Faraday’s ice-pail experiments
A particularly successful application of the properties of electric field lines has been made in the explanation of Faraday’s ice-pail experiments on electrostatic induction. These experiments have nothing to do with ice, but owe their name simply to the fact that Faraday used an empty ice pail as a convenient hollow conductor.
In the first experiment a positively charged brass sphere E, supported by an
insulating silk thread, is lowered inside a hollow can standing on the cap of a goldleaf electroscope. A negative charge becomes induced on the inside of the can, and a positive charge on the outside of the can and the leaf (Fig. 32.22 (a».
So long as the sphere is well down inside the can and is not allowed to touch the can, it may be moved about inside without causing any alteration in the divergence of the leaf. When the charged sphere is allowed to touch the bottom of the can no change oecurs in the leaf divergence (Fig. 32.22 (b». On removing the sphere it is found to be completely discharged, and also there is no longer any charge on the
inside of the can. It is therefore concluded that the inducing charge on the sphere has exactly neutralized the opposite induced charge on the inside of the can, and hence the two are equal in magnitude. This result may be explained in terms of the properties of electric field lines as follows. Before the sphere is put into the can its field lines spread out in all directions and end on the walls, ceiling, furniture and so on of the room. When well down inside the can, all the field lines terminate on the inside of the can. Since field lines begin and end on equal and opposite quantities of charge, it follows that a total charge, equal and opposite to that on the sphere, must be induced on the inside of the can. Finally, when the sphere is allowed to touch the can the field lines shrink towards the point of contact and the charges at their ends neutralize one another. In addition to the field lines inside the can there are other lines which arise from the induced positive charge on the outside of the can, the leaf and the plate. These will terminate on the earthed case of the electroscope, where they induce an equal negative charge. The tension in the lines between leaf and case causes the leaf to diverge. The second ice-pail experiment is shown in Fig. 32.23. As in the previous experiment, the positively charged sphere B is lowered well down inside the can without touching, and the leaf divergence noted. The can and electroscope are now earthed momentarily by touching with the finger, with the result that the leaf collapses (Fig. 32.23 (b)). This occurs because the field lines between the earthed case of the electroscope and the outside of the can and the leaf-have shrunk towards the earthing point and the charges at their ends have neutralized one another. When the charged sphere is removed from the can the negative induced charge inside the can passes to the outside of the can and the leaf. Electric field lines are
now set up between the leaf and the earthed case and the leaf diverges by exactly the same amount as it did previously (Fig. 32.23 (e)). This means that the negative induced charge must be equal in magnitude to the positive induced charge. We finally conclude from these experiments that, when all the electric field lines from an inducing charge terminate on a conductor, the two charges induced on the conductor are each equal in magnitude to the inducing charge.