Electrical measurementsA laboratory manual . uld be nearly equal to each other,and that the resistances should be moderately large. The charge and discharge of long cables or of cablescoiled in tanks is much retarded by absorption and elec-tromagnetic induction. Hence when the time constantsof the two condensers compared are very different thebridge method may give a result largely in error, partic-ularly for rapid charge and discharge. To avoid thiserror the key K should be worked slowly. Example. Comparison of a subdivided condenser with one marked « microfarad, but found by an absolute dete
Electrical measurementsA laboratory manual . uld be nearly equal to each other,and that the resistances should be moderately large. The charge and discharge of long cables or of cablescoiled in tanks is much retarded by absorption and elec-tromagnetic induction. Hence when the time constantsof the two condensers compared are very different thebridge method may give a result largely in error, partic-ularly for rapid charge and discharge. To avoid thiserror the key K should be worked slowly. Example. Comparison of a subdivided condenser with one marked « microfarad, but found by an absolute determination to have acapacity of mf. Subdivisions. R, R., ^2 1046 7000 1042 7000 4140 7000 4151 7000 102. Comparison of Capacities by Gotts Method. — This is also a bridge method, but differs from the lastone in exchanging the places of the galvanometer andbattery. The arrangement is shown in Fig. 103. Two resistances B^ and B.^ are selected inversely pro- 220 ELECTRICAL ME A S portional to the supposed values of Ci and C,- Thekey Ki is then closed and clamped. After a few seconds key K2 is closed, and if anydeflection of the galvanometeroccurs, the condensers aredischarged by opening Ky andclosing K2. After readjustingRi or the operation is re-peated and continued till onclosing K2 with the batterystill in circuit no deflectionis produced. Since the two condensersare connected in cascade they must contain the samequantity and C\ Vi = C-^Tl, where Vi is the fall of poten-tial over i?i, and K, that over i?2-Hence a Vi Ry The battery remains in circuit except during the dis-charge of the condensers. For highest accuracy theresistances should be quite large and the capacitiesequal. The galvanometer key should be well insulated, aswell as the conductors leading to the condensers. It isnot necessarv to insulate the battery. 103. Correction for Absorption. —The last methodfurnishes a means of measuring the absorpt
Size: 1585px × 1576px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublisherbosto, bookyear1895
