336 DRS. J. AND B. HOPKINSON ON DYNAMO-ELECTRIC MACHJNERY. 
The magnetising coils are wound directly on the limbs, and consist of 11 layers on 
each limb of copper wire 2’413 mms. diameter (No. 13, B.W.G.), making 3260 con¬ 
volutions in all, the total length being approximately 4570 metres. The pole-pieces 
are bored to receive the armature, leaving a gap above and below, subtending an 
angle of 51° at the centre of the fields. The opposing surfaces of the gap are 8 mms. 
deep. 
The following table gives the leading dimensions of the machine :— 
cms. 
Length of magnet limb. =45 '7 
Width of magnet limb.. . = 22‘1 
Breadth of magnet limb. = 44'45 
Length of yoke. = 61'6 
Width of yoke. = 48’3 
Depth of yoke. = 23‘2 
Distance between centres of limbs. =38T 
Bore of fields. =27'5 
Depth of pole-piece. =25'4 
Width of pole-piece measured parallel to the shaft . . =48‘3 
Thickness of zinc base. =12 - 7 
Width of gap.. =12‘7 
The armature is built up of about 1000 iron plates, insulated one from another by 
sheets of paper, and held between two end plates, one of which is secured by a 
washer shrunk on to the shaft, and the other by a nut and lock-nut screwed on the 
shaft itself. The plates are cut from sheets of soft iron, having probably about the 
same magnetic permeability as the magnet cores. The shaft is of Bessemer steel, and 
is insulated before the plates are threaded on. 
The following table gives the leading dimensions of the armature:— 
eras. 
Diameter of core. = 24‘5 
Diameter of internal hole. = 7’62 
Length of core over the end plates . . . =50’8 
Diameter of shaft. =6’985 
The core is wound longitudinally according to the Keener von Alteneck principle 
with 40 convolutions, each consisting of 16 strands of wire D753 mm. diameter, the 
convolutions being placed in two layers of 20 each. The commutator is formed of 
40 copper bars, insulated with mica, and the connections to the armature so made 
that the plane of commutation in the commutator is horizontal, when no current is 
passing through the armature. 
Plate 18, fig. 3 shows a side elevation of the dynamo ; fig. 4 a cross-section through 
