in; and second, to imperfect elasticity of the dielectric. Speaking of
the extraordinary rise of pressure on the Deptford mains, he said he
had observed similar effects with other cables. In his experiments the
sparking distance of a 14,000 volt transformer was increased from 3/16
of an inch to 1 inch by connecting the cables to its terminals. No
difference was detected between the sparking distances at the two ends
of the cable, nor was any rise of pressure observed when the cables
were joined direct on the dynamo.

In his opinion the rise was due to some kind of resonance, and would
be a maximum for some particular frequency. Mr. Mordey mentioned a
peculiar phenomenon observed in the manufacture of his alternators.
Each coil, he said, was tested to double the pressure of the completed
dynamo, but when they were all fitted together, their insulation broke
down at the same volts. The difficulty had been overcome by making the
separate coils to stand much higher pressures. Prof. Rucker called
attention to the fact that dielectrics alter in volume under electric
stress, and said that if the material was imperfectly elastic, some
loss would result. The president said that, as some doubt existed as
to what Mr. Ferranti had actually observed, he would illustrate the
arrangements by a diagram. Speaking of condensers, he said he had
recently tried lead plates in water to get large capacities, but so
far had not been successful.

Mr. Swinburne, in replying, said he had not made a perfect condenser
yet, for, although he had some which did not heat much, they made a
great noise. He did not see how the rise of pressure observed by Mr.
Ferranti and Mr. Kapp could be due to resonance. Mr. Kapp's experiment
was not conclusive, for the length of spark is not an accurate measure
of electromotive force. As regards Mr. Mordey's observation, he