a Oe 
TRANSACTIONS OF THE SECTIONS. 21 
Table of Experiments upon the Magnetic Forces of Rods of different lengths. 
Mean value of one coil, | Mean value of one coil, 
Length of | Number | if the helix occupies | if the helix occupies 
the rods. | of coils. | the whole length. | only the extremities. 
Sy 946 7°334 7°560 
Q"5 789 6°993 7°264 
2 634 7°402 6°871 
1°5 474 7°880 7°491 
1 315 7°847 7°573 
0°5 163 7°766 7°691 
7537 7°408 
From these numbers it will be seen that the influence of one coil 
of the helix is nearly the same for all the rods, and that their length 
does not exercise any specific influence. It is only in proportion to 
the number of the turns or revolutions, and to the force of the current, 
that the rods can acquire a greater or less amount of magnetism. The 
small rods even appear to have a slight advantage over large rods, 
since it has been found by experiments that the actual force of rods of 
three feet bears to that of rods of half a foot the ratio of seventy-three 
to seventy-seven. It is also found that there is a gain of seventy-five 
to seventy-four when the whole length of the rods is covered, instead 
of simply collecting the same number of coils around the extremities. 
The differences between the observations and the simple laws are, as 
will be judged, quite inconsiderable for practical purposes, and will, in 
time, I hope, entirely disappear by a complete integration embracing 
the whole length of the rods, and founded upon the effect of an ele- 
mentary part of the current. I will now hasten on to the immediate 
object of my present address. In March 1839, M. Lenz and I pre- 
sented to the Academy of Sciences at St. Petersburgh, a report, which 
I shall present to the Association. It contains the result of the expe- 
riments by which we have been enabled to establish the remarkable 
law, that the attraction of the electro-magnets is proportional to the square 
of the force of the galvanic current, to the infiuence of which the rods of 
tron are submitted. This law is of the highest practical importance, 
as it serves for the basis of the whole theory of electro-magnetic ma- 
chines. 
Before proceeding, I may be permitted to make some remarks 
concerning an instrument which I laid before the Academy of Sciences, 
in the commencement of this year. It is destined to regulate the gal- 
vanie current, and is of value in many investigations of this kind. 
During my sojourn in London, Prof. Wheatstone has shown me an in- 
strument, founded on exactly the same principles as mine, and with very 
inconsiderable modifications and differences. Now, it is quite impossible 
that he should have had the least notice of my instrument; but as it 
is probable that its use may be greatly extended, I must add, that 
while I have only used this instrument for regulating the force of the 
