204 A. M. Mayer—Researches in Electro-Magnetism. 
On the a. condition of the interior surface of a tube.— 
Referring to fig. will be seen that the Ampérean currents on 
was therefore an extremely interesting point for ce eer" 
it has been the subject of over two hundre experiments, 
te is esd far the most difficult research mentioned in this 
the effect of the inductive action which ‘cok place when the 
core was withdrawn from the tube changed the magnetic rela- 
tions which existed while the core was in the tube. 
The following method was then devised. Two tubes of very 
soft iron were constructed, having both an internal diameter of 
“84 inch, and one an external diam. of 1-03 inch, the other of 
131 inch. aoe tubes were 9 in. long. One hundred and 
twenty well annealed iron-wires a little less than 9 in. long 
were tightly see ene by silken cord and then wrapped in 
six thicknesses of writing paper, so that the wire core fitted 
ay the interior of the tubes. 
The w ire core was now deprived as far as ue of magnet- 
‘ons assing a weak magnet over it, and could thus be so 
reduced that it caused a deflection of only 1° when placed 7°5 
in. W. of the center of a surveyor’s compass needle. By now 
