143 
of Eclinhurgh, Session 1882-83. 
reversals of the alternate-current as being the true cause. In the 
Griscom motor the current comes from the source of electricity to 
one arm of the commutator attached to the Siemens armature, 
whence it is carried by the coil of wire surrounding the armature to 
the other arm of the commutator. The current leaving this arm of 
the commutator goes round the arms of the field magnets and 
magnetises the poles. Suppose now that by the rotation of the 
armature the reversal of the commutator takes place at the same 
time as the reversal, of the current, then the two simultaneous 
reversals will cause the current always to circulate in the same 
direction through the coils, and one part of the armature will always 
be north and the other always south. But the current exciting the 
held magnets is constantly being reversed, although so rapidly as to 
allow of only a very slight magnetisation. Thus suppose that 
when the left side of the armature is north, the upper held magnet 
is north, it follows that when this one is south the other is south 
also, so that there is always repulsion and consequent rotation in a 
direction opposite to the hands of a watch. 
It will be seen now that when we tried to start the motor with 
alternate currents, we had only the feeble interaction of the two 
pieces of iron extremely feebly magnetised; but when we have 
synchronous rotation, we have the more powerful interaction of a 
continuously excited and strong magnet (the armature), and the still 
feeble action of reversed magnetism of the field magnets. It is easy 
to see that if the armature tends to go too quickly or too slowly, the 
attractions between the movable and fixed parts will tend to check 
this irregularity. This accounts for the maintenance of the syn- 
chronism. It is also clear that if the speed be too much diminished 
VOL. XIL 
K 
