Chap. XIX
Directions concerning the use of the electrical apparatus, with some practical rules for performing experiments with it to the best advantage.
The machine described in Chapter IV, or one similar to it, is capable of exhibiting the principal electrical phenomena, provided it be skilfully managed; but without such management it will constantly disappoint the electrician, and prove of little use. Let the following directions and observations, then, be attentively regarded.
1. Keep all the instruments as free as possible from dust and moisture.
2. When the weather is clear, the air dry and a little cold, the electric fire may be easily and copiously collected. But when the weather is hot or damp, the electrical machine is much less powerful.
3. Before the machine is used, the cylinder should be wiped clean, with a linen cloth that is soft, dry, and warm; after which a clean hot piece of flannel, or old silk handkerchief, may be applied with advantage.—This done, if the cylinder be turned pretty fast, when the prime conductor and other instruments are removed, the electricity, upon applying the knuckle or other conductor, will issue from the glass with a crackling noise, accompanied with sparks; this indicates the machine to be in good order, so that the electrician may proceed to perform his experiments. But if, when the cylinder is turned and the knuckle applied, no sparks be perceived, then the fault is most probably in the rubber. If so, it must be removed and held to a fire, so that its silk part may be dried. Then take a little tallow from a candle and just pass it over the leather of the cushion, after which spread upon it, a little amalgam, and force it as much as possible into the leather. Replace it, and let the cylinder be again wiped; the machine is fit for use.
4. Sometimes the electric matter will not be well collected, because the machine is not sufficiently supplied with it from the earth; which happens, when the table upon which the electrical machine is placed and to which the chain or wire of the rubber is connected, is very dry, and consequently a bad conductor. In this case, the best method is to connect the chain or wire of the rubber with some moist ground, or with the iron-work of a water-pump, if convenient. Thus the rubber will be supplied with as much of the electric fluid as is required.
5. When the cylinder is very hot (say above 110° of Fahrenheit’s thermometer,) it will not collect the electric fluid well.
6. When a sufficient quantity of amalgam has been accumulated upon the leather of the rubber, and the machine does not work well, then, instead of putting more upon it, a small quantity of that which is already on the leather must be taken off.
7. After the cylinder has been used for some time it will contract black streaks, which continually increase, and greatly obstruct its electric power.—These streaks must be taken off, and the glass frequently wiped to prevent their being again formed.
8. Coated jars, before they are used, ought to be made a little warm. If this be done, they will receive and retain the charge much better.
9. If one of the jars of a battery, as is sometimes the case, make a spontaneous discharge prematurely, it will of course discharge the whole battery; and in such case the faulty jar should be exchanged for one which is free from this defect.
10. In making the discharge of an electrical battery, or of a single jar, the electrician must be careful not to place the discharging-rod upon the thinnest part of the glass, as that may cause the bursting of the jar.
11. In large batteries, some of the jars frequently burst in the discharge. To remedy this inconvenience, Mr. Nairne says that the discharging-rod should never be made of a good conducting substance, except the circuit be at least five feet long. But here it may be remarked, that the length of the circuit weakens the force of the shock proportionably; the highest degree of which is in many instances required. When a coated phial is cracked, either by a spontaneous discharge or otherwise, the outside coating must be removed from the fractured part; then make it moderately hot by holding it near the fire; in this situation apply burning sealing-wax to the part, so as to cover the fracture completely, taking care that the thickness of the wax be rather more than the thickness of the glass; lastly, cover all the sealing-wax, and part of the glass beyond it, with a composition made of four parts of bee’s-wax, one of rosin, one of turpentine, and a little oil of olives: which composition must be spread upon a piece of oiled silk, and applied in the form of a plaister. In this manner jars which have been broken may be repaired effectually.
12. When a jar, and especially a battery, has been discharged, the wires ought not to be touched with the hand before the discharging-rod has been applied a second, and even a third time; as there generally remains a residuum of the charge, which is sometimes very powerful. This residuum is in a great measure occasioned by the electricity, which, when the jar is charging, spreads itself over the uncoated part of the glass, and which is not discharged at first, but gradually returns to the coating after the first discharge is made.
13. When an experiment is to be performed which requires only a small part of the apparatus, the remaining part should be removed from the table.—Candles should never be placed near the prime-conductor; for the effluvia of their flames carry off much of the electric matter.
14. One or two inches of the lower part of a Leyden phial should be coated with some thick paint, in order to prevent the amalgam, which is often scattered upon the table, from corroding the tin-foil, and thereby diminishing the charge.
15. When a prime-conductor is used, those sparks are strongest which are taken from the extremity farthest from the cylinder.
16. The longest sparks are drawn from any conductor along an electric substance. Thus, if the conductor be supported by pillars of glass or baked wood, the longest sparks may be taken close to the pillar. If the conductor be bent a little inward, so as to make the surface concave, a particularly large and undivided spark may be drawn from that place: but where the surface is convex, the spark is more apt to be divided and weakened.
17. It sometimes happens that cylindric or globe machines do not work well, owing to the air within them being too much rarefied by the heat of the cement, when the caps are fixed on. To remedy this, a small hole may be bored through one of the caps, so as to admit air into the cylinder or globe.
18. If the electric by any means become scratched, the working of the machine will be greatly impeded, if not altogether prevented. This is accounted for upon the principle that smooth and rough glass electrify differently when excited by the same rubber, and the two different states destroy one another. This may be remedied by filling up the scratches with a little tallow.
EPITOME
OF
ELECTRICITY.