198 REPORTS ON THE STATE OF SCIENCE.—1913. 
same material, and it is probable that many unpublished tests would show 
even larger variations. 
This variation in the apparent endurance strength of test pieces cut 
from the same bar may be due to an actual variation in the material, to 
local weak spots in the structure of the bar, or to differences in the amount 
of surface damage in machining or grinding the test pieces, or they may be 
due to the test piece really being treated differently in testing, such as one 
piece being run more out of truth than another. 
While there is no doubt that in the case of some materials there may 
really be a difference in different parts of the same bar, yet there is some 
evidence that the apparent variation in endurance strength is not always 
due to this alone. In the alternating shock or Repeated Impact Tests of 
Dr. Stanton * different test pieces from the same bar of steel gave prac- 
tically identical endurance results. If the variation in endurance of 
different test pieces from the same bar when tested in an alternating 
stress-testing machine is due to variation in the material, it is remarkable 
that alternating shock tests should not be affected in the same way, for the 
impact tests referred to are really alternating stress tests with suddenly 
applied stresses, which are not calculated in tons per square inch. 
Alternating stress in practice is often accompanied with repeated shocks 
or occasional shocks or vibrations. In a testing machine such shocks or 
vibrations are as far as possible eliminated, but it may be that these or the 
stresses caused by these affect the endurance. 
Suggestions for Experimental Work.—There seems to be room for a 
great deal of purely experimental work on alternating stress and endur- 
ance ; to help to clear up some of the immediate difficulties of the subject I 
would suggest :— 
(1) An elaborate series of alternating stress endurance tests all with the 
same material on all the alternating stress-testing machines in use, at the 
very least twenty test pieces to be tested in each machine, and special 
precautions to be taken to ensure uniformity in the material. 
APPENDIX I. 
Brief Review of the Papers in the Bibliography on Internal Friction, 
Hysteresis, Effects of Magnetism, Temperature, and Oscillatory Discharge. 
By Dr. F. Rocers. 
The estimate (in 45) of the area of the hysteresis loop in ‘ static’ tests, 
as being, say, 10 to 20 per cent. greater than in alternating tests at a speed 
of average frequency of 136 alternations per second, is of much interest, and 
it would be of value if direct confirmation could be otherwise obtained. 
It is of great importance to inquire whether this hysteresis at stress well 
below the elastic hmit, determined statically by a good extensometer, is 
(a) simply a matter of local permanent set beyond the elastic limit owing to 
microscopically visible or other want of homogeneity ; or (b) strain of 
some sort which is as homogeneously distributed as purely elastic strain is 
commonly supposed to be, and therefore possibly dependent upon inter- 
molecular distances, or even upon orientations of molecules or of relatively 
small groups of molecules. Since magnetisation of a bar is accompanied 
by a change of length, and since straining of a bar assists the bar to take 
* Proc. Inst. Mech. Engs., No. 4, 1908. 
