Fig. 21.
The Bobbins, on which the wire that serves to carry the magnetising current is to be wound, next demand our attention. They may be turned out of boxwood, ebony, or ebonite, or out of any hard wood strong enough and dense enough to allow of being turned down thin in the body, a very necessary requirement to bring the convolutions of wire as near the coil as possible without touching it. Some amateurs use the turned ends of cotton reels or spools, and glue them on to a tube of paper formed on the cores themselves. If this tube be afterwards well covered with melted paraffin wax, the plan answers admirably, but of course the bobbins become fixtures on the magnets. There are some persons who are clever enough to make firm bobbins out of brown paper (like rocket cases), with reel ends, that can be slipped off and on the magnet cores. To these I would say, "by all means at your command, do so if you can." The size of the bobbins for a 2½ in. bell should be: length 1¾ in., diameter of heads ¾ of an in., the length increasing ¼ of an in. and the diameter ⅛ of an in. for every additional ½ in. in the diameter of the bell. The holes throughout the bobbins should be of a size to fit the iron cores exactly, and the cores should project ⅛ of an inch above the end of the bobbins when these are fitted on. The wire to be wound on the bobbins is sold by all dealers in electrical apparatus. It is copper wire, covered with cotton or
with silk, to ensure insulation. Mention has already been made of what is meant by insulation at [§ 3], but, in order to refresh the reader's memory, Mr. G. Edwinson's words are quoted here. "To insulate, as understood by electricians, means to protect from leakage of the electric current, by interposing a bad conductor of electricity between two good conductors, thus insulating[11] or detaching them from electric contact."
The following list will enable my readers to see at a glance the value of the substances mentioned here as conductors or insulators, the best conductors being arranged from the top downwards, and the bad conductors or insulators opposed to them in similar order, viz., the worst conductors or best insulators being at the top:—
| Conductors. | Insulators. |
|---|---|
| Silver. | Paraffin Wax. |
| Copper. | Guttapercha. |
| Iron. | Indiarubber. |
| Brass. | Shellac. |
| All Other Metals. | Varnishes. |
| Metallic Solutions. | Sealing Wax. |
| Metallic Salts. | Silk and Cotton. |
| Wet Stone. | Dry Clothing. |
| Wet Wood. | Dry Wood. |
| Oil, Dirt and Rust. |
See also the more extended list given at [§ 5] for a more complete and exact classification.
It will be seen, on reference to the above, that copper is a good conductor, being excelled by silver alone in this respect; and that silk and cotton are bad conductors. When, therefore, a copper wire is bound round with silk or with cotton, even if two or more strands of such a covered wire be superimposed, since these are electrically separated by the non-conducting covering, no escape of electricity from one strand to the other can take place, and the strands are said to be insulated. If the copper wire had been coiled naked round a bobbin, each convolution touching its neighbour, the current would not have circled round the whole length of the coils of wire, but would have leapt across from one coil to the other, and thus the desired effect would not have been obtained. A similar result, differing only in degree, would occur if a badly insulating wire were used, say one in which the covering had been worn in places, or had been badly wound, so as to expose patches of bare copper wire. If the insulation of a wire be suspected, it should be immersed in hot melted paraffin wax, and then hung up to drain and cool. The size of wire to be used on a 2½ in. bell should be No. 24 B. W. G., the size falling two numbers for each ½ in. increase in the diameter of the bell. In these wires the higher the number, the finer the size, No. 6 being 1/5 and No. 40 being 1/200 of an inch in diameter. Silk-covered wire has an advantage over cotton-covered wire, inasmuch as the insulating material occupies less space, hence the convolutions of wire lie closer together. This is important, as the current has less effect on the iron if removed further