quantity r'Q/P. This is in general the more convenient way of disturbing the balance of the bridge, as it is often necessary in order to get a perfect balance to adjust R to less than an ohm by placing a resistance in parallel with a portion of it. This auxiliary resistance then serves to make the required change of R.

=

To carry out the experiment, arrange the given coil S to form the fourth arm of the Post Office bridge; Q being the ratio arm adjoining S. Make Q-10 and P = 1000. 1000. Connect the galvanometer through a commutator to the junction between P and Q, and to that between R and S. Introduce also a commutator into the battery circuit, so that the current may easily be reversed. It is important that the battery should be one giving a constant electromotive force, e.g. a Daniell or storage cell. Leclanché cells are too variable. Adjust the zero of the galvanometer to the centre of the scale.

Leave the galvanometer circuit open, pass the current through the coil and notice whether the galvanometer shews a deflection. If it does, the coil acts directly on the needle and its position should be changed so that the action ceases. This is most easily secured by placing the coil with its plane nearly horizontal and tilting it till the magnetic field due to it is vertical at the galvanometer. Now close the galvanometer circuit and balance for steady currents.

When the balance is approximately made, break the battery circuit and take an observation of the throw, which should be between 10 and 20 cms. as measured on the scale. If the throw is too large, the electromotive force of the battery should be diminished or a resistance inserted in the battery branch. If it is too small, the galvanometer should be made more sensitive, and if that is not possible the electromotive force of the battery must be increased. Having obtained an approximate value of the throw, the change of resistance r' in the arm R, which gives a steady deflection of between 11 and 12 times that of the throw, should be found.

If the galvanometer has a small logarithmic decrement and takes a long time to come to rest a small coil in series with an auxiliary battery and key may be fixed behind it, and con

nection may be made and broken in such a way as to bring the suspended system to rest.

The above preliminary experiments having been concluded, note the position of rest of the galvanometer needle. Obtain as accurate a balance of the bridge as possible, if necessary using an auxiliary resistance as a shunt in a portion of R. If there is a slow creeping of the spot of light while the steady current passes through the coil, it is probably due to the heating action of the current, which slowly increases its resistance. If that is the case, adjust the balance so that the spot of light is deflected a little to the side opposite to that towards which the creeping takes place. Wait till owing to the heating action the spot of light occupies accurately the zero position; then suddenly break the battery circuit and observe the throw, reading both the first deflection (d) and that which follows it on the same side (d). Make the battery circuit again, change the resistance R by the addition of r' and observe the steady deflection. Break the galvanometer and battery circuits and read the zero.

Reverse the commutator in the battery circuit and repeat the observations, both for the throw and the steady deflection.

Reverse the galvanometer circuit and again repeat the observations, first with the battery commutator in one and then in the other position.

Four sets of readings are thus obtained the means of which must be combined in the final result.

Determine the time of oscillation of the galvanometer needle by observing the time of say 50 swings if the time be short and the logarithmic decrement small; or if the time be long, observe the instant of passage through zero for say six swings, allowing an interval of six, then observe six more and combine in the usual way.

If it is desired to determine the logarithmic decrement this should now be done by one of the methods explained above, care being taken that the sensitiveness of the galvanometer is the same as that used during the experiment. The logarithmic decrement for the same galvanometer is proportional to its time of oscillation.

Tabulate your results as follows:

2 January, 1898.

Inductance of coil A. Galvanometer No. 327.

P1000. Q=10. 2 Daniell Cells.

Balance obtained with R = 2194.

On breaking circuit

First swing to right (mean of four observations) = 11.04 cms.

SECTION LXXIII.

LEAKAGE AND ABSORPTION IN CONDENSERS.

Apparatus required: Condensers, one of which is specially selected to shew leakage and absorption, high resistance galvanometer, discharge key, cells.

Unless the plates and terminals of a condenser are well insulated, the condenser is found gradually to lose its charge. The rate at which this loss takes place is generally greater the greater the charge which the condenser possesses.

To determine the extent to which leakage takes place in a condenser C (Fig. 127), connect it through a "condenser key" K to one or two Leclanché cells L and a galvanometer G, as shewn in the figure.

G

C

K

ᅦ는

Fig. 127.

When the movable part of the key is down, the condenser is in connection with the cells and is charging; when the disc marked “insulate on the key is pressed, the moving arm of the key takes up the position shewn in fig. 127, and terminals of the condenser are insulated from each other; when the disc marked "discharge" is pressed, the arm moves up to the upper contact in the figure, the terminals are connected together through the galvanometer, and the condenser discharges itself.

If the capacity of the condenser is C, the charge Q which an electromotive force E imparts to it is equal to EC.

Owing to the passage of the discharge through the galvanometer, the needle is deflected, and the extent of the first swing a from the position of rest is proportional, within the degree of accuracy required for the present purpose, to the amount Q of electricity discharged through the galvanometer.

Take several observations of the swings after charging and discharging have been performed in the least possible time. In order that the condenser may be properly discharged between each observation, allow the key to remain at the discharge

position for three minutes before again charging. The mean of these deflections may be taken to represent the charge of the condenser due to the applied electromotive force before leakage has had time to occur. They ought not to be less than 20 large scale-divisions.

Now take several observations, charging the condenser for an instant only as before, but insulating for ten seconds, then discharging, and allowing three minutes before again charging.

Repeat the observations for instantaneous charges followed by intervals of 30 seconds, 1, 2, 3, 4, and 5 minutes' insulation, and plot the results in the form

of a curve, taking times of insulation as abscissae and deflections as ordinates.

Take logarithms of the

30

Leakage Curves

3

Time insulated

Fig. 128.

Charged 5 mts. 1 mt.

1 sec.

5 mts.

deflections, subtract consecutive logarithms from each other, divide by the differences in the time of insulation, and tabulate the results as follows:

If the leakage at any instant were proportional to the charge in the condenser at that instant, the numbers in the last column would be equal. Their gradual diminution shews that the leakage increases more rapidly than the charge.