| SINGLE SILK | 6.265 5.831 5.344 | DOUBLE SILK | 8.165 7.731 7.344 -------------------------------------------- TURNS PER | LINEAR INCH | SINGLE | COTTON | 95.000 100.700 106.000 | DOUBLE | COTTON | 66.80 68.80 71.20 | SINGLE SILK | 151.05 163.04 177.65 | DOUBLE SILK | 116.85 122.55 129.20 -------------------------------------------- TURNS PER | SQUARE INCH | SINGLE | COTTON | 9025.0 10140.5 11236.0 | DOUBLE | 4462.2 4733.6 5069.8 COTTON | | SINGLE SILK | 22820.0 26700.0 31559.0 | DOUBLE SILK | 13655.0 15018.0 16692.0 -------------------------------------------- OHMS PER | CUBIC INCH | SINGLE | COTTON | 567.0 811.0 1113.0 | DOUBLE | 305.5 409.8 545.5 COTTON | | SINGLE SILK | 1557.0 2266.0 3400.0 -------------------------------------------

If the length and the depth of the winding space of the coil as well as the diameter of the core are known, it is not difficult to determine how much bare copper wire of a given size may be wound on it, but it is more difficult to know these facts concerning copper wire which has been covered with cotton or silk.  Yet something may be done, and tables have been prepared for standard wire sizes with definite thicknesses of silk and cotton insulation.  As a result of facts collected from a large number of actually wound coils, the number of turns per linear inch and per square inch of B.&S. gauge wires from No. 20 to No. 40 have been tabulated, and these, supplemented by a tabulation of the number of ohms per cubic inch of winding space for wires of three different kinds of insulation, are given in Table IV.

Bearing in mind that the calculations of Table IV are all based upon the “diameter over insulation,” which it states at the outset for each of four different kinds of covering, it is evident what is meant by “turns per linear inch.”  The columns referring to “turns per square inch” mean the number of turns, the ends of which would be exposed in one square inch if the wound coil were cut in a plane passing through the axis of the core.  Knowing the distance between the head, and the depth to which the coil is to be wound, it is easy to select a size of wire which will give the required number of turns in the provided space.  It is to be noted that the depth of winding space is one-half of the difference between the core diameter and the complete diameter of the wound coil.  The resistance of the entire volume of wound wire may be determined in advance by knowing the total cubic contents of the winding space and multiplying this by the ohms per cubic inch of the selected wire; that is, one must multiply in inches the distance between the heads of the spool by the difference between the squares of the diameters of the core and the winding space, and this in turn by .7854.  This result, times the ohms per cubic inch, as given in the table, gives the resistance of the winding.