of Edinburgh, Session 1870 - 71 . 
491 
of diminished resistance is nearly as decidedly felt as that of in- 
creased resistance in Wilde’s machine on closing the electric coil 
circuit. The same feeling is not so decided in the case of the 
magnetic coil, and this, no doubt, arises from its smaller dimensions ; 
at any rate, there is no additional force needed. Whether this action 
has its origin in an essential difference in the action of permanent 
magnets and electro-magnets in these circumstances, or in some 
peculiarity of construction, is immaterial to the present inquiry, 
for to all appearance the armature currents cost no additional 
energy, but are got entirely from the waste heat of the armature. 
The core of the armature (fig. 4 a) is 11 inches long and 2J inches 
in diameter. The main longitudinal cut or groove 
is If inch wide and \ inch deep. The small cut 
is f of an inch wide and f of an inch deep.* In 
the large cut is wound the electric coil, consisting 
of a cable of 8 silk-insulated wires, of an inch 
iu diameter, and 82 feet long. The magnetic coil 
in the small cut is made of a cable of four such 
wires, 46 feet in length. The electric coil thus 
contains about four times as much wire, and offers about the same 
electric resistance as the magnetic coil. 
The two grooves leave four protruding ends at each end of the 
armature. To these are screwed a bronze cap and spindle of re- 
volution (figs. 4 and 5, which are on a larger scale than fig. 4 a). 
Fig. 4. Fig. 5. 
A collar of wood (a) is fixed next to the spindle, and on this collar 
two ferrules of iron (//fig. 5) are put, separated by the wood to 
prevent contact. To these ferrules the wires from the coils ( + - ) 
are soldered, care being taken to prevent unnecessary contact. A 
cylindrical collar (C C fig. 4) turns on the ferrules, and can be 
turned round and fixed in any position by screws (s s fig. 4). The 
collar is made up of three parts, two pieces of iron (one is shown 
* In the figure both cuts to be shown clearly appear of the same size. 
C o 
Fig. 4 a. 
