Making a Medical Coil.—Procure a well-seasoned walnut board about 21½ in. long, 3 in. wide, and ⅜ in. thick. From this cut one length 12 in. long for the base board a, and 3 pieces 3 in. square (like b) for the coil heads; when cut, a fillet 8 in. long must be nailed or screwed on the two sides of the base board (as shown in a); these fillets should be ¼ in. square section. Corresponding square nicks must be cut of two of the square heads (as shown at x in c). All the woodwork when thus squared and finished should be soaked for ¼ hour in melted paraffin wax, and then rubbed dry while still warm.
Obtain a thin brass tube (known in the trade as “triblet tubing”) about ½ in. diameter, 4½ in. long; turn up a short plug and button to fit one end of this tube and serve as a handle (see d). This may be fastened to the tube by driving in 3 fine brass brads, and filing off the heads flush with the tube.
109. Home-made Medical Coil
Cut up about 100 lengths of straight iron wire (best soft annealed) No. 22 gauge, say, about 4½ in. in length; fill the brass tube with them as tight as you can fit them; cut them all to the same length (they must protrude a little beyond the tube). Now draw out about 2 in. of the iron bundle and wrap it tightly round with twine, leaving about ½ in. free. Draw more out, and continue wrapping until you have wrapped to within ½ in. at each end of the bundle. Tie the string, and withdraw the bundle from the brass tube. Melt a little solder in a ladle, dip the ends of the iron bundle into soldering fluid (zinc dissolved in hydrochloric acid), and then at once into the melted solder. Allow the bundle to cool; file off the superfluous solder, so that the bundle will just enter freely into the tube. It should appear like e when the string has been removed.
The next operation is to make a good stout paper tube, also about 4½ in. in length, into which the brass tube d can slide easily. To make this, put a few turns soaped writing paper round the tube No. 1, then roll and glue seven turns of good stout brown paper, 4½ in. in length, round this writing paper, or else it will be difficult to draw out the tube. This paper tube f must be allowed to dry thoroughly while still on the brass tube d. When quite dry, it must be slipped off, the writing paper lining drawn out, and then it must be soaked for a few minutes in melted paraffin wax.
The iron bundle should also be allowed to stand in melted paraffin wax for some time, and then stood up to drain in a warm place. This will prevent rusting. When quite cold, all superfluous paraffin having been removed, a strip of brown paper, ½ in. wide, is rolled round one extremity of the iron bundle, until it is of such a diameter as to fit tightly into the paper tube f. This paper strip must be cut off at this point and glued tightly round the end of the iron bundle. The brass tube d is then slipped over the iron bundle until it just reaches the little paper collar just made. The brass tube and bundle together are pushed, button end first, into the paper tube f; and when the paper collar round the iron bundle is just about to enter the paper tube, it is to be well served with hot glue and forced into the tube. The whole must now be allowed to dry and set thoroughly.
Taking one of the 3 in. heads (the one which has not any nicks in the sides), bore a centre hole with a brace and centre bit, just large enough for the paper tube f, with its iron core, to fit tightly (see b). Putting a little thin good hot glue round the free extremity (the end opposite to that at which the brass enters), push it into the hole in the square head, until it projects about ⅛ in. on the other side. This must be allowed to dry thoroughly before proceeding to the next operation.
Now proceed to wind the primary coil. To this end, take about ½ lb. No. 24 silk-covered copper wire, and wind it round the tube, as shown at g, from end to end, in continuous layers, taking care to put a sheet of paraffined paper between each layer, and also to baste each layer with melted paraffin wax before winding on another. About 4 layers will thus be got on, and an even number of layers must be aimed at, so as to get the 2 ends of the wire at the same extremity, and able to fasten them under the binding screw y. To effect this, before screwing down the said screws, the ends of the copper wire are stripped of their covering and wound once round the screw of the binder. Free ends of wire, at least 6 in. in length, must be left for attachments, &c. This is shown at h.
This primary coil, with its iron core, sliding brass tube regulator, &c. may now be fastened to the base board by means of 2 screws from underneath, as shown at i, at 4 in. from one end, and therefore 8 in. from the other. One of the free ends of the primary wire is brought to one of the binding screws v, while the other connects to the clapper z. A short piece of wire connects the platinum screw pillar w, to the other binding screw, which is not visible, as it is behind the platinum pillar. At this point it will be well to try the working of the primary coil. For this purpose, couple up the 2 binding screws on the base board with a good bichromate cell. Connect the two binding screws u in i, with the 2 brass handles intended for use. Screw up the platinum screw w till the clapper z begins to vibrate. Now hold the handles in your hand. As long as the brass tube j is entirely over the iron core, little or no sensation is perceptible. If an assistant pulls out the tube, little by little, the current will be found to increase in strength until the regulator tube is quite out.