Balco is Carpenter's brand name for this alloy consisting of 70% nickel and 30% iron. It is also sold by Harrison as "Hytemco". Its principal use is for RTDs (resistance temperature detectors). An RTD changes its electrical resistance as a predictable function of temperature, making it useful as a thermometer with electrical output. Balco has a temperature coefficient of resistance of .00518, nearly as high as that of pure nickel (.00672) but with much better linearity. It is also mechanically strong and fairly corrosion resistant.
Pure platinum is preferred for the most precise RTD elements in spite of its price and lower temperature coefficient (.00385). This is because of its long-term stability and repeatability.