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To order to test whether the inequality is due to absorption. of the charge by the dielectic of the condenser, the effect of variation in the time of charge should now be found. The condenser should be charged each time for 10 seconds instead of an instant only, and the rest of the experiment carried out as previously. Then the charging should be continued for 30 seconds and afterwards for 1 and for 5 minutes.

Tables of results for each time of charge should be given and leakage curves drawn alongside the one drawn previously.

If absorption by the dielectric occurs, the absorbed charge should make its reappearance after discharge on the insulated conductors of the condenser. To test this, if the condenser has no short circuiting key, arrange one in parallel with the condenser. Charge the condenser for 10 seconds, discharge for an instant through the short circuiting key and then insulate for 5 seconds. On now pressing the discharge key, the galvanometer will be deflected owing to the passage through it of the "residual" discharge.

Repeat the observations, allowing the condenser to remain insulated 10, 20, 40, and 60 seconds before taking the residual discharge.

To find the influence of the time of charging on the magnitudes of the residual discharges, charge the condenser for 20, 40, and 60 seconds, insulate in each case for 5, 10, 20, 40, 60 seconds and take the first residual discharges as before.

Express the results by curves and in tabular form, as follows:

Condenser A. First Residuals after charge for given time, discharge, and subsequent insulation for given time.

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To test for successive residuals, charge again for 20 seconds, then discharge through the short circuiting key. Insulate for 10 seconds, take the first residual, insulate for 10 seconds more and take the second residual, and so on till no more charge remains in the condenser.

If the galvanometer is not sufficiently damped to allow of the observations to be taken so rapidly after each other, take the residuals after intervals of 20 or more seconds. If the galvanometer takes much longer to come to rest, after taking the first residual in the way described, recharge the condenser, discharge, take the first residual through the short circuiting key, and the second residual through the galvanometer. Similarly for successive residuals.

Tabulate as follows and draw a curve with the magnitudes of the successive residuals as ordinates, and times of insulation since the first discharge, as abscissae.

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Take a second condenser not specially chosen to exhibit leakage and absorption, and test whether the leakage and absorption are small, as they ought to be in a good condenser.

SECTION LXXIV.

COMPARISON AND USE OF CONDENSERS.

Apparatus required: Condensers, discharge key, high resistance mirror galvanometer, cells.

It has been stated in the previous section that when a condenser is discharged through a galvanometer, the extent of the first swing from rest is proportional to the quantity of electricity discharged. If Q is this quantity, E the potential of the battery used to charge the condenser, and C the capacity of the condenser, then Q = CE.

The method of discharge may therefore be used either to compare the capacities of a number of condensers charged by the same battery and discharged through the galvanometer, or to compare the electromotive forces of a number of cells used to charge the same condenser.

Apply the method, using a Leclanché cell and a high resistance galvanometer which swings without too much damping, to determine the capacities of the condensers provided, assuming that of the standard condenser to be given in microfarads. The condensers should be placed in circuit in turn, charged for an instant, then discharged, and the extent of the swing of the galvanometer needle observed.

In order to see whether leakage has any effect on the observations, take discharges from each condenser after 10 and 20 seconds' insulation, as well as instantaneous discharges.

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Select the condenser which shews least leakage, charge it in turn by means of a Leclanché, a Daniell, a storage and a standard cell. Take two discharges through the galvanometer with each cell, and assuming the electromotive force of the standard to be for a Clark 1·433, for a Cadmium 1019 volts, calculate those of the other cells.

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SECTION LXXV.

DETERMINATION OF THE CAPACITY OF
A CONDENSER.

Apparatus required: Condenser, high resistance galvanometer, resistance coils, voltaic cell.

When coulombs of electricity are discharged through a galvanometer the needle is deflected and swings through an angle a which, neglecting the damping, has been proved (pp. 348, 9) to be given by the equations

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where H/G is the constant of the suspended needle, and N/AH' that of the suspended coil galvanometer.

If the quantity Q is the charge of a condenser of capacity C, due to an applied electromotive force E, then Q=CE and therefore

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The damping may be taken into account by introducing the factor

1 H 1 N

eNo2 on the right-hand side. The quantity or EG EAH may

be determined or eliminated, by observing the steady deflection of the galvanometer produced by the electromotive force E acting through known resistances.

Let a circuit be arranged as in Fig. 130, the cell which has served to charge the condenser having its terminals connected to a resistance R, which should be large compared to that of the cell, so that the difference of potential at the ends of R, may

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