650 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
tially on certain definite properties of matter in regard to which different metals 
have remarkably different qualities. Thus in electro-magnetic engines the electric 
conductivity of the coils through which the current passes, and the magnetic induc- 
tive capacity and retentiveness of the iron cores of the electro-magnets, are essentially 
involved : and as essentially, when permanent magnets are used, the magnetic 
properties of steel, loadstone, or other bodies possessing strong retentiveness for 
magnetism. In the simple conversion of any kind of energy into heat by means of 
electric currents in metals, their electric conductivities are essentially and solely 
concerned. 
The inverse thermo-electric transformation of energy into an evolution and 
absorption of heat, at loealities of different temperature, in quantities differing from 
one another hy the thermal equivalent of the work spent in maintaining the current*, 
depends essentially on certain distinct properties of metals in regard to which their 
various qualities are shown by the differences of their positions in the thermo-electric 
series at different temperatures; and the accessory circumstances of such operations 
are influenced by the electric and thermal conductivities of the metals used. The 
same properties are involved in the direct thermo-electric transformation of energy 
in which electric currents, sustained by the communication of heat in a hot locality 
and the abstraction of a less quantity of heat in a locality lower in temperature, either 
produce any mechanical action, or are allowed to waste all their motive power in the 
frictional generation of heat'f-. 
* See “ Dynamical Theory of Heat, Part VI. Thermo-electric Currents,” §§ 105, 110. 
t “ .... a current cannot pass through a homogeneous conductor without generating heat in overcoming 
resistance. This elFect, which we shall call the frictional generation oy has been discovered by Joule to be 
produced at a rate proportional to the square of the strength of the current ; and, taking place equally with the 
current in one direction or the contrary, is obviously of an irreversible kind” (Dyn. Th. Heat, § 104). This 
definition was given merely to render circumlocution unnecessary in frequently referring to a mode of electric 
action which bore an obvious analogy to the action of a common fluid generating heat by friction among its 
particles as a dynamical equivalent to Avork spent upon it from without in forcing it to circulate in a tube, or 
otherwise keeping it in motion. It appears to me highly probable, however, that what I have, with reference 
only to recognized electric currents, defined as the frictional generation of heat, is precisely the mode of action 
by which all the heat is generated in every case when two solids are rubbed together. Certainly when two 
bad conductors of electricity are rubbed together, a portion of the heat of friction is generated in visible electric 
flashes ; and a charged Leyden battery contains, in potential energy, a dynamic equivalent for a portion of the 
heat of friction between rubber and glass never made till the battery is discharged. As certainly a portion of 
the heat of friction between a metal and a bad conducter of electricity is invisibly generated by electric currents 
through a very minute depth of the metallic substance beside its rubbed surface. The first effect of chemical 
forces of affinity, as Joule has so powerfully demonstrated in a variety of cases, is to press electricity into 
motion ; which motion may either subside into heat close to the locality of the combination (as when rough 
zinc is dropped into dilute sulphuric acid), or, reactively resisting the chemical combination, may transmit the 
work to a locality distant from the source, and may there either generate heat in a permanent metallic or other 
undecomposable conductor, or may, without any generation of heat at all, be wholly spent in effecting decom- 
positions against chemical affinities infinitely little less powerful than those from which it proceeds, or in raising 
weights. So it appears highly probable that the first effect of the force by which one solid is made to slide upon 
