Discharger
One last piece of apparatus—the simplest in construction—remains to be made. Bend a piece of stout brass wire into a semicircle (A, [Fig. 7]); then, having flattened it at the middle, pass through a hole at the end of a wooden handle (B, [Fig. 7]), and fix in position by means of a tiny glued wedge (C, [Fig. 7]). Next twist the rod at both ends into loops, and wrap tinfoil round so as to form metal knobs (D, [Fig. 7]). Brass balls may be used instead. This arrangement is now complete, and we may commence experimenting.
11. Twist a stout piece of copper wire surmounted by a brass or tinfoil ball round the outside coating of a Leyden jar (A, [Fig. 8]), so that the two knobs are about 3 inches apart. Suspend a pith ball (B, [Fig. 8]), made from dried wood pith, by silk thread (C, [Fig. 8]) from some support above, so that it hangs normally midway between the two knobs.
Next excite the resin surface of the electrophorus by rubbing with a dry rabbit’s fur, muff, or flannel; and, holding the cover of this instrument by the handle’s extreme end, place it upon the resin. Remove immediately and bring near to the knob, which connects with the interior of the Leyden condenser, at the same time touching its outer tinfoil surface with the finger. A spark should pass between the electrophorus cover and the Leyden jar, whereupon, if the former and finger be removed, and the pith ball allowed to drop into position, this will oscillate violently to and fro between the knobs.
Fig. 8.—Oscillating pith ball.
Fig. 9.—Sparks from discharger and Leyden jar.
12. Construct a Leyden jar with knob terminal similar to that previously described, but cover part way up the outside with gold paint (A, [Fig. 9]), instead of tinfoil. Charge this condenser by means of the electrophorus cover, as described in [experiment 11], and if sufficient induction does not take place with one contact, replace the metal disc on the resin and pass a spark to the Leyden condenser knob several times, taking care to touch the gold paint coating with the finger on each occasion. If now the semicircular discharger, held by the wood handle, be brought so that one knob touches that of the Leyden condenser whilst the other moves over the gold-paint surface, long series of sparks, differing greatly in appearance from any previously produced, will pass between the gilt and the discharger.
13. Excite a stick of sealing-wax by rubbing with flannel, and bring it against a pith ball suspended by silk. The sealing-wax was negatively charged, and the pith ball is now in a like condition. To prove this, approach the flannel which excited the wax, and had consequently acquired a positive charge; inasmuch as the pith ball is immediately attracted and we know that positively excited bodies have an affinity for those negatively affected, the pith ball must be of the latter character.
If, further, a glass rod be excited by rubbing with silk and brought near to the negative pith ball, the latter will again be attracted, showing the glass to be positively charged.
On the other hand, the silk rubber repels a pith ball which has been in contact with excited sealing-wax, in the same way as it will repel the latter substance itself if suspended freely, thus indicating that the silk is similarly, i.e. negatively charged.
The phenomenon of positive and negative electricity may be somewhat differently demonstrated by means of the electroscope. Touch the terminal of this instrument with rubbed sealing-wax. The gold leaves diverge. Moreover, they may be thrown farther asunder by bringing the electrophorus resin near, so that evidently the charge of this instrument is also negative. Perhaps as a diversion it may be noted that resin is one of the chief ingredients of sealing-wax. The electroscope leaves remain apart even when the electrified body has been removed, but they may be made to collapse by touching the knob with the finger or bringing an excited glass rod near. The former action “earths” (i.e. connects, via the person’s body, with the house walls and the earth) the gold leaves and allows the electrical charge to escape, whilst the latter operation counteracts the negative charge by virtue of the positively excited glass.
14. If the electrophorus and electroscope be placed as shown in [Fig. 10], the gold leaves being already held asunder by a negative charge from sealing-wax, and the cover of the first instrument be lifted, whilst a strip of cardboard bridges from its upper surface to the disc terminal of the electroscope (A, [Fig. 10]), the instrument’s leaves will fall together again. From this behavior it may be gathered that the charge on the upper surface of the electrophorus cover is positive, or opposite in character to that induced by rubbing on the resin cake.
Fig. 10.—An experiment with the electrophorus and electroscope.
15. Stand a charged Leyden jar and the electroscope close together on the table. They must not be so near, however, that the gold leaves are affected by the condenser’s presence.
If, now, a few pieces of sulphur, spread upon the Leyden jar’s disc terminal, be ignited so as to burn freely, the gold leaves will immediately diverge slightly, indicating that part of the electric charge, which is being rapidly dissipated by way of the flame into the atmosphere, has traced a path to the electroscope.
Fig. 11.—The swinging mannikin.
16. An amusing variation of [experiment 11], wherein a pith ball was rendered restless between the two terminals of a Leyden condenser, is constituted by carefully modeling a little seated figure out of pith and sealing-wax. The mannikin is threaded on a silk strand, so that the legs hang down as though seated on a swing, and is then suspended by tying the two silk ends to a support above. [Fig. 11] indicates the arrangements.
Two Leyden jars are next required, of opposite influences. To effect this, charge one by the ordinary method of touching its terminal several times with an electrophorus cover, being careful on each occasion to “earth” the outer surface, whilst the other condenser is charged by presenting its outer surface to the electrophorus cover, and each time touching the knob terminal with the finger, i.e. “earthing” it.
