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XXXI. The Bakerian Lecture. — On the Electro-dynamic Qualities of Metals*'. 
By Professor William Thomson, M.A., F.R.S. 
Received February 28, — Read February 28, 1856. 
1. An electrified body may be regarded as a reservoir of potential energy, and any 
material combination in virtue of which bodies can receive charges of electricity is a 
ource of motive power. The development of mechanical effect from the potential 
energy of electricity, or through electric means from any source of motive power, 
may take place in a great variety of ways. For instance, electro-statical attractions 
and repulsions may become direct moving forces (as in “ Franklin’s Spider”), to do 
work in the discharge of an electrified conductor or in the continued use of a con- 
tinuous supply of electricity ; or the forces of current electricity may, as in any kind 
of electro-magnetic engine, become working forces on bodies in motion ; or the whole 
energy of the discharge may, as discovered by Joule, be converted into heat, which 
again may be transformed into other kinds of energy ; or the heat evolved and 
absorbed by electricity, in a circuit of two different metals, at the places where 
it crosses the junctions from one metal to the other, being a thermal result of 
dynamic moment'f- when the junction at which heat is evolved is at a higher tempe- 
rature than the junction at which heat is absorbed, may be used in a thermo-dynamic 
engine. Again, a thermo-electric current is a dynamic result derived from a definite 
absorption of heat in one locality and a definite evolution of heat in a locality of 
lower temperature. 
2. Of these various kinds of action, all except the first mentioned, depend essen- 
* The author has to acknowledge much valuable assistance in the various experimental investigations 
described in this paper, from his assistant Mr. M'^Farlane, and from M. C. A. Smith, Mr. R. Davidson, 
Mr. F. Maclean, Mr. John Murray, and other pupils in his laboratory. 
t Either an evolution of heat at a temperature higher, or an absorption of heat at a temperature lower, than 
that of the atmosphere, may be taken advantage of to work an engine giving mechanical effect from heat ; by using 
the atmosphere in one case as a recipient for discharged heat, in the other as the source of the heat taken in. 
Or an evolution of heat at any temperature and an absorption of heat at any lower temperature, may be taken 
advantage of for the same purpose, in a limited material system, neither taking heat from nor parting with heat 
to any external matter. Hence such a double thermal effect may be said to possess “ dynamic moment.” See 
the author’s “ Account of Carnot’s Theory of the Motive Powder of Heat,” §§ 4 to 11, Trans. Roy. Soc. Edinb. 
Jan. 2, 1849 ; also his “ Dynamical Theory of Heat,” §§ 8, 13, 23 to 30, Trans. Roy. Soc. Edinb., March 17, 
1851, and “ Dynamical Theory of Heat, Part VI. Thermo-electric Currents,” § 102, Trans. Roy. Soc. Edinb., 
May 1, 1854. The series of articles under the general title “ Dynamical Theory of Heat,” have been republished 
in a succession of Numbers of the Philosophical Magazine, viz. §§ 1 to 80, Vol. July to Dec. 1852 ; §§ 81 to 
96, Vol. Jan. to June, 1855 ; §§ 97 to 181, Vol. Jan. to June, 1856. 
