Absolute standards of inductance . icient thick-ness. The inside edge was now trimmed with a special tool, thecylinder being turned on its own axis. In the University coil twostrands were wound on at once, and the windings were pressedclose together during the process. A uniform tension was mechan-ically maintained on the wire. The windings remained parallel tothe original ones, so that after six or seven hundred turns theycoincided almost exactly with the rim at the other end. The wires go through the marble in the center of the plugs andimmediately return through the same holes. This allows
Absolute standards of inductance . icient thick-ness. The inside edge was now trimmed with a special tool, thecylinder being turned on its own axis. In the University coil twostrands were wound on at once, and the windings were pressedclose together during the process. A uniform tension was mechan-ically maintained on the wire. The windings remained parallel tothe original ones, so that after six or seven hundred turns theycoincided almost exactly with the rim at the other end. The wires go through the marble in the center of the plugs andimmediately return through the same holes. This allows the coilto be subdivided and yet does not impair the uniformity of the wind-ing ; indeed it is difficult to find the spots in question where thisdivision takes place. In the University coil, in which the winding is double, there arein reality four separate and distinct coils alike in pairs. The ter-minals to these four coils are brought up to a hard rubber plug nearone end of the cylinder on the inside. The wiring on the inside is. Fig. 6.—Scheme of Winding. i ] Absolute Standards of Inductance. loi right-angled, so that the inductances of these portions may be cal-culated and added to the total. They are, however, very small. Itis thus seen that any combination of the four coils may be made, thecurrent flowing in either direction in any section. This gives alarge number of different substandards whose values are standard of mutual inductance is also furnished by this coil,since if the self-inductance of the whole coil is known, as well asof the smaller coils, the following relation holds: from which J^^g may be calculated. The coil in the possession of the Bureau of Standards is subdi-vided into three smaller coils all unequal. Calling these coils i, 2,and 3, the following values have been calculated: L, ^(1+2) -^(1+2+3) A A2+3) L, which may be derived the following: A« A-. A-a A-3 -^1+2-3 A-2+3 -^-1+2+3-^*^1 2 M,, M^ 3 ^(1+2)3 ^1(2+3), and others. The
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