The invar base‐line tapes used by the U.S. Coast and Geodetic Survey are tested at the Bureau of Standards. Since these tapes are used in the fundamental geodetic surveys of this country, including international boundary lines, it is essential that every precaution be used in the standardization.It is now possible to measure the change in the distance between the end graduation lines of such a tape when a small weight is placed on the tape, even if the weight is as small as an ordinary wire paper clip. This effect can be calculated mathematically from the weight of the paper clip and there is agreement between the observations and the calculations to about the one fifty‐millionth part of the total length of the t