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.
Coil No. 2. 2400 feet (.6 pound) No. 31 B & S, about 350 ohms, divided into 2400 feet, 1500 feet, and 900 feet.
Coil No. 3. 750 feet (1 pound) No. 22 B & S in one coil, or two divisions of 500 and 750 feet, respectively; approximate resistance of wire, 125 ohms.
Coil No. 4. It may be necessary to obtain currents of extremely high tension, in which case a coil may be prepared of 5000 feet No. 38 B & S, or No. 40 B & S preferably.
The finer the wire, the less current and the most sedative effect; the coarser the wire, the more current with corresponding increased painful action.
The spools, in fact as much of the framework as possible, should be made of hard rubber, to which a fine finish can be given, although mahogany, rosewood, or even stained oak can be used. On each side of the right-hand spool heads a flat brass spring is screwed, making the contact for the secondary wires on brass strips screwed on top of the track rods. These secondary connections can be made by means of flexible cords to binding posts, but the sliding contact is preferable. The primary coil P is firmly held in the left spool head, and consists of a core of No. 22 B W G soft iron wires, insulated and wound with three layers of No. 20 B & S magnet wire. The outside of this coil is neatly enclosed in a hard rubber tube to permit of the secondary coils sliding freely upon it. It is better, however, for the secondary coils not to touch the primary tube. The vibrator, or contact breaker, should be of the adjustable form shown in Fig. 17. The adjusting screw for the contact breaker can be mounted in a brass lug carried by the spool head.
Connections of this coil are substantially the same as those of the first-described medical coil. This apparatus is well worthy of elaboration; it should be fitted with a ribbon vibrator as well as an adjustable speed slow vibrator, a switch controlling either. A great variety of secondary coils can be made, those of coarse wire taking the place of the current from the contact breaker. The vibrators should be operated from an independent battery, although in the last coil described the magnet may be wound with the same size wire as the primary and then be in series with it. The secondary spools can be made of stained hard wood ends fitted on to fibre tube, which latter is easily procurable. Particular attention should always be paid to the spools and heads; if not properly made, they may come apart, and a disastrous unravelling of the wires ensues.
Fig. 17.