of Edinburgh, Session 1870 - 71 . 
489 
nal current, and the second the external current; and the coils 
furnishing them I shall designate the magnetic coil and the electric 
coil respectively. The action of the magnetic coil is based on 
Siemens’ and Wheatstone’s principle of reciprocal increase. Wlien 
a Siemens’ armature revolves between the poles of an electro¬ 
magnet, what feeble magnetism there may be in the iron core 
generates a feeble current in the armature coil. This current, by 
a commutating arrangement of revolving collar and springs, is sent 
into the coils of the electro-magnet, which in consequence rises in 
power. It is now able to excite a stronger armature current, 
thereby rendering itself still more powerful, and this mutual action 
goes on until the driving force is insufficient to continue the 
action. Ladd has ingeniously turned this principle to account in 
his machine, the magnetic coil of which furnishes electricity for 
the electro-magnet, and this last is thereby rendered competent to 
generate electricity in the electric coil available for external use. 
Wishing to make a machine to give off a current equal to a few 
cells of Bunsen, I thought of trying the following deviation from 
Ladd’s construction:—Instead of having two separate armatures 
revolving on the same axis, I thought one might serve, in which 
two coils were inserted, the one at right angles to the other. In 
the revolution of a Siemens’ armature there are two polarities, so 
