THE CHARGES OF ELECTRIFIED BODIES

Experiment 6

17.] Take any deep vessel of metal,—a pewter ice-pail was used by Faraday,—a piece of wire gauze rolled into a cylinder and set on a metal plate is very convenient, as it allows any object within it to be seen.

Set this vessel on an insulating stand, and place an electroscope near it. Connect one elec- trode of the electroscope permanently with the earth or the walls of the room, and the other with the insulated vessel, either permanently by a wire reaching from the one to the other, or occasionally by means of a wire carried on an ebonite rod and made to touch the vessel and the electrode at the same time. We shall generally suppose the vessel in permanent connection with the electroscope. The simplest way when a gold-leaf electroscope is used is to set the vessel on the top of it.

Take a metal ball at the end of an ebonite rod, electrify it by means of the electrophorus, and carrying it by the rod as a handle let it down into the metal vessel without touching the sides.

As the electrified ball approaches the vessel the indications of the electroscope continually increase, but when the ball is fairly within the vessel, that is when its depth below the opening of the vessel becomes considerable compared with the diameter of the open- ing, the indications of the electroscope no longer increase, but remain unchanged in whatever way the ball is moved about within the vessel. This statement, which is approximately true for any deep vessel, is rigorously true for a closed vessel.

Fig. 9.

This may be shewn by closing the mouth of the vessel with a metal lid worked by means of a silk thread. If the electrified ball be drawn up and let down in the vessel by means of a silk thread passing through a hole in the lid, the external electrification of the vessel as indicated by the electrometer will remain unchanged, while the ball changes its position within the vessel. The electrification of the gold leaves when tested is found to be of the same kind as that of the ball.

Fig. 10.

Touch the outside of the vessel with the finger, so as to put it in electric communication with the floor and walls of the room. The external electrification of the vessel will be discharged, and the gold leaves of the elec- troscope will collapse. If the ball be now moved about within the vessel, the electroscope will shew no signs of electrification; but if the ball be taken out of the vessel without touching the sides, the gold leaves will again diverge as much as they did during the first part of the experiment. Their electrification however will be found to be of the opposite kind from that of the ball.

Experiment VII.

To compare the charges or total Electrification of two electrified balls. 18.] Since whatever be the position of the electrified bodies within the vessel its external electrification is the same, it must depend on the total electrification of the bodies within it, and not on the distribution of that electrification.

Hence, if two balls, when alternately let down into the vessel, produce the same divergence of the gold leaves, their charges must be equal. This may be further tested by discharging the outside of the vessel when the first ball is in it, and then removing it and letting the second ball down into the vessel. If the charges are equal, the electroscope will still indicate no electrification. If we wish to ascertain whether the charges of two bodies, oppositely elec- trified, are numerically equal, we may do so by discharging the vessel and then letting down both bodies into it. If the charges are equal and opposite, the electroscope will not be affected.

Experiment 8

When an electrified body is hung up within a closed metallic vessel, the total electrification of the inner surface of the vessel is equal and opposite to that of the body.

19.] After hanging the body within the vessel, discharge the external elec- trification of the vessel, and hang up the whole within a larger vessel connected with the electroscope.

The electroscope will indicate no electrification, and will remain unaffected even if the electrified body be taken out of the smaller vessel and moved about within the larger vessel. If, however, either the electrified body or the smaller vessel be removed from the large vessel, the electroscope will indicate positive or negative electrification.

When an electrified body is placed within a vessel free of charge, the ex- ternal electrification is equal to that of the body. This follows from the fact already proved that the internal electrification is equal and opposite to that of the body, and from the circumstance that the total charge of the vessel is zero.

But it may also be proved experimentally by placing, first the electrified body itself, and then the electrified body surrounded by an uncharged vessel, within the larger vessel and observing that the indications of the electroscope are the same in both cases.

