The wire is guided, when winding, by the fingers. If it is wrapped with a piece of felt and held between the thumb and forefinger it will run without friction and not cut the fingers. It is necessary that the wire should be closely watched for kinks, etc. which would cause the wire to break. Oftentimes the wire is broken but is held together by the insulation. Therefore each "pie" should be tested for continuity when completed. This is best accomplished by means of a galvanometer and battery. All imperfect "pies" should be rejected, as one of them would cause serious trouble if embodied in the coil. In soldering the secondary wires, acid must not be used as it soon corrodes the fine wires. Rosin is the best flux for this purpose. When building a small coil with a "layer" winding it is absolutely necessary that the wire should be wound on in smooth even layers. In a built-up secondary having "pies" not greater than 1/4 inch in thickness such great care is not necessary.
Fig. 31. Methods of Connecting the Secondary Sections.
Fig. 31 shows the methods of connecting up the pies or pancakes. In A, the inside of one section is connected to the outside of the next, and so on. The maximum voltage which can exist between the adjacent sections in this case is equal to the e.m.f. generated by one "pie" and is equal throughout. In B, the coils are connected alternately inside and out. The voltage ranges from zero at the points where they are connected, to a value equal to twice the e.m.f. developed by one section. It would seem that there would be a saving in insulation space of one-half in the first case, but it is not so since the connecting wire passes between the "pies" and therefore the insulation must be twice as thick or exactly equal to that in the second case. The latter method (Fig. 31 B) is the best and most convenient. When the "pies" are connected in this manner the current must flow through alternate sections in opposite directions. To accomplish this it is not necessary to wind every alternate coil in an opposite direction, but merely to turn them around and connect them with the direction of their windings reversed as shown by the arrows and the bevels in Fig. 31. The connections between the sections must be very carefully soldered.
After the secondary is assembled the coil should be placed in a tight receptacle or tank containing melted paraffin. The tank is then connected to an air pump or aspirator and the air exhausted. The diminution of pressure causes any air bubbles in the windings to expand and be pumped out. After standing a while, the pressure of the atmosphere is readmitted and the place of the bubbles will be occupied by paraffin which has been forced in under pressure.
Mounting.—A coil for wireless work is best mounted as shown in Fig. 32 and used with an independent interrupter. The coil may then be placed under the operating table or on the wall, out of harm's way, and the interrupter on the table, where it is handy to the adjustment of the operator.
Fig. 32. Coil Case.
The case is simply a rectangular hardwood box large enough to contain the completed coil. Two binding posts mounted on the side of the box connect with the primary winding and two on the top of the box lead to the secondary terminals. The box is filled with boiled oil or melted paraffin and sealed up by screwing on the lid. If desirable, the secondary binding posts may be mounted on the top of a short piece of hard rubber rod as illustrated in the drawing.