As each coil will not be of very high resistance, the continuity of the wire can be readily tested by means of a few cells of battery, connecting one end of the coil to one pole of the battery, and the other pole of the battery and coil end touched to the tongue. If a burning sensation is experienced, the connection is not broken. Where possible the coils should be measured as to their resistance on a Wheatstone bridge.
When the requisite number of coils has been prepared, they are assembled in the following manner (Fig. 9): The coils, having their aperture diameter graded, are placed in order, and starting with the one having the largest hole, it is slipped over the primary protection tube T, one end being brought out through a hole in the reel end drilled vertically or between the reel end and the coil. A couple of paper rings are then slipped on the tube, and another coil placed over them, having its ends connected as in Fig. 3. This process is continued until all the coils are in place. The annular space between the coils and the tube T (Fig. 9) is filled in with melted paraffin and the coils gently pressed together, so as to form a compact mass, paraffin being poured over the outside of the whole combination. Before winding any wire used in this work it must be perfectly dry, which end can be accomplished by subjecting the whole spool to a short period of baking in a moderately warm oven.
The accompanying table gives the length of No. 36 silk-covered wire that will fill a linear space equal to one thickness of the wire in different-sized rings. This size wire wound tight will give 125 turns per linear inch. Therefore on a ring having a middle aperture of 1½ inches and an outside diameter of 4 inches, there will be 156 turns, or a total length of 1347 inches. This is obtained as follows: 1½ inches × 3.1416 = 4.7124 (or 4.712); 4 inches × 3.1416 = 12.5664 (or 12.56); (4.712 + 12.56)∕2 = mean circumference—viz., 8.635 inches.
This mean × number of turns in thickness of ring between the two circumferences—viz., 156 = 1347 inches.
Table of Secondary Windings.
| No. 36 Silk-Covered Wire.125 Turns per Linear inch. 13,306 Feet per Pound | 1½″ Aperture Diameter, 4.712″ Aperture Circumference. | 2″ Aperture Diameter, 6.283″ Aperture Circumference. | 2½″ Aperture Diameter, 7.854″ Aperture Circumference. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Outside diameter | 4″ | 5″ | 6″ | 4″ | 5″ | 6″ | 5″ | 6″ | 7″ |
| Outside circumference | 12.56 | 15.70 | 18.84 | 12.56 | 15.70 | 18.84 | 15.70 | 18.84 | 21.99 |
| Mean circumference | 8.635 | 10.20 | 11.78 | 9.421 | 10.99 | 12.56 | 11.78 | 13.35 | 14.92 |
| Turns between circumferences | 156 | 219 | 282 | 125 | 188 | 250 | 156 | 219 | 282 |
| Distance between aperture and outside, in inches | 1.25 | 1.75 | 2.25 | 1 | 1.50 | 2 | 1.25 | 1.75 | 2.25 |
| Length of wire, in inches | 1347 | 2234 | 2650 | 1178 | 2066 | 3140 | 1838 | 2924 | 4207 |
To obtain the length of wire necessary for a ring occupying more than the space of one turn on the primary insulating tube, multiply the length before obtained by the number of turns in the space it occupies. Thus a flat ring one-tenth of an inch thick would equal 1347 inches × 12.5.
This rule is necessarily only approximate, owing to the way the wires bed on each other from their cylindrical section. In actual practice, when the wire is run through the paraffin bath not more than 50 per cent of the calculated wire will occupy the space. And the thickness of the paper rings must also be added when figuring the total length of the coil. In the iron-clad transformers or induction coils of highest efficiency used in the alternating current electric light system, the rule for determining the windings of the coils is based on the ratio of the turns of wire in the primary to the turns in the secondary, the electromotive force in the primary, and the lines of force cut by the windings.
The secondary ends can be attached to binding posts mounted on the reel ends. Unless these reel ends be very high and clear the outside of the coil considerably, it is better to mount the binding posts on the top of the hard rubber pillars. A neat plan is to mount on the top of the coil a hard rubber plate reaching from reel end to reel end, and place the binding posts on that.
A discharger consists of two sliding metal rods with insulated handles passing through pillars attached to the secondary coil. The inside ends of these rods is provided with screw threads for the ready attachment of the balls, points, etc., which are to be used. The substance to be acted upon is laid on a rubber or glass table midway between the rod pillars and slightly below the level of the rods.