motive variation is increased from (0·1—0·25 volt) to (0·1—0·81 volt). They will give a good general idea of the phenomena (fig. 55). A series of readings for cyclic variations taken with a single point receiver will be given later.
First, I tried the effect of cyclic variation through a small range, from 0·1 to 0·25 volt. It will be seen that the forward and return curves do not coincide, but enclose a small area. The receiver, in as far as conductivity is a criterion of its physical state, does not regain its original condition. There remains a residual conductivity variation, just as in iron there is a residual magnetism after it has been subjected to a cyclic variation of magnetising force. The residual magnetism disappears on tapping, just as the effect of residual conductivity can be dissipated by the same means.
In curve B, where the range of electromotive variation is still larger, from 0·1 to 0·43 there is a greater divergence between the forward and return curves, and the area enclosed is further increased. An interesting effect, due to lag, will be noticed at its far end; though the platform was on its return course, producing a diminution of E. M. F., the current nevertheless continued for a short time to rise. It is thus seen that whatever be the change to which the conductivity variation is due, it lags behind the impressed electromotive variation.
The curves C, D and E, further show that by increasing the range of electromotive variation, the area enclosed between the forward and return curves becomes considerably increased.
I give below detailed readings taken on two occasions with a single point iron receiver, (1) when the range of
