Electricity for public schools and colleges . e plate I acts more or less as a conductingplate ; and then, like a conducting plate, it screens B from theaction of X to a greater or less degree {see Chapter X. § 17). Or again we can, by waiting, charge a Leyden jar from a sourceof constant potential with a larger charge than it would receive inthe first instance ; the specific inductive capacity of the glass appearingto increase with time. As explained in Chapter VI. § 10, there is a penetration of thecharge, and we findresidual charges. The true specific in-ductive capacity wouldbe that obtain
Electricity for public schools and colleges . e plate I acts more or less as a conductingplate ; and then, like a conducting plate, it screens B from theaction of X to a greater or less degree {see Chapter X. § 17). Or again we can, by waiting, charge a Leyden jar from a sourceof constant potential with a larger charge than it would receive inthe first instance ; the specific inductive capacity of the glass appearingto increase with time. As explained in Chapter VI. § 10, there is a penetration of thecharge, and we findresidual charges. The true specific in-ductive capacity wouldbe that obtained be-fore any such actionon the dielectric hadtaken place. § 3. Cavendisks Me-thod.—Cavendish de-termined, with a degreeof accuracy that is sur-prising when we con-sider the apparatusthat was at his dis-posal, the value of a for various dielectrics. His general method was as follows. He prepared a series ofcondensers constructed with glass and tin-foil. These he employedas arbitrary standards, with which he compared condensers in which. Earth. Fig. i. Earth. CH. IX. SPECIFIC INDUCTIVE CAPACITIES 12 various other dielectrics were used. In this way he obtained theinductive capacity of these materials as compared with that of thestandard glass. Then he determined the ratio borne by the inductivecapacity of this glass to that of air, and so obtained finally the specificinductive capacity {see definition) of the dielectrics employed. In fig. i. D represents in section the condenser in which the dielec-tric was the substance whose specific inductive capacity was represents one or more standard condensers. The upper plates ofboth were charged from the same source, and therefore to the samepotential ; and the under plates were put to earth or were at zeropotential. If the charges upon the upper plates were + a and + brespectively, then those upon the lower plates would be (very nearly)- a and - b respectively ; the charges on the upper plates exceedingthose on the lower
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