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Galvanometers Suitable for the Comparison of Capacities

The capacities of two condensers are compared most easily by comparing the quantities of electricity required to charge them to the same difference of potential, being directly proportional to these quantities.

Now the quantity of electricity required to charge a condenser to a given difference of potential will not depend on the resistance of the conductor through which the charge passes. The same total quantity will pass through the wire whatever be its resistance; the time required to charge the condenser will be greater if the resistance be greater, but, even if the resistance be many thousand ohms, the time of charging will be extremely small.

The effect produced on the galvanometer needle by a given quantity of electricity will be proportional to the number of turns of the wire of the galvanometer; thus for the present purpose the galvanometer should have a very large number of turns. This, of course, increases its resistance; but, then, this increase does not produce any evil effect. A galvanometer of five or six thousand ohms may conveniently be used. The time of swing of the needle should be considerable; a period of from two to three seconds will give fair results.

For the comparison of two capacities the damping does not matter greatly; it will affect all the throws in the same manner. If, however, it be required to express the capacity of a given condenser in absolute measure, it will be necessary to use a galvanometer in which λ can be measured with accuracy. The time of swing, too, since it requires to be accurately measured, should be greater than that mentioned above.

Last Update: 2011-03-19