THE SMALL EFFECT OF TENSION. 19 



metal was increased 95 per cent above that of the billet ; " and Burr says : 23 

 " Wire is the strongest form in which iron can be used to resist tensile 

 stress." 



It is evident, then, that by pulling a wire of small cross-sectional area it is 

 possible to exert the maximum of stress on the surface of a test-piece. 



The apparatus (shown in figure 4, page 10) employed in our experiments 

 was exceedingly simple. The upper end of the wire was twisted around an 

 iron support and the pull was exerted by weights in a basket attached to the 

 lower end. In the middle the wire was narrowed at one place, and this was 

 surrounded by a tube of normal ferrous sulphate and connected with a nor- 

 mal calomel electrode. The results were at first highly irregular. The 

 electromotive force of the wires used (pure piano wire and pure soft iron 

 wire) varied over a range of 0.005 v lt without apparent cause. Merely 

 tapping the stretched wire sometimes caused a momentary rise of from 0.003 

 to 0.004 v lt. Such electrical disturbances from mechanical jarring have 

 been observed by von Helmholtz. In one case the addition of 5 kilograms 

 to the load caused a net rise of about 0.0007 v0 ^ an d the next 5 brought it 

 down again by the same amount, while one more brought it up again by 

 0.0005. 



Calculating the theoretical change of potential, we get 0.0017 volt for the 

 total work done, and half of this, or 0.0009 volt, as the change to be expected 

 according to Barus. The data for this calculation are: Tensile strength, 

 25 kilograms ; elongation, 9.3 per cent ; diameter of wire, 0.08 cm. 



Hence the accidental disturbances exceeded the maximum theoretical effect 

 and entirely masked the latter. 



Only one regularity was noticeable, namely, that at the instant at which 

 the load was increased there was a momentary drop in electromotive force. 

 Starting with no load at all and the potentiometer reading 0.7145 volt, 24 a 

 10 kg. load was applied, and at the same instant the galvanometer swung 

 18 to 20 divisions to the right, and then instantly swung back again very 

 rapidly to the permanent reading, 0.7125 volt. Exactly how much fall of 

 potential this swing to the right indicates could be determined only with a 

 ballistic galvanometer, but it may have been as much as 0.015 volt. The 

 addition of another kilogram had a similar effect, except that the swing was 

 only 12 to 15 divisions to the right and it returned more slowly to 0.7125. 

 The third 10 kg. caused a smaller swing still, and a much slower return to 

 0.7125. About that value was always reached after standing a few minutes. 



23 Elasticity and resistance of the materials of engineering, p. 241. 



"This reading, being taken with the help of a normal calomel electrode, must have 

 0.052 volt added to it in order to be comparable with the other potentials recorded in 

 the paper. It thus becomes 0766, the same as before. The normal electrode has less 

 resistance and allows more sensitive readings than the decinormal. 



