DRS. J. AND E. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 
335 
The amended formula now becomes 
I 
A, 
I 
4 'rrnc — l 1 f( — ) + 2l z j +/ 3 /( A ) + kf\ j-) + 2l 5 f 
A, 
Jvl\ 
A/ 
,/j; 
A, 
where l v is the mean length of lines of force in the wrought-iron yoke, A,, the area of' 
the yoke, l- a and A 5 corresponding quantities for the pole pieces, the last two terms 
being introduced for the forces required to magnetise the yoke and the two pole- 
pieces. 
Fig. 2. 
We now repeat the graphical method of construction exactly as before, the actual 
observations of induction in armature and current being plotted on the same diagram, 
Sheet II. (Plate 17), in which curve G represents the force required to magnetise the 
yoke, and curve H that required to magnetise the pole pieces. Before discussing 
these curves further, and comparing the results with those of actual observation, 
it may be convenient to describe the machine upon which the experiments have 
been made, confining the description strictly to so much as is pertinent to our present 
inquiry. 
Description of Machine. 
The dynamo has a single magnetic circuit, consisting of two vertical limbs, extended 
at their lower extremities to form the pole pieces, and having their upper extremities 
connected by a yoke of rectangular section. Each limb, together with its pole-piece, 
is formed of a single forging of wrought iron. These forgings, as also that for the 
yoke, are built up of hammered scrap and afterwards carefully annealed, and have a 
magnetic permeability but little inferior to the best Swedish charcoal iron. The yoke 
is held to the limbs by two bolts, the surfaces of contact being truly planed. In 
section the limb is oblong, with the corners rounded in order to facilitate the winding 
of the magnetising coils. A zinc base, bolted to the bed-plate of the machine, 
supports the pole-pieces. 
