432 The Royal Society Sorree. 
dimensions, being 10 inches in diameter, is driven round © 
within the bore, there being a free space of about I-20th of 
an inch allowed between them. Two armatures have been 
furnished, which are interchangeable with each other, ac- 
cording to the purpose required; one being for the produc- 
tion of heat, the other for light. The intensity armature is 
coiled with an insulated conductor, consisting of a bundle 
of 13 copper wires, the same as those coiled round the sides 
of the electro-magnet, the rope being 376 feet in length, 
and weighing 232lbs. The quantity armature is enveloped 
within the folds of an insulated conductor, consisting of four 
plates of copper, each 67 feet in length, 6 inches in width, 
and 1-16th of an inch in thickness. The plates are super- 
posed in metallic contact, so as to form a single copper: 
plate, one quarter of an inch in thickness, 67 feet in length, 
and nearly wide enough to occupy the entire space between 
the segmented sides of the armature. This division of the 
conductor into four plates is made for the greater conve- 
nience of binding round the armature. The inner extre- 
mity of it is held in intimate contact with the body of the 
armature by flat-headed screws, and the convolutions are 
insulated from one another by placing between them a 
band of thick cotton, the edges being also insulated from 
the sides of the armature by thin pieces of wood. The 
outer extremity of the conductor is terminated by a thick 
copper stud which connects it with the insulated half of 
the commutator. The convolutions in all the armatures 
are prevented from spreading out by the high centrifugal 
force of the rapid revolutions by being retained in place 
by broad brass bands. The whole of the large armatures 
are cased in wood to save them from accidental injury. 
The weight of the laminated conductor of the quantity-ar- 
mature is 344 lbs., and the total weight 1s more than a quar- 
ter ofaton. Each of the large armatures is accurately 
balanced, so as to avoid whilst working in the,bore of the 
magnet cylinder, those excessive vibrations which would 
otherwise be produced if any inequality in the weights of 
the ends existed, when the armature was being driven at 
the rate of 1,500 revolutions per minute. As the same 
double action takes place here as in the former case, 3,000 
waves of electricity are thus propagated, and are conducted 
by large copper springs to the poles proper of the machine 
and thence connected with two insulated plates of copper 
at the under side of a separate experimental table. Upon 
the upper side of this table are two movable brass studs 
