INDUCTION OF MAGNETISM BY ELECTRICAL CURRENTS. 
295 
Postscript. 
Since the above was written, I have made the subjoined experiments on the eiec- 
tro-magnetic attraction of contact. A cylinder of wrought iron, 9 inches long and 
4 inches in diameter, had a hole 3 inches in diameter bored along its axis. The thick- 
ness of the metal of the hollow cylinder thus formed was exactly half an inch. This 
was cut longitudinally into two exactly equal pieces, the surfaces of which were then 
carefully finished. Each of these semicylinders was wound with 25 feet of covered 
copper wire -j^th of an inch in diameter, and making fifteen complete convolutions. 
One of the semlcylindrical electro-magnets thus formed was firmly secured with its 
flat surfaces upwards ; and the other, with its surfaces downwards, was suspended 
to the beam of a balance sensitive to 2 or 3 grains when several pounds were in 
each scale. A cup containing mercury was affixed to one of the terminals of the wire 
of the subelectro-magnet, into which a terminal of the wire of the suspended magnet 
dipped. And similarly, a mercury cup attached to the other terminal of the sus- 
pended electro-magnet was dipped into by a wire in connexion with the voltaic bat- 
tery, so as to counteract any effect on the balance which might be produced by the 
other mercury cup. Each semicylindrical electro-magnet was thus acted upon by 
the same current of electricity, and the resulting attractions are tabulated below, 
each recorded number being the mean of four experimental determinations, two with 
the current in one direction, and the other two with it in the reverse direction. 
Table VI. 
Attraction in contact of two semicylindrical electro-magnets. 
Intensity 
of 
current. 
Attraction 
in lbs. 
avoirdupois. 
Attraction 
of 
magnetic set. 
Attraction divided 
by the 4th power of 
the current. 
Attraction of magnetic 
set divided by the 4th 
power of the current. 
•0410 
•0.365 
•0045 
12917 
1592 
•0690 
•242 
•0185 
10676 
816 
•1013 
1-203 
•0835 
11424 
793 
•1388 
5-595 
•3280 
15074 
883 
•2074 
17-937 
2-5095 
9694 
1356 
•2364 
32-812 
4-9685 
10506 
1590 
•3682 
not observed 
17-5 
952 
•7013 
not observed 
40-25 
166 
1 
2 
3 
4 
5 
The numbers in column 5 show that the magnetic set obtained by a weak current 
is nearly proportional to the square of its intensity. On inspecting column 4, it will 
also appear that the magnetism existing during the flow of the current follows the 
same law so nearly, that we may infer that it possesses the character of the magnetic 
set. Experiments that I have recently made on the attraction of ordinary electro- 
magnets for their armatures lead to the same conclusions. 
December 21, 1855. 
