492 Prof. Oliver Lodge on the Paths of 
circuit is complete, except for just the metallic skin, and 
everything goes on i the outer field as if the energy-paths 
really started from both sides of the zinc-copper junction, 
and some of them cut through the metal of the external 
circuit with degradation into heat. 
Note, however, that the heat-energy in a wire is not really 
thus carried through the air: it creeps along the surface of 
the wire, some of it perpetually diving into the wire, and be- 
coming converted into heat. 
The energy which liberates hydrogen from the copper-acid 
surface, moreover, does not reach it in the way Mr. Poynting 
has supposed. Some of it crawls over the current the whole 
distance ; all of it goes some distance with the current. 
In the electrolyte, however, this 1s not so; and the heat 
which there makes its appearance is produced at the expense 
of energy which has arrived via the air from the zine-copper 
junction. ‘This is on the supposition that no contact-force 
exists between the electrolyte surrounding the plates and the 
dielectric medium (air) enveloping the rest of the circuit. 
In cases where such a contact-force does exist, some energy 
will creep along the air-liquid surface, just as it does over the 
surface of a metal. 
In the diagram one of the five lines here spoken of is 
doubled back upon itself, so as never to leave the surface of 
the zinc: the spaces between the lines are the real paths of the 
energy of course, not the lines themselves; the lines only 
indicate the directions across which no energy goes. There 
are five spreading-out energy-paths, then, and it is most 
instructive to watch the energy arising at the zinc-acid surface 
flow in its first narrow channels, then broaden out through 
space, and then cramp itself again into its tube, by which 
it reaches the copper surface, and is transformed into energy 
of chemical separation ; or, again, to watch that in the 
channels touching the copper fritter itself gradually away 
into heat. 
Energy-paths in Secondary Circuits. (Plate V.) 
The four figures in Plate V. are intended to indicate the 
transfer of energy to a secondary circuit, 7. e. one in which an 
induced current is being generated. 
Fig. 1 shows a steady electromagnetic field disturbed by the 
presence of a stationary copper channel. The energy simply 
flows round it, as Prof. Poynting has said. Fig. 2 shows a field 
no longer steady, but with the potential changing: the energy- 
paths moving, say, toward the right. Hach line, as it comes up, 
