104 
That this increase of magnetism was attended by an 
increase of dynamic electricity in the wires surrounding the 
armature and electro-magnet was manifest from the bril- 
liancy of the spark at the commutator and the increased 
deflection of a galvanometer needle placed within the 
influence of the electro-magnetic circuit ; but the quantity 
of electricity was not sufficient to ignite wires; and was 
comparatively small in proportion to the mechanical force 
expended in producing it. 
The apparatus used by Dr. Wheatstone in his experiments, 
which was also shown in operation at the meeting, was very 
similar in construction, and identical in principle, with that 
of Siemens’, though each of these experimentalists appears 
to have worked out his results independently. 
Instead of the long and thin wire used by Siemens in his 
machine, Wheatstone employed coils of thicker wires, and 
the wire on the electro-magnet was of the same diameter, 
and about eight times the length of that on the armature. 
The coils of the electro-magnet had consequently about 
eight times the resistance of the armature circuit. In this 
arrangement, the wire surrounding the armature being the 
generating circuit, it follows from a well-known property of 
the electric circuit, that the amount of dynamic, or useful 
effect, which the current is capable of exerting when the 
resistance of the coils of the electro-magnets is in circuit, is 
about one-nintli of the total amount which the armature 
coil is capable of evolving when the electro-magnet is excited 
to the same degree by a separate battery or other electro- 
motor. 
This peculiarity of the machine was made evident by Dr. 
Wheatstone in a very striking manner by means of the 
following well-devised experiment : — Two branch wires were 
led off from the extremities of the armature coil, and 
when the electro-magnet was fully excited the free ends 
of the branch wires were bridged across by means of a fine 
