The magnetic and electric properties of the binary and ternary alloys involving iron, silicon and aluminium have been measured, and the relations of these properties to their respective concentrations have also been investigated. Those alloys containing 6_??_11 per cent of Si, 4_??_8 per cent of Al show specially excellent magnetic properties and their representative alloy has been shown to contain 9.62 per cent of Si, 5.38 per cent of Al. Some of its physical constants are as follows : the initial permeability, 35100; the maximum permeability, 117500; the magnetic hysteresis-loss, 28 egrs/cm³ per cycle: the coercive force, 0.022 oersteds; and the specific electric resistance, 81 microhms /cm³ at 20°.