When a thin plate of mica is put between tin foils, it heats excessively; and the fall of potential over the air films separating the mica and foil is great enough to cause disruptive discharge to the surface of the mica.  There appears to be a luminous layer of minute sparks under the foils, and there is a strong smell of ozone.  In a dielectric which heats, there may be three kinds of conduction, viz., metallic, when an ordinary conductor is embedded in an insulator; disruptive, as probably occurs in the case of mica; and electrolytic, which might occur in glass.  In a transparent dielectric the conduction must be either electrolytic or disruptive, otherwise light vibrations would be damped.  The dielectric loss in a cable may be serious.  Calculating from the waste in a condenser made of paper soaked in hot ozokerite, the loss in one of the Deptford mains came out 7,000 watts.  Another effect observed at Deptford is a rise of pressure in the mains.  There is as yet no authoritative statement as to exactly what happens, and it is generally assumed that the effect depends on the relation of capacity to self-induction, and is a sort of resonator action.  This would need a large self-induction, and a small change of speed would stop the effect.  The following explanation is suggested.  When a condenser is put on a dynamo, the condenser current leads relatively to the electromotive force, and therefore strengthens the field magnets and increases the pressure.

[Illustration:  T_{1} and T_{2} are large transformers; t_{1} and t_{2} are small transformers or voltmeters V_{1} and V_{2}.  The numbers 1, 4, 1, 25, represent their conversion ratios.]

In order to test this, the following experiment was made for the author by Mr. W.F.  Bourne.  A Gramme alternator was coupled to the low pressure coil of a transformer, and a hot wire voltmeter put across the primary circuit.  On putting a condenser on the high pressure circuit, the voltmeter wire fused.  The possibility of making an alternator excite itself like a series machine, by putting a condenser on it, was pointed out.  Prof.  Perry said it would seem possible to obtain energy from an alternator without exciting the magnets independently, the field being altogether due to the armature currents.  Mr. Swinburne remarked that this could be done by making the rotating magnets a star-shaped mass of iron.  Sir W. Thomson thought Mr. Swinburne’s estimate of the loss in the Deptford mains was rather high.  He himself had calculated the power spent in charging them, and found it to be about 16 horse power, and although a considerable fraction might be lost, it would not amount to nine-sixteenths.  He was surprised to hear that glass condensers heated, and inquired whether this heating was due to flashes passing between the foil and the glass.  Mr. A.P.  Trotter said Mr. Ferranti informed him that the capacity of his mains was about 1/3 microfarad per mile, thus making 2-1/3 microfarads for the seven miles.  The heaping up of the potential only took place when transformers were used, and not when the dynamos were connected direct.  In the former case the increase of volts was proportional to the length of main used, and 8,500 at Deptford gave 10,000 at London.