354 
DRS. J. AND B. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 
armature. It will be observed that the magnetic circuit is divided. The pole-pieces 
are of cast iron and are placed above and below the armature, and are extended 
laterally. The magnet cores are of wrought iron of circular section and fit into the 
extensions of the cast iron pole-pieces, so that the area of contact of the cast iron is 
greater than the area of section of the magnet. The magnetising coils consist of 
2196 convolutions on each limb of copper wire, No. 17, B.W.G., in No. 1 machine, 
and 2232 convolutions in No. 2 machine. The pole-pieces are bored to receive the 
armature leaving a gap on either side subtending an angle of 41° at the axis. 
The bearings are carried upon an extension of the lower jDole-piece. 
The following table gives the principal dimensions of the magnets in No. 1 
machine : — 
cms. 
Length of magnet limbs between pole-pieces . . 26‘0 
Diameter of magnet limb.8‘6 
Bore of fields.257 
Width of pole-piece parallel to the shaft . . . . 247 
Width of gap between poles.8'6 
The armature is built up of plates as in the machine already described, and is 
carried from the shaft by a brass frame between the arms of which the wires pass. 
The principal dimensions are as follows :— 
cms. 
Diameter of core.247 
Diameter of hole through core.14'0 
Length of core over end plates.247 
The core is wound on Gramme’s principle with 160 convolutions, each consisting of 
a single wire, No. 10, B.W.G., the wire lying on the outside of the armature in a 
single layer. The commutator has 40 bars. 
This dynamo is compound wound, and is intended for a normal output of 105 volts 
130 amperes, at a speed of 1050 revolutions per minute. The resistance of the arma¬ 
ture is 0'047 ohm, and of the magnet shunt coils 537 ohms. 
There is here no yoke, and consequently A 4 , and l h do not appear in the equation. 
It is necessary to bear in mind that the magnetising force is that due to the 
con solutions on one limb, and that the areas are the sums of the areas of the two 
Limbs. In calculating induction from E.M.F. it is also necessary to remember 
that two convolutions in a Gramme count as one in a Hefner-Alteneck armature. 
A x ;—the section of the core is 245 sq. cms., allowances for insulation reduce this to 
220‘ 5 sq. cms. 
l x ;—this is assumed to be 10 cms., but it will be seen that an error in this value has 
a much more marked effect on the characteristic in this machine than in the other. 
A 3 ;—the angle subtended by the bored face of the pole-pieces is 139°, the mean of 
