of Edinburgh, Session 1870 - 71 . 
497 
experiment which I have not at my command. When I say that 
6 inches of soft iron wire g 1 ^ of an inch in diameter can he melted or 
ignited by it, I only mean to say that the arm of an ordinary man, 
working briskly for a second or two, can accomplish this, though it 
would he hard work for him to continue the same for a minute. 
A stronger arm than usual, or a more ardent labourer, would do 
much more than this. A battery of six Bunsen cells, each with an 
effective surface of 42 square inches, melted 5 inches of the same 
wire. With an induction coil a spark of 1^ inches can he got with 
an expenditure of labour that may be continued for a minute or two ; 
with intense exertion a spark of 5 or even more inches may be got. 
By working reasonably for a minute from 2J to 4 cubic inches of 
explosive gas can he got from a voltameter ; working very hard for 
a quarter of a minute at the rate of 6 inches or more may be 
obtained. To turn a handle some 100 times a minute, more espe- 
cially against some resistance, is not work that can he easily con- 
tinued for minutes ; and such machines, when driven by the hand, 
are only good for incidental, not continuous use. To keep down 
the pull on the hand with a resisting circuit, the commutating 
collar of the magnetic coil has to be turned round from its position 
of maximum effect. There is a certain speed at which the hand 
can best work, for slow and difficult motion is not so convenient 
nor attended by so good results as quick and easy motion. 
The machine is well adapted for an educational instrument, viz., 
for illustrating electro-magnetic action. If the electro-magnetic 
coil he joined with one cell of Bunsen, and the electric coil with 
five or six cells, the conditions of the machine are reversed; and now 
electricity produces motion, instead of motion producing electricity. 
The handle is made to go round with considerable velocity, and if 
the belt that connects the gearing with the handle he removed, the 
armature alone spins round at a great rate. If now the poles of 
the magnetic coil be joined, the armature instantly slows, and the 
slowing is all the more marked the less the resistance of the circuit 
offered. The current of this new circuit can raise to a white heat 
about a \ inch of fine platinum wire. It may be worth mentioning, 
that the current given off by the magnetic coil under these condi- 
tions is singularly steady, and that its strength is something like 
inversely proportional to the circuit resistance. This slowing of 
