]
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.
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