on a Permanent Electric Current. 165 
very slight effect of the kind looked for would not have been 
detected, though it is probable that if a reversal of the magnet 
had caused a change of four mm. in the position of the spot of 
light, this effect would have been apparent. 
e may therefore conclude that any change of relative po- 
tential on the quadrants of the electrometer caused by reversal 
of the magnet was probably less than 4 of that caused by 
reversing the connections of the electrometer with a Bunsen 
cell, as mentioned above. If now we estimate the difference 
of potential between the plugs A and B, connected with the 
eyden jars, to have been, as indicated by the length of the 
spark, equal to that which would be produced by 10,000 Bun- 
sen cells in series, we may conclude that any difference of 
potential between the other plugs C and D which was caused 
by the action of the magnet, must have been less than go4og5 
of the difference of potential between A and B. We must 
remember, however, that any change of potential on C and D 
had to be extended as well over the comparatively large area 
of the electrometer quadrants. Professor Rowland has roughly 
estimated the capacity of the quadrants as twenty times that of 
the plugs C ae D. If, therefore, these plugs had not been 
attached to the electrometer, any difference of potential between 
them due to the action of the magnet would have been twenty 
times as great as in the actual case, so that instead of 
we have of the difference of potential of A and B as the 
superior limit of the difference of potential of C and D which 
the magnet might possibly have produced, if C and D had not 
been connected with the electrometer. Representing the for- 
mer difference of potential by E, the latter by E’, and the 
strength of the magnetic field, about 4000 (cm.-grm.-sec.), by 
M, we have for this case of static induction in glass Ee’ if 
not zero, is less than +g5g400007" 
Turning to the analogous case of current electricity in the 
various metals and representing now by E the difference of 
potential of two points a centimeter apart in the direction of 
the current, and by EK’ the difference of potential of two points 
a centimeter apart in a direction at right angles to that of 
the current, while M has the same signification as before, 
ps may write, as a very rough estimate for the case of iron, 
ExM ~tovb00m While for tin the value of this ratio may be 
as small as popoboane: We may therefore conclude that the 
equipotential lines in the case of static induction in glass, if 
affected at_all by the magnet, are affected much less than the 
equipotential lines in the case of a current in iron, but we can 
not say that any such possible action in glass has been shown 
