insulated by being fixed on to elbow-bent rods of glass. These rods are set in stands that run, guided by rails, along a board three or four feet in length.

(ii.) A form of Peltier electrometer.—Insulated inside a glass shade, the method of insulation being indicated in the accompanying figure, is a frame of wire a b c d. Balanced on a steel point

is a magnetic needle n s, provided at the ends with gilt pith balls.

The apparatus is so placed that the needle, as directed by the earth's field, rests with the pith-balls just lightly touching the sides of the brass frame.

When the knob K is connected with a conductor at a + or potential, the frame and needle all become of a + or potential with respect to the earth; there is an electric field, and the needle is urged away from the frame towards the walls.

K

Or, in ordinary two-fluid language, if we connect K with a conductor that we have charged

FIG. ii.

or —, we

shall have the needle urged away from the similarly charged frame towards the walls of the room; and it finally rests in such a position that the couple due to the electrostatic field balances the couple due to the earth's magnetic field.

(iii.) We shall also use a

charged ebonite rod with which to test the sign of the charge that may be at any time causing the needle of the electrometer to be deflected; this testing having been explained earlier.

We may add that the electrometer can be made more or less sensitive by weakening or strengthening the magnetic field acting on the needle, by means of a controlling magnet, as explained in Chapter XVII. § 9.

(iv.) Source of electricity.-For charging the plates we shall use an electrophorus or frictional machine; and shall assume that it is so regular in its action that it is of a constant potential. That is, the machine will continue to give a charge to a conductor, cease to do so, or receive a charge back from the conductor, according as this latter is below, up to, or above, this fixed potential of the machine respectively.

The analogy would be a water-supply kept at a certain fixed level.

§ 2. Experiments with the two Condenser Plates. In the following experiments the two plates A and B are connected with the electrometers E, and E, respectively, and are insulated, unless the contrary is stated.

A

Experiment.—(i.) Let B be at some distance, say '5 mètre, from A, and let both plates be initially uncharged. Now charge A with (say) + electricity, until it will receive no further charge from the source. It will be found that

the needle Ex is deflected with electricity. At the same time the usual proof-plane test will show that on the face of B turned towards A there is - electricity; while Ep is deflected with + electricity.

In two-fluid language we should say that the charged body A induces a - charge on the face of B turned towards it, repelling the complementary to the further side of B and to E1 (see Chapter IV. § 14).

B

B

In potential language we should say that there is on B and Eg such a redistribution of the total charge (whose algebraic sum is zero) as to leave this system at one potential. Since the whole system is between a + charged body A and the earth, it must be. at a potential, since it would take work to bring our + unit from earth up to B. Hence + electricity tends to run from all parts of the system to the earth; and hence the needle of E1 is urged down an electric field between the brass frame and the walls.

B

Experiment.-(ii.) Put B to earth, by touching any part of it. It will be seen that Eß returns to zero deflexion, while E、 is deflected less than it was before, and further that A will now receive more charge from our source.

In two-fluid language we can say that we have allowed the repelled + to pass away. In consequence of this, A has further inductive action on B; so that there is more

on B than

before, and more + passes to earth. Corresponding to this larger charge on B, there is drawn over to the surface of A that faces B more of the + charge on A; and, as a consequence, E, is less deflected, and the whole will receive further charge.

A

(The reader may here look at the diagram given at end of this section. It represents roughly the effect that the presence of B has on the distribution of the charge on A.)

B

In potential language we may say that we have put B and E, to zero potential. There is therefore no field of force between the brass frame of E, and the walls, and hence the needle has no couple deflecting it.

Now, there was in (i.) on the side of B turned towards A a certain charge. This by itself would have made B of a potential; but, owing to the presence of the + charge on A, B was yet at a + potential. In (ii.) we have B at zero potential, since it is to earth. It seems clear, then, that there must now be a largercharge on B than there was before, since it is as close to the + charge of A as it was before and is yet at zero potential.

For a similar reason to that indicated in Chapter V. § 8, note, the presence of this larger charge near A will lessen the work required to bring a ' + unit' up to A. In other words the poten tial of A will be in this case (ii.) lower than it was in case (i.).

