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.
Windings for Secondary.
The following windings for removable or interchangeable secondary coils are those most in use.
Coil No. 1. 4500 feet (.375 pound) No. 36 B & S, approximating 1800 ohms. This may be led out in three divisions by means of switch on coil head. First division, 4500 feet; second division, 3000 feet; third division, 1500 feet.