Al 
of energy, or as an argument against perpetual motion. Lenz’s law is, of 
course, when the actions are properly understood, a consequence of New- 
ton’s: third law of motion. 
Discoveries similar to those of Faraday as to induced currents were 
made almost simultaneously by Henry in this country. We have in the 
discoveries of Faraday and Henry the fundamental information required 
for nearly the whole of our recent developments in dynamo-electric gen- 
erators and electric motors, but it was reserved for the next generation to 
develop them. This development we owe in no small degree to the splen- 
did exposition of Faraday’s discoveries and their consequences, contained 
in Maxwell's book on electricity and magnetism. 
Going back for a minute to 1822, we have to notice another important 
discovery; namely, the thermo-electric couple by Seebeck. There followed 
almost immediately the important experiments of Cumming, who showed 
that the thermo-electric order of the metals is not the same at all fempera- 
tures. The next important discovery in thermo-electricity was that by Pel- 
tier of the heat generated at the junction of two metals when a current is 
formed across it against the e. m. f. of the junction. In later years we 
have the classic researches of Thomson (Kelvin), who added thermo-elec- 
tric convection and the specific heat of electricity, and gaye the thermo- 
electric diagram method of representing results. This method was after- 
ward used and extended by Tait, who added a good deal to our knowl- 
edge of thermo-electric data. Among the large number of others who have 
worked in this field, we may mention Becquerel. Magnus, Matthieson, 
Leroux, and Avenarius. Thermo-electric batteries of considerable power 
have been made by Clamond and others. 
In 1827 the celebrated law giving the relation between e. m. f. resist- 
ance and current was published by Ohm in a paper on the mathematical 
theory of the Galvanic circuit. The theory has been sometimes criticised, 
but it seems to be absolutely certain that the law is almost exact, and it 
has proved to be of the greatest importance in the further development of 
the subject of electric measurements. The subject had, about the middle 
of the century, reached a stage in which it was possible to develop almost 
completely the mathematical theory as we now have it. Most of the work 
since Iaraday’s time has been largely directed toward quantitative meas- 
urements and the furnishing of exact data to answer questions as to how 
much in various cases. F. E. Neumann discovered what he called the po- 
tential function (now called the coefficient of self and mutual induction) 
