The influence of an addition of chromium on the rigidity modulus and its temperature coefficient of the alloys of iron, cobalt and nickel has been determined. It has been found that, at the section of 10 per cent of chromium,there are two maximum portions of the positive value of the temperature coefficient of rigidity modulus, one of which originates in the composition of elinvar and the other from the composition of co-elinvar. These maximum portions coincide at the composition of about 14 per cent of nickel, about 35 per cent of cobalt, 10 per cent of chromium and rest of iron; showing that co-elinvar is closely connected with elinvar and the nickel and the cobalt contained in those alloys can be substituted almost linearly for each other.