620 Mr. A. I. Steven on Anomalous 



seem to be what give rise to the so-called anomalous effects, 

 for even i£ the two opposite stresses immediately follow one 

 another, the supposition that the processes of bending and 

 unbending should exactly neutralise each other could 

 scarcely be justified. If they did, then there would require 

 to be some relation between the position of the neutral 

 axis, and the yield-points for extension and compression ; for 

 instance, if the neutral axis were in the centre (as does not 

 seem to be the case when there is a large overstrain) then 

 the yield-points for compression and extension would require 

 to lie symmetrically about the point of zero load, and if a 

 change took place in the position of either, then the alteration 

 of one would require an equal change in the other. 



The most suitable material for an investigation of these 

 effects experimentally is some kind of soft metal, and in the 

 following experiments a soft iron wire was used. It had 

 been intended for electrical purposes, and after the insulation 

 had been removed with sandpaper, had an average diameter 

 of -0364 cm. 



The apparatus used was of quite a simple nature. The- 

 wire hung horizontally from a rigid support at one end of a 

 table and from a spring balance at the other end. The spring 

 balance was attached to a spiral screw working horizontally 

 in a fixed nut, so that the tension might be varied continuously. 

 The index of the balance was sharp, and by observing its 

 image in the well burnished scale and thus avoiding parallax, 

 the increase in load could be measured accurately to at least 

 1 per cent. The extension of a length of 150 cm. was 

 measured by means of two microscopes : — the one which 

 measured the extension nearer the balance travelled hori- 

 zontally and was actuated by a micrometer screw graduated 

 to *005 mm.; while the other, which observed any motion of 

 the fixed support, was provided with an eyepiece scale, and 

 was carefully calibrated beforehand. The best index-marks 

 were found to be two small pendulums of about 5 cm. length 

 consisting of a single silk fibre with a little bob of .-oft wax r 

 the bob being allowed to hang in a small vessel of water 

 to damp their oscillations. The fibres were easily attached 

 to the wire at their proper places by a thin film of melted 

 wax, and when viewed through the microscope against a 

 dark background were immensely superior to the scratch on 

 the wire advocated by many writers of text-books on practical 

 Physics. A certain amount of initial tension (corresponding 

 to the ^ lb. mark on the spring balance) was required to 

 keep the wire straight, and this tension is the zero of tension 

 on the diagrams illustrative of this paper. 



