18 REPORT—1840. 
On the Principles of Electro-Magnetical Machines. 
By Prof. Jacos1, of St. Petersburgh. 
I have the honour to present to the British Association an historical 
sketch of the laws which regulate the action of electro-magnetic 
machines, laws which will enable us to determine in a precise manner 
the important question of the application of this remarkable force as a 
moving power. Since the commencement of my labours, which had partly 
a purely practical tendency, I proposed to myself to fill up as much as 
possible the blank which still remained in our knowledge of electro- 
magnetism. With the assistance of M. Lenz, I prosecuted the labours, 
which were the more arduous as they had but few precedents in the 
direction which I considered it necessary to follow, and we began to 
examine carefully the laws of electro-magnets. The report, which 
contains the results of our researches, was read in June 1838, before 
the Academy of Sciences at St. Petersburgh. I take the liberty of 
repeating here, very briefly, the contents of this first report. The 
problem which we sought to determine may be stated as follows: If a 
nucleus of malleable iron and a voltaic battery of a certain surface is 
given, into what number of elements should this surface be divided ? 
what should be the thickness of the wire of’ the helix which surrounds 
the nucleus ? and, lastly, what number of turns should this helix have, 
in order to produce the greatest amount of magnetism? I will not 
dilate here upon the manner in which we have proceeded, or upon the 
degree of certainty which belongs to the laws established according to 
our observations. I take the liberty of appending to this statement the 
report in question, and will proceed to explain the particular laws: Ist. 
The amount of magnetism engendered in malleable iron by galvanic 
currents is in proportion to the force of those currents. 2ndly. The 
thickness of the wire twisted into a helix, and surrounding a rod of 
iron, is absolutely of no consequence, provided that the helix have the 
same number of turns, and the current be of the same force. This law 
extends also to the case in which ribbons of copper were employed 
instead of wire. Nevertheless I must notice, that in order to obtain a 
current of equal force, it is necessary to employ a voltaic apparatus of 
greater force, if small wires which offer a greater resistance are employed. 
3dly. If the current remain the same, the influence which the diameter 
of the helix exercises may be neglected in the majority of practical 
cases. 4thly. The total action of the electro-magnetic helix upon the 
rod of iron, is equal to the sum of the effects produced by each coil 
separately. Adopting these laws,and submitting them to calculation 
according to the formula of M. Ohm, the importance of which formula 
has but lately begun to be appreciated by some British philosophers, 
we have established the formula which contains all the particular con- 
ditions required to obtain the maximum amount of magnetism, which 
may be expressed in the following extremely simple manner, viz. the 
maximum of magnetism is always obtained when the total resistance of 
the conducting wire, which forms the helix, is equal to the total resistance 
of the pile. On referring to the remarkable law of the definite action 
