THE WIREWOUND RESISTOR The first commercial resistors made were formed by wrapping a resistive wire around a non-conducting rod (see Figure 3). The rod was usually made of some form of ceramic that had the desired heat properties since the wires could become quite hot during use. End caps with leads attached were then placed over the ends of the rod making contact to the resistive wire, usually a nickel chromium alloy. The value of wirewound resistors remain fairly flat with increasing temperature, but change greatly with frequency It is also difficult to precisely control the value of the resistor during construction so they must be measured and sorted after they are built.