Experiment 9

When an electrified body is placed within a closed vessel and then put into electrical connection with the vessel, the body is completely discharged. 20.] In performing any of the former experiments bring the electrified body into contact with the inside of the vessel, and then take it out and test its charge by placing it within another vessel connected with the electroscope. It will be found quite free of charge. This is the case however highly the body may have been originally electrified, and however highly the vessel itself, the inside of which it is made to touch, may be electrified.

If the vessel, during the experiment, is kept connected with the electro- scope, no alteration of the external electrification can be detected at the mo- ment at which the electrified body is made to touch the inside of the vessel. This affords another proof that the electrification of the interior surface is equal and opposite to that of the electrified body within it. It also shews that when there is no electrified body within the surface every part of that surface is free from charge.

Experiment 10

To charge a vessel with any number of times the charge of a given electrified body

21.] Place a smaller vessel within the given vessel so as to be insulated from it. Place the electrified body within the inner vessel, taking care not to discharge it. The exterior charges of the inner and outer vessels will now be equal to that of the body, and their interior charges will be numerically equal but of the opposite kind.

Now make electric connection between the two vessels. The exterior charge of the inner vessel and the interior charge of the outer vessel will neutralise each other, and the outer vessel will now have a charge equal to that of the body, and the inner vessel an equal and opposite charge.

Now remove the electrified body; take out the inner vessel and discharge it; then replace it; place the electrified body within it; and make contact between the vessels. The outer vessel has now received a double charge, and by repeating this process any number of charges, each equal to that of the electrified body, may be communicated to the outer vessel.

To charge the outer vessel with electrification opposite to that of the electrified body is still easier. We have only to place the electrified body within the smaller vessel, to put this vessel for a moment in connection with the walls of the room so as to discharge the exterior electrification, then to remove the electrified body and carry the vessel into the inside of the larger vessel and bring it into contact with it so as to give the larger vessel its negative charge, and then to remove the smaller vessel, and to repeat this process the required number of times.

We have thus a method of comparing the electric charges of different bodies without discharging them, of producing charges equal to that of a given electri- fied body, and either of the same or of opposite signs, and of adding any number of such charges together. 22.] In this way we may illustrate and test the truth of the following laws of electrical phenomena.

Fig. 11.

I. The total electrification or charge of a body or system of bodies remains always the same, except in so far as it receives electrification from, or gives electrification to other bodies.

In all electrical experiments the electrification of bodies is found to change, but it is always found that this change arises from defective insulation, and that as the means of insulation are improved, the loss of electrification be- comes less. We may therefore assert that the electrification of a body cut off from electrical communication with all other bodies by a perfectly insulating medium would remain absolutely constant.

II. When one body electrifies another by conduction the total electrification of the two bodies remains the same, that is, the one loses as much positive or gains as much negative electrification as the other gains of positive or loses of negative electrification. For if the electric connection is made when both bodies are enclosed in a metal vessel, no change of the total electrification is observed at the instant of contact.

III. When electrification is produced by friction or by any other known method, equal quantities of positive and of negative electricity are produced.

For if the process of electrification is conducted within the closed vessel, however intense the electrification of the parts of the system may be, the electrification of the whole, as indicated by the electroscope connected with the vessel, remains zero.

IV. If an electrified body or system of bodies be placed within a closed conducting surface (which may consist of the floor, walls, and ceiling of the room in which the experiment is made), the interior electrification of this surface is equal and opposite to the electrification of the body or system of bodies.

V. If no electrified body is placed within the hollow conducting surface, the electrification of this surface is zero. This is true, not only of the electri- fication of the surface as a whole, but of every part of this surface.

For if a conductor be placed within the surface and in contact with it, the surface of this conductor becomes electrically continuous with the interior surface of the enclosing vessel, and it is found that if the conductor is re- moved and tested, its electrification is always zero, shewing that the electrifi- cation of every part of an interior surface within which there is no electrified body is zero.

By means of Thomson’s Quadrant Electrometer it is easy to measure the electrification of a body when it is a million times less than when charged to an amount convenient for experiment. Hence the experimental evidence for the above statements shews that they cannot be erroneous to the extent of one-millionth of the principal electrifications concerned.