88 On the Galvanic Current. 
maximum effect of the induced eurrent, or the volume and noise 
of the’spark were so much increased, although the time required 
appeared to be the same. It was only from the great contrast 
manifested between almost “no current,” and a full development 
of a current which produced a secondary spark of nebular form, 
having an average diameter of three inches. I may remark here 
that the form of the spark depends entirely upon the manner of 
breaking the circuit. When the wires are separated slowly the 
spark is nebular in form, but when they are separated suddenly 
and carried to a considerable distance apart, the spark is linear or 
nearly so, and I have obtained it sometimes when a bar of soft 
iron was inclosed, from siz to eight inches in length, and I have 
no doubt that by following out the novel suggestion of my friend, 
Mr. Lane, viz., to shoot away the connecting wires by a cannon 
ball, a spark of a foot or more in length, might be obtained. 
About twelve years ago I noticed in this Journal a nebular spark 
of nearly one inch in breadth from a powerful magneto-electric 
machine, and a linear spark of #th inch in length from the same 
machine, but do not remember that they excited any special at- 
tention at that time. ‘Their very great magnitude in my present 
experiments, however, renders them objects of intense interest and 
beauty. When a soft iron bar is inclosed in the coils, the neb- 
ular spark is frequently spread out from five to six inches in 
breadth. But to return to the subject. Measuring the time of 
the rise and fall of the current by the metronome, it was found 
to be from 3th to 4th of asecond; when the coils were used alone 
and when a soft iron bar was included, the time was from $ to 3th 
of asecond.* The presence of the iron bar increases the initial 
induced current, and thus delays the rise of the battery currents, — 
and the terminal secondary is also increased and thus sustains or 
prolongs the duration of the battery current. 
It is very interesting to close the circuit in the coil surrounding 
the iron bar, and watch the subsidence of the current in the coil 
after the battery current has been entirely removed from it. On 
opening the circuit by separating the ends of the coil, sparks may 
