434 Mr. H. H. Hall on the Rotation of the 
nected in the same way, a weaker current will flow from a’ 
to b’ through the wire. 
= g —> 6. 
a 
G — 
— eae 
4 ue Bf ae 
a Z 
as, o 
— Z 7 
Le! 
6 Ts 
— 4/ — 
Vig. 5 represents a metal strip, having along its middle a 
slit longer than the diameter of the magnetic poles, between 
which the slit is placed, 7 and / being corresponding equi- 
potential lines when the strip is in its normal unstressed 
condition. 
Let a magnetic force act as before. Hach of the lines J, / 
in fig. 5 is rotated about its centre through the same angle as 
{in figs. 3 and 4. The result is shown in fig. 6. 
If now a and b be connected by means of a wire a current 
will flow from a to 6 through the wire. If a and Jb’ be 
connected in the same way a current will flow from 0’ to a/ 
through the wire. 
To get Mr. Bidwell’s case, imagine a narrow bridge of metal 
to be left crossing the slit between a’ and U’/. Such a bridge 
would serve as a shunt to the wire which we have imagined 
connecting a’ and b’, but would not reverse the relative poten- 
tials of these two points. The lines U, /, as Sir Wm. Thomson 
pointed out at the Philadelphia meeting, would now take a 
form something like that shown in fig. 7. 
As the bridge across the slit becomes wider such lines as J 
will become more nearly straight, till, finally, the condition of 
things represented in fig. 4 will be practically reproduced. 
On the other hand, if the slit in fig. 5 were short compared 
with the width of the poles of the magnet, a reversal such as 
Mr. Bidwell detected should not occur. 
Mr. Tomlinson’s Suggestion. 
Tn the ‘ Philosophical Magazine’ for May 1884, Mr. Tom- 
linson stated that certain relations which he had discovered 