By this method, whilst the charge at the knob of one Leyden jar is positive, that at the terminal of the other is negative. Place these two instruments at equal distances on either side of the “swing” support, and at such an elevation that the pith figure may just touch the brass knobs.
Then, if the swing be started going, it should continue to oscillate for a considerable time. As the pith figure approaches, say, the positive knob, it acquires a negative charge and is thereby attracted. Directly it touches, however, this negative state is counteracted, a positive charge supersedes it, and repulsion between pith figure and positive knob ensues. The pith doll then swings over towards the negative knob and—being positively influenced—is of course attracted until contact takes place. Then negative supersedes positive in the pith figure, repulsion again results, and a similar cycle of changes is repeated. Thus the figure continues to oscillate until the charges of the jars have been exhausted or have leaked away.
17. In the same manner as the alternate attraction and repulsion of a light body has been used to maintain oscillation, so similarly an electrostatic motor may be constructed embodying the same principles. Cut out a 16-pointed star of cardboard, 8-inch diameter, and, having glued a 1⁄4-inch slice of cork in the middle of each side (A, [Fig. 12]), pass a knitting-needle through the star’s center at right angles (B, [Fig. 12]). Both ends of this spindle rest in conical cavities (C, [Fig. 12]), which may be punched or drilled in small brass discs to act as bearings, and the apparatus is mounted as shown.
Fig. 12.—Electrostatic motor.
It is important that the cardboard should be perfectly dry, and with this end in view it may well have previously received a saturation in paraffin wax. When set hard, stick a brass pin bent at right angles into each point of the star, and set Leyden jars of different charges beneath any two diametrically opposite points. The motor, being given a start, should continue to revolve for some time owing to the succeeding attraction and repulsion of the metal pins.
The following description is of a small machine which may be fairly easily constructed, and when in good working order will give far better results than the electrophorus. [Fig. 13] shows the reference letters and general arrangement. Two cork bungs (A, [Fig. 13]), previously saturated with paraffin wax or shellac varnish, are fixed into the ends of a cylindrical lamp chimney, and through a hole in the exact center of each passes a wooden rod (B, [Fig. 13]), which must be glued in place. Each end is supported in a wooden bearing (C, [Fig. 13]), black lead being used as a lubricant, and to the protruding one a small handle (D, [Fig. 13]) is fitted, wherewith the glass may be revolved. Next a long pad of wash leather (E, [Fig. 13]), stuffed with horse-hair, is fixed so as to press gently against the cylinder, whilst a silk flap (F, [Fig. 13]) about 2 inches wide hangs over the top surface of the glass. A brass chain (G, [Fig. 13]) is also fixed to the wash-leather cushion and, in the ordinary way, connects to “earth” either direct or through the operator. The wash-leather cushion is well black-leaded where it touches the glass.
Fig. 13.—Frictional machine (more useful than the electrophorus for producing electric charges).
On the opposite side of the cylinder a metal comb (H, [Fig. 13]) is supported, the teeth being 1⁄16-inch away from the glass, whilst the back of the comb is soldered to a brass knob (I, [Fig. 13]) fixed on a wood pedestal. Any noticeable edges at the base of the brass globe should be filed off until only round surfaces are presented.
Care should be taken to dry the machine thoroughly, and if then the handle be turned regularly, as indicated by the arrow, the brass knob should rapidly become charged to such an extent that sparks may be drawn with the knuckles, possibly with discomforting results!
If the brass ball possess any sharp edges or, alternatively, if a length of wire girdle it so that one end projects radially about one inch, then the electricity will leak from these extremities in the form of a glow discharge as fast as it is developed. This frequently creates such a disturbance in the surrounding air as to distinctly blow a candle flame when held at the discharging point.
In 1752 Faraday invented an arrangement of attraction and repulsion by which three bells were kept ringing. To the electrical machine conductor was attached a metal support from the ends of which hung two bell domes on brass chains, whilst from the center another was supported by a silk strand and connected by a brass chain to earth. Between the bells small brass clappers were hung by silk threads, as shown in [Fig. 14].
The action in working was for the machine to charge its conductor and hence the two outer bell domes, which then attracted their brass clappers until contact took place. This caused repulsion, so that the clappers swung over to the center dome, where discharge to earth ensued and a condition obtained when the cycle of events might be repeated.
Fig. 14.—Faraday’s bell chimes.
Having thus arrived, via the simplest phenomena, at a piece of mechanism which will work, our experiments in the field of static electricity must conclude; but rather, let it be noted, on account of a dearth of apparatus than from any scarcity of phenomena to be studied. If therefore the reader foresees an opportunity of pursuing the subject further, such for example as the use of a well-equipped laboratory, let him not hesitate to snatch the advantage. To mention two facts demonstrable by further experiment: (1) Frictional electricity is produced in excessively small quantities at a very high pressure. For this reason sparks in air may be produced frictionally of such a length as cannot be obtained direct from batteries. (2) Friction itself is not a necessary element in the production of electrical energy, but is adopted in order to bring every portion of the rubbed surfaces into perfect contact. Thus far and no further. We must say “Au revoir,” and splash down a full stop.
[1] For the experiments in this and the following chapter I am indebted to the assistance rendered by Mr. Stanley S. Barnard.—C. H. B.