inside diameter, and 3 millimetres thick, upon which are screwed two
flanges FF, 24 centimetres square, the space between the flanges being
about 3 centimetres. The secondary, SS, of the best gutta
percha-covered wire, has 26 layers, 10 turns in each, giving for each
half a total of 260 turns. The two halves are wound oppositely and
connected in series, the connection between both being made over the
primary. This disposition, besides being convenient, has the advantage
that when the coil is well balanced--that is, when both of its
terminals T_1 T_1 are connected to bodies or devices of equal
capacity--there is not much danger of breaking through to the primary,
and the insulation between the primary and the secondary need not be
thick. In using the coil it is advisable to attach to _both_ terminals
devices of nearly equal capacity, as, when the capacity of the
terminals is not equal, sparks will be apt to pass to the primary. To
avoid this, the middle point of the secondary may be connected to the
primary, but this is not always practicable.

The primary PP is wound in two parts, and oppositely, upon a wooden
spool W, and the four ends are led out of the oil through hard rubber
tubes tt. The ends of the secondary T_1 T_1 are also led out of the
oil through rubber tubes t_1 t_1 of great thickness. The primary and
secondary layers are insulated by cotton cloth, the thickness of the
insulation, of course, bearing some proportion to the difference of
potential between the turns of the different layers. Each half of the
primary has four layers, 24 turns in each, this giving a total of 96
turns. When both the parts are connected in series, this gives a
ratio of conversion of about 1:2.7, and with the primaries in
multiple, 1:5.4; but in operating with very rapidly alternating
currents this ratio does not convey even an approximate idea of the
ratio of the E.M.Fs. in the primary and secondary circuits. The coil