Experiments 1-2: Electrification by Friction

Experiment 1: Electrification by Friction

1. Take a stick of sealing-wax, rub it on woollen cloth or flannel.

Then bring it near to some shreds of paper. The shreds of paper will move, the lighter ones will raise themselves on one end, and some of them will leap up to the sealing-wax.

Those which leap up to the sealing-wax sometimes stick to it for awhile, and then fly away from it suddenly.

Therefore, in the space between the sealing-wax and the table, there is a region where small bodies, such as shreds of paper, are acted on by forces.

These cause them to assume particular positions and to move sometimes from the table to the sealing-wax, and sometimes from the sealing-wax to the table.

These phenomena, with others related to them, are called electric phenomena.

The bodies between which the forces are manifested are said to be electrified. The region in which the phenomena take place is called the electric field.

Other substances may be used instead of the sealing-wax:

• ebonite
• gutta-percha
• resin or shellac
• amber

These phenomena were first noticed in amber. The word electric is derived from the Greek amber.

The substance on which these bodies are rubbed may also be varied. The cat’s fur excites them better than flannel.

Only those parts of the surface of the sealing-wax which were rubbed exhibit these phenomena.

Some parts of the rubbed surface are more active than others. In fact, the distribution of the electrification over the surface depends on the previous history of the sealing-wax.

This in a manner so complicated that it would be very difficult to investigate it.

There are other bodies, however, which may be electrified, and over which the electrification is always distributed in a definite manner. We prefer, therefore, in our experiments, to make use of such bodies.

The fact that certain bodies after being rubbed appear to attract other bodies was known to the ancients. In modern times many other phenomena have been observed, which have been found to be related to these phenomena of attraction.

They have been classed under the name of electric phenomena, amber, ἤλεκτρον, having been the substance in which they were first described.

Other bodies, particularly the loadstone and pieces of iron and steel which have been subjected to certain processes, have also been long known to exhibit phenomena of action at a distance.

These phenomena, with others related to them, were found to differ from the electric phenomena, and have been classed under the name of magnetic phenomena, the loadstone, μάγνης, being found in Magnesia∗

These 2 classes of phenomena have since been found to be related to each other, and the relations between the various phenomena of both classes, so far as they are known, constitute the science of Electromagnetism.

The name of Magnesia has been given to two districts, one in Lydia, the other in Thessaly. Both seem to have been celebrated for their mineral products, and several substances have been known by the name of magnesia besides that which modern chemists know by that name.

The loadstone, the touchstone, and meerschaum, seem however to have been the principal substances which were called Magnesian and magnetic, and these are generally understood to be Lydian stones.

Experiment 2: Electrification of a Conductor.

2. Take a metal plate of any kind (like a tea-tray, turned upside down).

Support it on 3 dry wine glasses.

Place on the table a plate of ebonite, a sheet of thin gutta-percha, or a well-dried sheet of brown paper.

Rub it lightly with fur or flannel, lift it up from the table by its edges and place it on the inverted tea-tray, taking care not to touch the tray with your fingers.

The tray is now electrified. Shreds of paper or gold-leaf placed below it will fly up to it.

If the knuckle is brought near the edge of the tray, a spark will pass between it and the knuckle. Then the tray will be discharged, no longer exhibiting electrical phenomena.

If a metal rod, held in the hand, is brought near the tray, the phenomena will be nearly the same.

The spark will be seen and the tray will be discharged. But the sensation will be slightly different.

If, however, instead of a metal rod or wire, a glass rod, or stick of sealing- wax, or a piece of gutta-percha, be held in the hand and brought up to the tray there will be no spark, no sensation, and no discharge.

The discharge, therefore, takes place through metals and through the human body, but not through glass, sealing-wax, or gutta-percha.

Bodies may therefore be divided into 2 classes:

1. Conductors

These transmit the discharge

2. Non-conductors

These do not transmit the discharge.

In electrical experiments, those conductors, the charge of which we wish to maintain constant, must be supported by non-conducting materials.

In the present experiment, the tray was supported on wine glasses in order to prevent it from becoming discharged.

Pillars of glass, ebonite, or gutta-percha may be used as supports, or the conductor may be suspended by a white silk thread.

Solid non-conductors, when employed for this purpose, are called insulators. Copper wires are sometimes lapped with silk, and sometimes enclosed in a sheath of gutta-percha, in order to prevent them from being in electric com- munication with other bodies. They are then said to be insulated.

The metals are good conductors; air, glass, resins, gutta-percha, caoutchouc, ebonite, paraffin, &c., are good insulators; but, as we shall find afterwards, all substances resist the passage of electricity, and all substances allow it to pass though in exceedingly different degrees.

For the present we shall consider only two classes of bodies, good conductors, and good insulators.

Experiment 3: Positive and Negative Electrification

3. Take another tray and insulate it as before.

After discharging the first tray, remove the electrified sheet from it and place it on the second tray.

Both trays are now electrified. If a small ball of elder pith suspended by a white silk thread be made to touch the first tray, it will be immediately repelled from it but attracted towards the second.

If it is now allowed to touch the second tray it will be repelled from it but attracted towards the first. The electrifications of the two trays are therefore of opposite kinds, since each attracts what the other repels.

If a metal wire, attached to an ebonite rod, be made to touch both trays at once, both trays will be completely discharged.

If two pith balls be used, then if both have been made to touch the same tray and then hung up near each other they are found to repel each other, but if they have been made to touch different trays they attract each other.

Hence bodies when electrified in the same way are repelled from each other, but when they are electrified in opposite ways they are attracted to each other.

If we distinguish one kind of electrification by calling it positive, we must call the other kind of electrification negative. We have no physical reason for assigning the name of positive to one kind of electrification rather than to the other. All scientific men, however, are in the habit of calling that kind of electrification positive which the surface of polished glass exhibits after having been rubbed with zinc amalgam spread on leather.

This is a matter of mere convention, but the convention is a useful one, provided we remember that it is a convention as arbitrary as that adopted in the diagrams of analytical geometry of calling horizontal distances positive or negative according as they are measured towards the right or towards the left of the point of reference.

In our experiment with a sheet of gutta-percha excited by flannel, the electrification of the sheet and of the tray on which it is placed is negative; that of the flannel and of the tray from which the gutta-percha has been removed is positive. In whatever way electrification is produced it is one or other of these two kinds.

I find it convenient to fasten the other end of the thread to a rod of ebonite about 3 mm diameter. The ebonite is a much better insulator than the silk thread, especially in damp weather.