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July, i914 .]
CURRENT TOPICS. 129 sions are that the apparent dielectric strength of insulating materials in thin sheet form is materially higher with small than with large electrodes. This is probabaly true of solid, liquid, and gaseous dielectrics, though the variations differ widely. With paper and cloth, and with electrodes varying between the limits of size given above, the variations may amount to 50 per cent. ; with oil, under the same conditions, the variation is more than iooo per cent. The electrostatic stress is concentrated between sharp points, and failure of the dielectric soon takes place; but if the needle points are rounded off, so that the electrodes have an appreciable area, the puncture value becomes much higher than with large electrodes. This means that the testing of thin dielectric materials must be done on some specified plan, in so far as the area of the electrodes is concerned, if any comparison is to be made possible between various materials. Other variables, such as temperature, moisture of the atmosphere, and the rate of application of the testing voltage, also "should be specified ; but these latter points are not dealt with in the present paper.
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