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IV. On some Applications of Dynamical Principles to Physical Phenomena. 
By J. J. Thomson, M.A., F.P.S., Fellow of Trinity College, Cambridge, and 
Cavendish Professor of Experimental Physics. 
Received December 16, 1884,—Read January 8, 1885. 
§ 1. The tendency to apply dynamical principles and methods to explain physical 
phenomena has steadily increased ever since the discovery of the principle of the 
Conservation of Energy. This discovery called attention to the ready conversion of 
the energy of visible motion into such apparently dissimilar things as heat and electric 
currents, and led almost irresistibly to the conclusion that these too are forms of 
kinetic energy, though the moving bodies must be infinitesimally small in comparison 
with the bodies which form the moving pieces of any of the structures or machines 
with which we are acquainted. As soon as this conception of heat and electricity was 
reached mathematicians began to apply to them the dynamical method of the Con¬ 
servation of Energy, and many physical phenomena were shown to be related to each 
other, and others predicted by the use of this principle; thus, to take an example, the 
induction of electric currents by a moving magnet was shown by von Helmholtz to 
be a necessary consequence of the fact that an electric current produces a magnetic 
field. Of late years things have been carried still further; thus Sir William 
Thomson in many of his later papers, and especially in his address to the British 
Association at Montreal on “ Steps towards a Kinetic Theory of Matter,” has devoted 
a good deal of attention to the description of machines capable of producing effects 
analogous to some physical phenomenon, such, for example, as the rotation of the 
plane of polarisation of light by quartz and other crystals. For these reasons the view 
(wjfich we owe to the principle of the Conservation of Energy) that every physical 
phenomenon admits of a dynamical explanation is one that will hardly be questioned 
at the present time. We may look on the matter (including, if necessary, the ether) 
which plays a part in any physical phenomenon as forming a material system and 
study the dynamics of this system by means of any of the methods which we apply to 
the ordinary systems in the Dynamics of Rigid Bodies. As we do not know much about 
the structure of the systems we can only hope to obtain useful results by using 
methods which do not require an exact knowledge of the mechanism of the system. 
The method of the Conservation of Energy is such a method, but there are others 
which hardly require a greater knowledge of the structure of the system and yet are 
capable of giving us more definite information than that principle when used in the 
