Metals used as resistive materials should have the characteristics of high electrical conductivity, sufficient mechanical strength, not easy to oxidize, not easy to corrode, easy to process and weld. The resistance material has a great influence on the resistance performance. Generally the resistance materials used in the high-end precision resistor are all high precision type. This kind of precision alloy material can make the performance of precision resistance more superior, the precision higher and temperature drift smaller.


Performance requirements of resistive materials
Copper, aluminum and its alloys are the most commonly used conductive metals, and other metals, such as precious metals, are used in some special occasions. Copper is the first widely used resistive material with good electrical conductivity and mechanical properties.
The conductivity of aluminum is about 62% of that of copper and the density is 33% of that of copper. The specific strength of aluminum is about 30% higher than that of copper. At the same time, the aluminum cross-sectional area is 168% of the copper cross-sectional area, while the aluminum weight is only 54% of the copper.
 
The development of modern science and technology requires that some electrical resistance materials not only have good electrical conductivity, but also have high strength, hardness and heat resistance, corrosion resistance, wear resistance and other comprehensive characteristics.
As a result, conductive copper alloys, aluminum alloys and composite metal conductors for different purposes have developed rapidly. The electric conductivity of conductive alloy is lower than that of pure metal, but the strength and heat resistance of the alloy are improved significantly.
 
A composite conductor made of two or more different metals by a certain composite process has many unique advantages, such as high strength, high hardness, high elasticity, good wear resistance, heat resistance, corrosion resistance, thermal conductivity, special magnetic and thermal expansion coefficient, etc.



Resistance of conductive metals
 
Resistance is an important index to measure the conductivity of metal and the main parameter of selecting conductive metal. The resistance of metal is called resistivity. In alternating current, alternating magnetic field is produced in the conductor, and the current is not uniformly distributed on the whole section of the conductor. The closer to the surface, the greater the current density, which is the skin effect. In order to reduce the influence of skin effect, the diameter of single wire and the structure of strands or hollow lines can be reduced. When the frequency is high, the surface can be coated with high conductive silver and other materials.

 

Main factors affecting resistance
 
The resistance of the metal rise with the increase of temperature. In general, that resistance is in a certain proportion to the temperature. When the temperature reach a certain range, the resistance performance begins to decrease, and as the temperature rise, resistance is likely to be broken down or burned down.
 
Resistance increase caused by cold deformation of pure metal. It is generally not more than 4%, For alloy, it is slightly higher.
 
Cold deformation of metal increases strength and hardness, decreases electrical conductivity and plasticity. Annealing can restore its performance. The mechanical properties of metals become worse when annealing temperature is too high or heating time is too long, but the change of resistance is not significant.