DRS. J. AND E. HOPKINSON ON DYNAMO-ELECTRIC MACHINERY. 
337 
the centres of the magnets; fig. 5 a section of the core of the armature, in a plane 
through the axis of the shaft. 
The dynamo is intended for a normal output of 105 volts 320 amperes at a speed 
of 750 revolutions per minute. The resistance of the armature measured between 
opposite bars of the commutator is 0'009947 ohm, and of the magnet coils 
16*93 ohms, both at a temperature of 1 3‘5° Centigrade; Lord Rayleigh’s deter¬ 
mination of the ohm being assumed. 
We have now to estimate the lengths and areas required in the synthesis of the 
characteristic curve. 
A x ;—from the length of the core of the armature (50'8 cms.) must be deducted 
3‘4 cms. for the thickness of insulating material between the plates ; the resultant 
area is, on the other hand, as has already been stated, slightly augmented by the 
presence of the steel shaft. A : is taken as 810 sq. cms. 
4this is assumed to be 13 eras.: i.e., slightly in excess of the shortest distance 
(12'6 cms.) between the pole-pieces. 
Ao;—the angle subtended by the bored face of the pole-piece at the axis is 129°, 
the breadth of the pole-piece is 48*3 cms., the diameter of the bore of the field is 
27‘5 cms., and, as already stated, the diameter of core 25‘5 cms., thus, the area of 
pole-piece is 1513 sq. cms., and the area of 129° of the cylinder at the mean radius 
of 13*0 cms. is 1410 sq. cms; this value is taken for A 2 in the curves drawn on 
Sheet I. (Plate 16). In Sheet II. (Plate 17) A 3 is taken as 1600, an allowance of 
190 sq. cms. being made for the spreading of the field at the edges of the pole- 
190 . .1-2 
pieces, or —^=1*2 cm. all round the periphery, that is — = 0*8 of the distance 
from iron of pole-pieces to iron of core. 
4 is 1*5 cm. 
A 3 is a little uncertain as the forgings are not tooled all over, it is here taken as 
930 sq. cms., but this value may be slightly too high. 
4 is 91 ’4 cms. 
A 4 is 1120 sq. cms. 
4 is 49 cms., being measured along a quadrant from the centre of the magnet, 
thus :— 
Fig. 3. 
A 5 is 1 230 sq. cms., intermediate between the area of magnet and face of pole-picce. 
Z 5 is 11 cms. 
v was determined by experiment as described below, and its value is taken as 1*32 ; 
MDCCCLXXXVI. 2 X 
