‘sections h and h’, h’ being in fact the projec- 
30 Law of the Induction of an Electric Current upon ttself. 
through metallic wires, on the presumption that the resistance to 
conduction is slight compared with the electro-motive and indue- 
tive forces concerned, it being understood however, that they are 
offered only as conclusions to be hereafter submitted to the test 
of experiment. © 
17. In discharging a Leyden jar through a wire, the current once 
generated by which the discharge is effected, is not to be checked 
except by the exertion of a similar amount of electro-motive force 
in the contrary direction to counterbalance or neutralize the in- 
duction at the cessation or falling of the current. Hence the cur- 
rent rushes on as if it had momentum, until it charges the coat- 
ings of the jar oppositely to their original charge, when a second 
discharge in the opposite direction to the first commences, and 
thus a series of vibrations ensues, as first suggested by Prof. Henry, 
fore receive strong support from numerous unpublished experi- — 
ments of Prof. Henry, in which he informs us he has shown the — 
existence of these vibrations in the discharge of a jar. ; 
18. It may not be uninteresting to trace 
some geometrical relations. In fig. 8, let abcd 
ea portion of a conducting wire, of which oe 
def is a cross section, and on def imagine a cc 
hemisphere dmc to be erected, and let an 
pher e erected, y 
longitudinal prismatic element or fibre no of n 
the wire be cut by the hemispherical surface 
and by the cross section so as to form the ad 
a 
a 
tion of the small spherical surface h upon ps te ae 
the plane def. Now if h’ be constant, the 
area of h it will readily be seen is inversely 
proportional to the shortest chord that can be 
drawn in the circle def through h’. Hence 
the rate of development of the current con- 
ducted by the elementary prism no is propor- 
tional to the area of the section of the latter 
by 
th 
>" 
