The thermal expansion coefficient, and rigidity modulus and its temperature coefficient were measured with ternary alloys of Cobalt, Iron and Molybdenum. The influence of an addition of Nickel on the above properties was also examined to ascertain whether the rule of the small expansibility of Invar proposed previously by one of the present authors is valid in the above alloy systems or not, and whether the effect of the addition of Nickel observed with alloys of Cobalt, Iron and Chromium and of Cobalt, Iron and Vanadium can be observed with the above alloys or not.It was found that the temperature coefficient of rigidity modulus shows maximum positive values at compositions in the neighborhood having the minimum thermal expansion coefficient in the ternary system, that is, at the composition having 55 percent Cobalt and 12.5 percent Molybdenum, and in the quaternary system with an addition of Nickel, the contents of Cobalt and Molybdenum at the composition showing the minimum thermal expansion coefficient and the maximum positive temperature coefficient of rigidity modulus shift to the side of lower Cobalt and lower Molybdenum contents with an increase of Nickel content. Thus, there are certain composition ranges in the systems of the ternary and, quaternary alloys in which the alloys show Elinvar type properties as expected, and they were named "Moelinvar". Some examples of "Moelinvara" are as follows, an alloy containing 50 percent Cobalt and 17.5 percent Molybdenum having a temperature coefficient of rigidity
