There is a considerable variation in the method of applying silk insulation to the finer wires, and it is in the finer sizes that the errors, if any, pile up most rapidly.  Yet the table throughout is based on data taken from many samples of actual coil winding by the present process of winding small coils.  It should be said further that the table does not take into account the placing of any layers of paper between the successive layers of the wires.  This table has been compared with many examples and has been used in calculating windings in advance, and is found to be as close an approximation as is afforded by any of the formulas on the subject, and with the further advantage that it is not so cumbersome to apply.

Winding Calculations. In experimental work, involving the winding of coils, it is frequently necessary to try one winding to determine its effect in a given circuit arrangement, and from the knowledge so gained to substitute another just fitted to the conditions.  It is in such a substitution that the table is of most value.  Assume a case in which are required a spool and core of a given size with a winding of, say No. 25 single silk-covered wire, of a resistance of 50 ohms.  Assume also that the circuit regulations required that this spool should be rewound so as to have a resistance of, say 1,000 ohms.  What size single silk-covered wire shall be used?  Manifestly, the winding space remains the same, or nearly so.  The resistance is to be increased from 50 to 1,000 ohms, or twenty times its first value.  Therefore, the wire to be used must show in the table twenty times as many ohms per cubic inch as are shown in No. 25, the known first size.  This amount would be twenty times 7.489, which is 149.8, but there is no size giving this exact resistance.  No. 32, however, is very nearly of that resistance and if wound to exactly the same depth would give about 970 ohms.  A few turns more would provide the additional thirty ohms.

Similarly, in a coil known to possess a certain number of turns, the table will give the size to be selected for rewinding to a greater or smaller number of turns.  In this case, as in the case of substituting a winding of different resistance, it is unnecessary to measure and calculate upon the dimensions of the spool and core.  Assume a spool wound with No. 30 double silk-covered wire, which requires to be wound with a size to double the number of turns.  The exact size to do this would have 8922. turns per square inch and would be between No. 34 and No. 35.  A choice of these two wires may be made, using an increased winding depth with the smaller wire and a shallower winding depth for the larger wire.