44 
THE CANTOR LECTURE, 1890 . 
Electrical engineering, the latest and most vigorous offshoot of 
applied science, embraces many branches. The dynamo for generating 
electric currents, the motor for transforming their energy back into 
work, the arc lamp, the electric bell, the telephone, the recent electro¬ 
magnetic machinery for coal mining, for the separation of ore, and many 
other electro-mechanical contrivances, come within the purview of the 
electrical engineer. 
In every one of these, and in many more of the useful applications 
of electricity, the central organ is the electro-magnet. By means of 
this simple and familiar contrivance—an iron core surrounded by a 
copper wire coil—mechanical actions are produced at will, at a distance 
under control, by the agency of electric currents. These mechanical 
actions are known to vary with the mass, form, and quality of the iron 
core, the quantity and disposition of the copper wire wound upon it, 
the quantity of the electric current circulating around it, the form, 
quality, and distance of the iron armature upon which it acts. But the 
laws which govern the mechanical action in relation to these various 
matters are. by no means well known, and, indeed, several of them have 
long been a matter of dispute. Gradually, however, that which has 
been vague and indeterminate becomes clear and precise. The laws of 
the steady circulation of electric currents, at one time altogether 
obscure, were cleared up by the discovery of the famous law of Ohm. 1 
Their extension to the case of rapidly interrupted currents, 2 such as 
are used in telegraphic working, was discovered by Helmholtz; whilst 
to Maxwell is due their further extension to alternating, or, as they 
are sometimes called, undulatory currents. All this was purely electric 
work. But the law of the electro-magnet was still undiscovered; the 
magnetic part of the problem was still buried in obscurity. 
Gradually, however, new light dawned. It became customary, in 
spite of the mathematicians, to regard the magnetism of a magnet as 
something that traverses or circulates around a definite path, flowing 
more freely through such substances as iron than through other 
relatively non-magnetic materials. Analogies between the flow of 
electricity in an electrically-conducting circuit, and the passage of 
magnetic lines of force through circuits possessing magnetic con¬ 
ductivity, forced themselves upon the minds of experimenters, and 
compelled a mode of thought quite other than the previously accepted. 
So .far back as 1821, Gumming experimented on magnetic conductivity. 
The idea of a magnetic circuit was more or less familiar to Ritchie, 
Sturgeon, Dove, Dub, and De La Rive, the last-named of whom 
explicitly uses the phrase, a closed magnetic circuit.” Joule found the 
maximum power of an electro-magnet to be proportional to ff the least 
sectional area of the entire magnetic circuit,” and he considered the 
resistance to induction as proportional to the length of the magnetic 
circuit. Indeed, there are to be found scattered in Joule's writings on 
the subject of magnetism some five or six sentences, which, if collected 
1 Namely; that the current through a given conductor is proportional to the force that drives 
it, or that the opposition force exerted by a conductor upon a current is simply proportional to 
the strength of that current: or, quite briefly, a current through a given conductor is proportional 
to the JE.M.F. which drives it. 
2 This expression must not be taken as implying that Ohm’s law (which is true only of steady 
currents) is applicable to currents that are not steady. . ' 
