Measurement of a Quantity of Electricity

We have seen that if two points be maintained steadily at different potentials, and connected by a conductor, a current of electricity flows along the conductor and will produce a steady deflexion in a galvanometer, if there be one in the circuit. If, however, the difference of potential between the points be not maintained, the flow of electricity lasts for an exceedingly short time, sufficient merely for the equalisation of the potential throughout the conductor. A quantity of electricity passes through the galvanometer, but the time of transit is too short to allow it to be measured as a current in the ordinary way. The needle is suddenly deflected from its position of equilibrium, but swings back again through it directly, and after a few oscillations, comes to rest in the same position as before; and it is necessary for our purpose to obtain from theoretical considerations the relation between the quantity of electricity which has passed through the galvanometer and the throw of the needle.