Experiment. — (iii.) Re-insulate B, and approach it nearer to A. We shall find that EB again diverges with +, while EA falls a little in its deflexion. And further, as we should expect from the falling off of the deflexion of EA, we can give A more charge from our source.

In two-fluid language we have a further separation of + and

on B, the + being repelled to the regions (such as E,) remote from A. There is also in A a further concentration of the charge on the face turned toward B, and consequently a weakening of the charge in E,. We should thus expect to be able to give more charge to the whole, A and E.

In potential language we have moved B higher up the elec trical hill' that exists between A and the earth, or into a region of higher potential than before. If, before, its charge of - electricity was such as to keep it just at zero potential, this charge will be insufficient to keep it at zero now that it is nearer to the + charged A. It will therefore again become of a + potential, or + electricity

will be ready to flow from it to the earth.

The electrometer

needle is again in an electric field, and is again deflected.

At the same time the potential of A will be lowered for a reason similar to that given in case (ii.).

Experiment.(iv.) Next put B to earth. deflected, EA falls slightly.

We find that EB is no niore

The discussion of this will be similar to that of Experiment (ii.). Experiment. (v.) Again insulate B and remove it further from A. The needle EB is deflected with this time, while EA is deflected more strongly with +.

In two-fluid language the removal of A and B from each other causes some of the + and electricities, concentrated on the opposed faces of A and B respectively, to pass away from these faces to the other portions of the conductors, and so to E and E In potential language the potential of A will be reason similar to that given in cases (ii.) and (iii.). a stronger field between the wire frame of the electroscope and the walls, and the needle will be more deflected.

raised, for a There will be

With respect to B, the charge that kept it at zero potential when in the nearer presence of A, is too much for this purpose when A is removed to a greater distance; it renders B in sign of potential; and the charged needle of the electrometer is in an electric field running down from the walls to the frame, and therefore moves up this field away from the brass frame.

It is important to note that by charging A when B is near, and by then removing B, we can get in A a charge of a much higher electrical potential, or level, than that of the source from which we charged A initially.

Experiment. (vi.) When A B are very close and B is to earth, the charge that can be given to A [roughly measured by counting the number of sparks given] will be seen to be very large indeed as compared with the charge that can be given to A when isolated.

The proof-plane, if it can be used, will show that this charge is mainly on the face of A turned towards B; while on the opposed face of B is an opposite charge of equal magnitude (see § 4).

If B be pushed still nearer, a brilliant blue spark will be seen to span the gap between A and B, and a strident 'crack' will be heard.

If a person holding B touch A, this discharge will pass through him and will give him a shock. Such an experiment must, however, be tried with caution.

The accompanying figures show roughly the cases of A charged when isolated, and of A and B acting as a condenser, respectively. The crowding of the signs + and - is meant to indicate the concentration of charge.

§ 3. Discussion of the Terms 'Bound' and 'Free.'-We have seen that when a body is charged with a certain quantity, say + Q, of electricity, there is induced on surrounding objects an equal

A

++++

++++

FIG. ii.

FIG. iii.

B

The

quantity of opposite sign, or Q (see Chapter IV. § 16). distribution of this quantity + Q on the conductor depends on its form and on the relative positions of surrounding bodies.

It is found to be part of the essential nature of 'electricity' that a + or charge does not exist alone; it is always the one or other side of an electrical field.

Now the charge on the plate A (see figure at end of § 2) induces on the whole an equal and opposite charge on the walls, &c., and the greater part of this will be on that surface of B which is opposed to A; since B is by far the nearest part of the earth that is presented to A.

If we now cut off B from the earth and insulate it, without otherwise disturbing the system, and then put A to earth, what will happen?

To begin with we may say roughly, but as we shall see in § 6 not quite exactly, there will pass away from A that part of the charge that can, by so passing away, get at and neutralise the corresponding part of the induced - charge. If B were still to earth, then all the charge would pass from A, and the two sides of the field (the equal and opposite + and charges) would close in and leave no field or charge; the + and would entirely 'neutralise' each other. Rut B has, we suppose, been insulated again before A is put to earth. Hence, to speak roughly, that part of the charge on A which answers to the charge induced on B, will have no tendency to pass away, since it cannot thereby get at the charge on B, which is the opposite side of its field. But if we stood on B, insulated from the earth, and then touched A, we should withdraw from A all that part of the charge answering to

G