and rigid. The electric locomotive or EMU is connected to the contact network through the frame type pantograph, from which the power is taken. The early electrified railway used DC power supply with relatively low voltage. The motor of locomotive or EMU is directly connected to the main line of power grid, and controlled by the resistance and relay in parallel or series on the motor.
Generally, the voltage of trams and subways is 600 V and 750 V, while that of Railways is 1500 V and 3000 v. Vehicles used to use rotary converters to convert alternating current to direct current. Semiconductor rectifiers are generally used to do this work. Some electrified locomotives use transformers and rectifiers to provide low-voltage pulsating direct current for motors, and adjust transformers to control motor speed. Others use thyristors or field-effect transistors to generate abrupt alternating current or variable-frequency alternating current to supply the locomotive's alternating current motors.
Single phase 25 kV 60 Hz power supply is used, while single phase 12.5 kV and 25 kV 60 Hz AC are usually used in the United States. In addition, 20 kV 50 Hz AC is used in Northeast and Hokkaido areas of Japan, and 20 kV 60 Hz AC is used in Beilu and Kyushu areas.