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