[Illustration:  Fig. 7.]

I wish now, in closing, to refer briefly to phenomena of moving lines of force, and to the effects of speed of movement.  In order to generate a given potential in a length of conductor we have choice of certain conditions.  We can vary the strength of field and we can vary the velocity.  We can use a strong field and slow movement of conductor, or we can use a weak field and rapid movement of the conductor.  But we find also that where the conductor has large section it is liable to heat from eddy currents caused by one part of its section being in a stronger field than another at the same time.  One part cuts the lines where they are dense and the other where they are not dense, with the result of difference of potential and local currents which waste energy in heat.  We cannot make the conductor move in a field of uniform density, because it must pass into and out of the field.  The conditions just stated are present in dynamos for heavy current work, where the speed of cutting of lines is low and the armature conductor large in section.

But we find that in a transformer secondary we can use very large section of conductor, even (as in welding machines) 12 to 15 square inches solid copper, without meeting appreciable difficulty from eddy currents in it.  The magnetic lines certainly cut the heavy conductor and generate the heavy current and potential needed.  What difference, if any, exists?  In the transformer the currents are generated by magnetic field of very low density, in which the lines are moving across the conductor with extreme rapidity.  The velocity of emanation of lines around the primary coil is probably near that of light, and each line passes across the section secondary conductor in a practically inappreciable time.  There is no cause then for differences of potential at different parts of the section heavy secondary.  Then to avoid eddy currents in large conductors and generate useful currents in them, we may cause the conductor to be either moved into and out of a low density field with very great speed, or better, we must cause the lines of a very low or diffused field to traverse or cut across the conductor with very high velocity.

It is a known fact that, in dynamos with large section armature conductors, there are less eddy currents produced in the conductors when they are provided with iron cores or wound upon iron cores than when the conductors are made into flat bobbins moved in front of field poles.  Projections existing on the armature between which the conductors are placed have a like effect, and enable us to employ heavy bars or bundles of wire without much difficulty from local currents.  The reason is simple.  In the armatures with coils without iron in them, or without projections extending between the turns, the conductor moves into and out of a very dense field at comparatively low velocity, so that any differences of potential developed in the parts of the section