60 BRIDGMAN. 



cium has been determined, ^^ and this gives very strong probabihty to 

 the view that its mechanism is also of the hthium type. It has been 

 found that in metalhc calcium the atoms of Ca occupy almost exactly 

 the same positions as the Ca atoms in Ca F2, the F atoms have merely 

 dropped out of the structure. The inference is strongly suggested 

 that the F atoms have been replaced by electrons, which do not give 

 an X-ray photograph because of their small mass, and that therefore 

 metallic calcium consists of interpenetrating lattices of atoms and 

 electrons. 



Let us now consider what these pictures of the mechanism lead us 

 to expect for the tension coefficient. It is in the first place evident 

 that we would expect the resistance of normal metals to increase in 

 the direction of stretch, both because the distance between the atoms 

 increases in this direction and because the amplitude of vibration in 

 this direction increases to compensate for the decrease in period due 

 to the weakening of the restoring force on the atoms due to their 

 increased distance apart. A detailed working out of the theory must 

 recognize in addition that changes in the positions of the atoms trans- 

 versely may affect the period longitudinally. Now of course it is a 

 fact that the resistance of normal metals increases in the direction of a 

 tension. The same reasoning would lead us to expect a decrease of 

 resistance in a direction transverse to the tension, and this also agrees 

 with the facts in the few cases known. 



Consider now bismuth. As tension is applied, the distance between 

 the atoms increases longitudinally. The same abnormality in the 

 force that compels an increase of amplitude when pressure is applied 

 now compels a decrease of amplitude, and just as in the case of pressure 

 the increase of amplitude causes a greater increase of resistance than 

 can be overbalanced by the decrease due to the approach of the atoms, 

 so now the decrease of resistance due to decreasing amplitude more 

 than overbalances the increase due to the increasing separation of the 

 atoms. The outstanding effect will be a decrease of resistance, which 

 is actually the case. 



It is otherwise, however, for metals of the lithium type. The 

 resistance is here determined by the channels between the atoms. 

 When a tension is applied, the channels are made narrower, because of 

 the lateral contraction, just as they are made narrower when a hydro- 

 static pressure is applied, and we should expect an increase of resist- 

 ance. This is actually the case for lithium. 



The fact that the resistance of calcium increases under tension 



18 A. W. Hull, Phys. Rev. 17, 42-44, 1921. 



