Medical Coil with Tube Regulation.
Fig. 14.
Figure 14 shows a coil with tube mode of regulation. The core C consists of a piece of iron tube, very thin, 4 inches long by ⅜ inch diameter, and filled with soft iron wires. One end of this core is firmly fixed in the left-hand bobbin head. The object of the iron tube is to prevent the sliding tube from catching in the iron wires, otherwise it can be dispensed with. Over this tube is slipped a brass tube T, ending in a handle H at the right-hand end; this must work easily over the core tube. The spool for the primary is now made up by fixing the other bobbin head on a paper or fibre tube and fastening its free end to the left-hand bobbin head, or the spool can be made in the usual way by glueing up two spool ends on a fibre or paper tube and securing the iron core firmly in one end, allowing room, of course, for the brass tube to slide in at the right-hand end. The primary winding is three or four layers of No. 20 B & S gauge cotton-covered magnet wire, the ends being brought out for future connection. Over this is now laid a few layers of paraffined paper, and ten or twelve layers of No. 36 B & S cotton-covered magnet wire is wound on for the secondary coil.
The contact breaker R is in no way different from the simple form described in Chapter II. Its construction can be readily seen from the figure.
A layer of cloth of the kind used in covering electromagnets is laid on over the secondary, and the coil is ready to be attached to the base. The base is seven inches long by three wide, and has little feet at its four corners to elevate it from the table and prevent abrasion of the connections underneath.
Fig. 15.
The connections are as given in Fig. 15. When in operation, the electrode cords being attached to binding posts, Nos. 1 and 2 are in circuit with the secondary coil only. When at Nos. 2 and 3 they receive the induced current or extra current in the primary, caused by the break of the battery circuit (see page 3).
Medical Coil with Interchangeable Secondaries.
This form of coil is the only one for practical medical work, and more space will be given to its construction than to the foregoing, which is suited only for limited use.
Fig. 16.
Fig. 16 shows side elevation of coil on base. The design can be largely varied, also it can be used either for a wall board, a cabinet top, or made to be carried in a case containing battery, electrodes, etc. S is one of the secondary coils, of which at least three should be provided. The dimensions are, of course, the same—namely, four inches long by 3½ inches wide over all. The spool ends are furnished with heel pieces, which slide under the brass track bar T. This accurately centres the coil and prevents it from working loose.